removal of some environmentally relevant heavy metals using low

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Environmental Engineering and Management Journal

March/April 2009, Vol.8, No.2, 353-372

http://omicron.ch.tuiasi.ro/EEMJ/

“Gheorghe Asachi” Technical University of Iasi, Romania

______________________________________________________________________________________________

REMOVAL OF SOME ENVIRONMENTALLY RELEVANT HEAVY METALS USING LOW-COST NATURAL SORBENTS Raluca Maria Hlihor, Maria Gavrilescu “Gheorghe Asachi” Technical University of Iasi, Faculty of Chemical Engineering and Environmental Protection, Department of Environmental Engineering and Management, 71 Mangeron Blvd., 700050, Iasi, Romania

Abstract The sorption of metal ions from aqueous solution plays an important role in water pollution control and in recent years there has been considerable interest in the use of low-cost adsorbents. Many researchers have tried to exploit naturally occurring materials as low-cost adsorbents, for removing of heavy metals. In this article the removal performance of various low-cost adsorbents derived from agricultural waste or natural material is evaluated. The metal adsorption capacities of low cost adsorbents presented vary, depending on the characteristics of the individual adsorbent, concentration of metal ions, pH, and contact time. Biosorption technology, utilizing natural materials and agricultural wastes either in natural form or modified form is highly efficient for the removal of metal ions from aqueous media and offers a cost-effective alternative compared to traditional chemical and physical remediation and decontamination techniques. The data obtained may be useful for environmental engineers in designing heavy-metal-containing wastewater treatment systems.

Key words: agricultural waste, biosorption, heavy metal, low cost adsorbents, natural material 1. Introduction Environmental pollution is an anthropogenic phenomenon, as a consequence of the industrialization process, and constitutes one of the major problems that has to be solved or controlled (Ünlü and Ersoz, 2006). Heavy metal are often discharged by a number of industries, such as mining, metal plating facilities, tanneries, which can lead to the contamination of freshwater and marine environment. Heavy metal ions are nowadays among the most important pollutants in surface and ground water (Brinza et al., 2009; Baysal et al., 2009; Gavrilescu, 2004). Metals are essential minerals for all aerobic and most anaerobic organisms, but it has been proven that in large amounts some of them, such as copper, lead, cadmium, or mercury, seriously affect human health. The human body cannot process and dispose the metals. As a result they are deposited in various internal organs and may cause adverse reactions and



serious damage to the body (Gavrilescu, 2004; WHO, 2007). Understanding the sorption of metal ions from aqueous solution is important in water and soil pollution control. Various methods for the removal of heavy metals from wastewaters include chemical precipitation, ion exchange, electro-dialysis, reverse osmosis, membrane filtration and absorption (Gavrilescu et al., 2009; Sadrzadeh et al., 2008). Adsorption was one of the recognized efficient processes of heavy metals removal from aqueous solutions, so that some studies have been carried out with the intention of developing new and especially cheap materials and sustainable adsorbents for the removal of heavy metals from wastewaters (Brinza et al., 2005; Gavrilescu et al., 2004; Pintilie et al., 2007). These materials should have high affinity, selectivity and capacity towards metals. Activated carbon is the most common used adsorbent; however, it is relatively expensive (Barkat et al., 2009; El-Sikaily et al., 2007; Etutu Ngoh, 2006; Ho et al., 2004; Schneider et al., 2007). Efficiency of peat in the

Author to whom all correspondence should be addressed: e-mail: [email protected]

Hlihor and Gavrilescu/Environmental Engineering and Management Journal 8 (2009), 2, 353-372

treatment of metal-baring industrial effluents has been reported by many researchers. This studies have suggested that peat is uniquely valuable as an inexpensive sorbent (Balan et al., 2008; Batista et al., 2008; Bulgariu et al., 2007; Kalmykova et al., 2008; Kicsi et al., 2006; Ho and McKay, 2004). Biosorption, as the property of certain biomolecules (or types of biomass) to bind and concentrate selected ions or other molecules from aqueous solutions is a rapid, reversible, economical and eco-friendly technology in contrast to traditional methods used for removal of heavy metals from aqueous streams (Gavrilescu, 2004; Tunali and Akar, 2006; Volesky, 2007). Biosorption achieved by some molecules and/or their active groups, such as dead biomass is passive and based mainly on the ‘‘affinity’’ between the (bio-)sorbent and sorbate, as opposed to a much more complex phenomenon of bioaccumulation based on active metabolic transport (Volesky, 2007). Since all biological material has an affinity for metals, the kinds of biomass potentially available for biosorption purposes are vast (Gadd, 2009). Natural materials or industrial and agricultural wastes are applied in biosorption technology to remove heavy metals from aqueous media. They offer an efficient and cost-effective alternative to traditional chemical and physical remediation and decontamination techniques. The use of agricultural residues or industrial by-product as biosorbents have been studied for years, although the cost of these processes proved to be sometimes expensive (Brinza et al., 2007; Doyurum and Çelik, 2006; Ghodbane et al., 2008; Lesmana et al., 2009; Nouri et al., 2007). Recently, application of a number of agricultural materials such as wheat steams (Tan and Xiao, 2008), chinese herb Pang Da Hai (Liu et al., 2006), grape bagasse (Farinella et al., 2008), modified and unmodified oil palm fruit fibre (Abia and Asuquo, 2007), peepul leaves, banana peels, peanut hulls, coir fibers, rice stem, teak saw dust, discarded tea leaves, mango leaves, rice husk, grass clippings (Gupta et al., 2008), tobacco dust (Qi and Aldrich, 2008), juniper bark and wood (Nouri et al., 2007), etc., have been reported for the removal of different heavy metals from aqueous solution. Studies of metal ion removal reported in the literature refer to either noncompetitive or single adsorbers or competitive adsorbers (Lesmana et al., 2009; Rowell, 2006). Noncompetitive adsorption describes the behavior of metal ion removal from an aqueous solution containing only one type of metal ion, the amount removed being about twice than that can be achieved by competitive adsorption. In the competitive sorption, the metal ions come into contact with an adsorbent and all the metal ions are adsorbed concurrently with varying degrees of achievement depending on their affinities for the functional groups on the sorbent (Rowell, 2006). Numerous researches are developed in order to find efficient and low-cost materials as sorbents (Li et al., 2008). A biosorbent can be considered low cost if 354

it requires little processing, is abundant in nature, or is a by-product or waste material from another industry (Gadd, 2009). For example, Kurniawan et al. (2006) discussed about the removal performance and cost effectiveness of various low-cost adsorbents derived from agricultural waste. The adsorption capacity of these low cost adsorbent are summarized and compared to those using activated carbon for the removal of heavy metals from metal-contaminated wastewater. Also, the removal of heavy metal ions from wastewater by chemically modified plant wastes was reviewed by Wan Ngah and Hanafiah (2008). An extensive list of plant wastes as adsorbents including rice husks, spent grain, sawdust, sugarcane bagasse, fruit wastes, weeds and others has been compiled in their research. Considering the major contribution of heavy metals to environmental pollution and their efficient elimination from environmental compartments, this paper attempts to summarize recent studies on the features of biosorption of this group of pollutants using natural sorbents. 2. Agricultural wastes One of the richest sources for low-cost sorbents is agriculture and especially the waste resulting from crops. The use of agricultural products and by-products has been widely investigated as a replacement for current costly methods of removing heavy metals from water and wastewater (Kumar, 2006). Due to abundant availability, agricultural waste poses little economic value and moreover, creates serious disposal problems (Kurniawan et al., 2006; Tilman et al., 2002). Agricultural materials, particularly those containing cellulose show potential metal biosorption capacity, so that diverse agro-waste have been successfully used to remove toxic heavy metals (Pb(II), Cd(II), Hg(II), Cu(II), Ni(II), Zn(II), Cr(III), and Cr(VI)) from contaminated industrial and municipal wastewaters using as is shown in Fig. 1 (Demirbas, 2008). The types of studied biomasses are different and depended on the type of agricultural production prevailing in the geographical areas where the studies were carried out (Nurchi and Villaescusa, 2008). The basic components of the agricultural waste materials biomass include hemicellulose, lignin, extractives, lipids, proteins, simple sugars, water hydrocarbons, starch containing variety of functional groups that facilitates metal complexation which helps for the sequestering of heavy metals (Sud et al., 2008). An analysis of the literature shows the very wide distribution of countries where the ecological application of biomasses to metal removal is being developed, and indicates a great interest in emerging countries such as India, Brazil, Turkey, Argentina and Nigeria (Fig. 2) (Demirbas, 2008; Kurniawan et al., 2008; Nurchi and Villaescusa, 2008; Sud et al., 2008).

Removal of some environmentally relevant heavy metals using low-cost natural sorbents

Fig. 1. Percentage interest in each metal ion in the literature on the use of agriculture biomasses for wastewater treatment

Modification of agricultural by-product could enhance their natural capacity and add value to the by-product (Kumar, 2006). Usually, these materials are previously conditioned in order to increase their sorption capacity or to give them the most appropriate form, so that they can be use in certain sorption systems or devices (Bailey et al., 1999). Biosorbents are prepared from the naturally abundant or waste biomass either by washing biomass with acids or bases, or even both, before final drying and granulation. Vieira and Volesky (2000) schematically summarized alternative process pathways to produce biosorbent materials, which are effective and sustainable in repeated long-term applications aimed mainly at removing metals from large quantities of toxic industrial metal bearing effluents (Fig. 3). 2.1. Removal of Cadmium

Fig. 2. Countries involved in studies on the use of agriculture biomasses for wastewater treatment

Cadmium is a non-essential element and one of the most hazardous trace elements, being considered a “priority metal” from the standpoint of potential hazard to human health (Alvarez-Ayuso and Garcia-Sanchez, 2007). Toxic metal ions such as Cd(II) can eventually reach the top of food chain and thus, become a risk factor for peoples health (Katircioglu et al., 2008). Cadmium, which is a widely used metal and extremely toxic in relatively low concentrations, is one of the heavy metals responsible for causing kidney damage, renal disorder, high blood pressure, bone fragility, and destruction of red blood cells (Abia and Asuquo, 2007).

Fig. 3. Most important operations for the treatment of different types of microbial biomass into usable biosorption materials (Vieira and Volesky, 2000)

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Tan and Xiao (2008) investigated the adsorption behavior of cadmium on ground wheat stems in aqueous solution to understand the physicochemical process involved and to explore the potentiality of wheat stems in wastewater treatment. The results showed that 0.1032 mmol of cadmium is adsorbed per gram of ground wheat stems. The results of the study indicated that wheat stems has great potential to remove cadmium ions from aqueous solution through a low-cost and eco-friendly way. Shin et al. (2007) compared the capacity of sorbents prepared from juniper wood (JW) and bark (JB) to adsorb cadmium (Cd) from aqueous solutions at different pH values. Adsorption kinetics, adsorption isotherms, and adsorption edge experiments were used to characterize adsorption behavior. Results from kinetics and isotherm experiments showed that JB had 3–4 times higher adsorption capacity for Cd than JW. In addition to higher capacity, JB exhibited a higher strength of adsorption (45.3 versus 9.1 L.mmol-1) and faster uptake kinetics (0.0119 versus 0.0083 g µmol-1min-1) compared to JW. For both these adsorbents, increasing Cd adsorption with increasing solution pH in the range of 2–6 suggests that surface carboxyl groups (RCOOH) might be involved in interaction with Cd. In the study of Al-Anber and Matouq (2008) olive cake was used as an adsorbent. It was generated during the squeezing of olive. Olive cake is an abundant and a low-cost adsorbent material on a large scale in many Mediterranean countries especially in Jordan. Jordan has a strong agricultural foundation that leaves behind 80,000 tones annually of olive cake wastes, with manure possibly being the most problematic one. The goal of this work was to study the capacity of using untreated olive cake to treat wastewater contaminated by the cadmium ion under different operating conditions (temperature, dose and pH). The adsorbent used in the study exhibited as good sorption at approximately pH 6 at temperatures 28, 35 and 45 C, respectively. The removal efficiency was found to be 66% at pH 6 and temperature 28 C. Researches have also been carried out by using wheat bran as sorbent, which proved to be much economical, effectual and more viable than conventional sorbents, according to Nouri et al. (2007). The ability of Neem oil cake (NOC), a biowaste material obtained as by-product of Neem fruit to remove Cu(II) and Cd(II) ions from aqueous solution was investigated by Rao and Khan (2009). Neem (Azadirachta indica) is a fast growing, usually evergreen plant, which reaches a height of 15–20m and a trunk girth of 1.5–3.5 m. Neem has been widely explored for solving various problems related to agriculture, public health, population control and environmental pollution. Neem has been recognized as a natural air purifier and it has been suggested that

356

the planting of Neem trees on roadside is an effective way to regulate traffic related pollution. The increase in initial concentration of Cu(II) and Cd(II) ions results in an increase in the sorption capacity, qe (mg g−1). The maximum sorption capacities at equilibrium were found to be 1.47, 2.4 and 4.94 mg g−1 at 15, 25 and 50 mgL−1 initial Cd(II) concentration, respectively. The equilibrium time was found to be 10, 50 and 60 min, respectively, for the initial Cd(II) concentrations of 15, 25 and 50 mgL−1 showing that equilibrium time depends upon the initial Cd(II) concentrations. Adsorption behavior of Ni(II), Cd(II) and Pb(II) from aqueous solutions by shells of hazelnut and almond, which are a very cheap and readily available material, was investigated for the removal of selected heavy metals from aqueous solutions by Bulut and Tez (2007). The shells of hazelnut (SH) and almond (SA) were obtained commercially and used for the preparation of adsorbent. It was washed several times with distilled water to remove surface impurities and then dried. The structural properties and surface chemistry of the shells were characterized using sorption of nitrogen and Boehm titration. The equilibrium adsorption capacity of shells was obtained by using linear Langmuir and Freundlich adsorption isotherms. The best correlation coefficients were obtained for the pseudo secondorder kinetic model. Ion exchange is probably one of the major adsorption mechanisms for binding divalent metal ions to the shells of hazelnut and almond. The selectivity order of the adsorbents is Pb(II) > Cd(II) > Ni(II). Most of the studies showed that agricultural waste either in natural form or modified form is highly efficient for the removal of cadmium metal ions. Summary of research work done has been compiled in Table 1. 2.2. Removal of Chromium Chromium exists in two stable oxidation states, Cr(III) and Cr(VI). Trivalent chromium (III) is an essential trace element for maintaining glucose, cholesterol, and fatty acid metabolism; its toxicity is relatively low. Hexavalent chromium (VI), however, is a carcinogenic and mutagenic agent that can inflict many health problems, including allergic contact dermatitis and other immunomodulatory diseases. Therefore, living organisms usually reduce Cr(VI) in vivo to Cr(III) to decrease its toxicity. In recent years, large quantities of chromium compounds have been used in industrial processes, leading to serious occupational problems and hazards to human health. Owing to the different toxicities of Cr(III) and Cr(VI), the distinction of chromium species is an essential aspect of understanding their diverse biological effects. Chromium species are excreted mainly by the kidneys and can be detected in the urine (Lee et al., 2008).

Removal of some environmentally relevant heavy metals using low-cost natural sorbents

Table 1. Summary of work of various researchers using variety of agricultural waste materials for the removal of Cadmium and other heavy metals Agricultural waste Calotropis procera Rice husk ash Styrax leaves Plum leaves Pomegranate leaves Walnut leaves Meddler leaves Wheat straw (Triticum aestivum) Sugar beet pulp

Metal ion Cd(II)

Adsorption model Freundlich

Cd(II) Ni(II) Cd(II)

Redlich–Peterson Freundlich Freundlich

Cd(II) Cu(II) Cd(II) Pb(II) Cd(II) Pb(II) Cd(II) Cr(III)

Langmuir

Adsorption capacity Batch-40mg/g Column mode50,5mg/g 0.0083mmol/g 0.0233mmol/g -

Removal capacity >90% 85% 85% 80% 78% 77% 87%

-

4.20 mg/g 4.16 mg/g 46.1 mg/g 43.5 mg/g 0.774 mmol/g 0.428 mmol/g -

Cd(II)

Freundlich

6.80 mg/g

-

Cd(II)

Langmuir Freundlich -

9.34 mg/g 5.82 mg/g -

85-90%

Cd(II)

Langmuir, Freunflich and D-R Freundlich Langmuir Freundlich Langmuir Langmuir

Cd(II)

Langmuir

Cd(II) Se(IV) Cd(II) Ni(II) Cd(II)

Langmuir Freundlich Langmuir

Cd(II)

-

Langmuir Langmuir Freundlich

Coffee husks (CH)

Cd(II) Cd(II) Co(II) Ni(II) Zn(II) Cd(II)

Eucalyptus Bark

Cd(II)

Grape bagasse Elaeis guineensis unmodified oil palm fruit fibre (UOPF) modified oil palm fruit fibre (MOPF) Nordmann fir leaves (Abies nordmanniana ) Compost cellulose pulp Carpobrotus edulis plant

Sporopollenin Maize leaf Maize (Zea mays) wrapper Orange peels Lemon peels Pine bark modified with Fenton reagent Modified rice husk Exhausted olive cake ash (EOCA) Activated carbon prepared from olive stone Euphorbia echinus Launea arborescens Senecio anthophorbium Carpobrotus edulis Papaya wood Orange peel cellulose

Cd(II) Cu(II) Pb(II) Zn(II) Cd(II) Cu(II) Pb(II) Cd(II) Cd(II)

Freundlich Langmuir

Langmuir Freundlich

Langmuir Freundlich Langmuir Freundlich

70–75% 80-85%

References Pandey et al., 2008 Srivastava et al., 2009 Salim et al., 2008

Dang et al., 2009 Pehlivan et al., 2008 Farinella et al., 2008 Abia and Asuquo, 2007

Serencam et al., 2008 Ulmanu et al., 2003 Chiban et al., 2008

0.0146 mmol/g 0.0195 mmol/g 0.0411 mmol/g -

94% 91% 99% 98% 94.65%

-

95%

0.55 mequiv./g 0.71 mequiv./g 30.2 mg/g

-

-

Adesola Babarinde et al., 2007 Adesola Babarinde et al., 2008 Santosh and Patil, 2008 Sukru and Dursun, 2008 El-Shafey, 2007

97%

Unlu and Ersoz, 2006

41.15 mg/g 45.92mg/g 8.38 mg/g 7.32 mg/g 1.7 mg/g

-

Elouear et al., 2008

80%

Kula et al., 2008

11-28 mg/g

>80%

Benhima et al., 2008

1.13 mol/kg 1.28 mol/kg 1.23 mol/kg 1.21 mol/kg 6.854 mg/g

94.9% -

Saeed et al., 2005 Li et al., 2008

65-85%

Oliveira et al., 2008

14.53 mg/g

-

Ghodbane et al., 2008

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Chromium is introduced into the natural bodies of water from industries like electroplating, leather tanning, cement industries, steel industries, and photography (Babu and Gupta, 2008). Fiol et al. (2003) used waste products from industrial operations, such as yohimbe bark, grape stalks, cork and olive stones for the removal of Cr(VI) from aqueous solutions. Equilibrium batch experiments at room temperature were performed. Metal uptake showed a pH-dependent profile and optimum uptake at initial pH (2.0–3.0). Slight influence of NaCl on metal uptake was observed. The sorption data fitted well to the Langmuir model within the concentration range studied. Grape stalks proved to be the most efficient sorbent followed by yohimbe bark, cork and olive stones. Babu and Gupta (2008) studied Cr(VI) removal from aqueous solutions using neem leaves. Neem leaves are activated by giving heat treatment and with the use of concentrated hydrochloric acid (36.5%). Batch adsorption studies demonstrate that the adsorbent prepared from neem leaves has a significant capacity for adsorption of Cr(VI) from aqueous solution. The parameters investigated in this study include pH, contact time, initial Cr(VI) concentration and adsorbent dosage. The adsorption of Cr(VI) is found to be maximum (99%) at low values of pH in the range of 1-3. A small amount of the neem leaves adsorbent (10 g/L) could remove as much as 99% of Cr(VI) from a solution of initial concentration 50 mg/L. The adsorption process of Cr(VI) is tested with Langmuir isotherm model. Application of the Langmuir isotherm to the system yielded maximum adsorption capacity of 62.97 mg/g. The most appropriate solution for controlling the biogeochemistry of metal contaminants is sorption technique, to produce high quality treated effluents from polluted wastewater according to Devaprasath et al. (2007). They evaluated the sorption capacity of Prosopis spicegera, a readily available tree leaves for removal of Cr (VI) from aqueous media. Adsorption studies were performed by batch experiments as a function of process parameters such as sorption time, pH, and concentrations of sorbate and sorbent. Freundlich model fitted best with the experimental equilibrium. The adsorption of Cr (VI) was found dependent on initial concentration of metal ion, pH, adsorbent dosage and agitation time. The maximum removal of Cr (VI) was observed at pH 2 (95%). Levankumar et al. (2009) investigated the removal of hexavalent chromium from aqueous solutions by Ocimum americanum L. seed pods. The powdered O. Americanum L. seed pods were reported as an effective adsorbent for the treatment of hexavalent chromium for the very first time. It was capable of removing 100% of chromium from the aqueous solutions of concentrations 100mg/L, 150mg/L and 200mg/L. The predicted maximum chromium adsorption capacity as 83.33mg/g showed that adsorbent prepared from the O. Americanum L.

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seed pods has reasonable chromium removal efficiency. The ability of low-rank Turkish brown coals to remove Cr(VI) from aqueous solutions was studied by Arslan and Pehlivan in 2007. Utilization of the brown coals for the treatment of aqueous solution containing Cr(VI) ions is gaining attention as a simple, effective and economical means of wastewater treatment. The mechanism of Cr(VI) ion binding to brown coals may include ion exchange, surface adsorption, chemosorption, complexation, and adsorption– complexation. The kinetics of Cr(VI) biosorption by brown coals was fast, reaching 50–90% of the total biosorption capacity in 60 min. The kinetics studies indicated that equilibrium in the adsorption of Cr(VI) on the brown coals was reached in 80 min of contact time between the brown coals and the solution. Cr(VI) adsorption on brown coals was described only by the Freundlich isotherm model. The adsorption of Cr(VI) increased with an increase in the concentrations of these metals in solution. The substantially lower cost, easily available of the lowrank coal indicates great potential for the removal of Cr(VI) ions from aqueous systems. The paper of Bansal et al. (2009) reported the feasibility of using pre-consumer processing agricultural waste to remove Cr(VI) from synthetic wastewater under different experimental conditions. For this, rice husk (agricultural waste obtained from the rice mills), has been used after pre-treatments (boiling and formaldehyde treatment). Maximum metal removal was observed at pH 2.0. The efficiencies of boiled and formaldehyde treated rice husk for Cr(VI) removal were 71.0% and 76.5% respectively for dilute solutions at 20 g L−1 adsorbent dose. According to the researchers this data can be used by small scale industries having low concentrations of Cr(VI) in wastewater using batch or stirred-tank flow reactors where standard material such as activated carbon is not available. Most of the studies showed that the chromium biosorption by agricultural waste materials is quite high and varies from 50 to 100%. Table 2 summarizes the work reported in literature for the removal of chromium by using agricultural waste materials. 2.3. Removal of Lead At present, lead pollution is considered a worldwide problem because this metal is commonly detected in several industrial wastewaters (Davydova, 2005). Lead is a more toxic element for human and animal lives. The presence of even low levels of lead in water is a concern primarily because it tends to bioaccumulate in the food chain (Prasad et al., 2008). Pb heads the list of environmental threats because, even at extremely low concentrations, lead has been shown to cause brain damage to children.

Removal of some environmentally relevant heavy metals using low-cost natural sorbents

Table 2. Summary of work done by various researchers using variety of agricultural waste materials for the removal of Chromium Agricultural waste Walnut shell Hazelnut shell Almond shell Spent brewery grains Not treated spent grains (NTSG) Treated spent grain (TSG)

Metal ion Cr(VI)

Adsorption model Langmuir

Cr(III)

Langmuir

Adsorption capacity 8.01 mg/g 8.28 mg/g 3.40 mg/g

Removal capacity 85.32% 88.46% 55.00%

References Pehlivan and Altun, 2008 Ferraz et al., 2005

17.84 mg/g

85%

13.87 mg/g -

80% 97%

Coconut coir pith

Cr(VI)

Thomas

Waste acorn of Quercus ithaburensis

Cr(VI)

Langmuir Freundlich Dubinin– Radushkevich Temkin Frumkin, Harkins–Jura Smith Thomas

31.48 mg/g

85%

55.65 mg/g

-

Langmuir Freundlich Temkin Langmuir Freundlich Langmuir Freundlich Langmuir Freundlich Elovich Langmuir Freundlich

-

89.99%

42.96 mg/g 43.81 mg/g 39.98 mg/g

99%

24 mg/g

97.3%

Wang et al., 2009

59.35 mg/g 56.32 mg/g 58.39 mg/g 40.80 mg/g 312.52 mg/g

55% 55% 52% 33% -

Wang et al., 2008

5.7 mg/g

81.7%

4.81 mg/g

76.5%

20.98 mg/g

99%

Demirbas et al., 2004

>80%

Mohan et al., 2006

Tea factory waste

Cr(VI)

Casuarina glauca tree leaves

Cr(III)

Citrus reticulate (kinnow) Treated oil palm fibre

Cr(III) Cr(VI) Cr(VI)

Walnut hull

Cr(VI)

Laminaria japonica P. yezoensis Ueda Rice bran Wheat bran Maize bran

Cr(VI)

Pre-boiled sunflower stem (BSS) Formaldehyde-treated sunflower stem (FSS) Cornelian cherry activated carbon Apricot stone activated carbon Almond shell activated carbon Activated carbon prepared from coconut shell fibers

Cr(VI) Cr(VI)

Cr(VI)

Langmuir Freundlich Freundlich Langmuir -

Suksabye et al., 2008 Malkoc and Nuhoglu, 2007

Malkoc and Nuhoglu, 2006 Abdel-Ghani et al., 2008 Zubair et al., 2008 Isa et al., 2008

Hasan et al., 2008 Jain et al., 2009

20.98 mg/g 19.98 mg/g Cr(III)

Langmuir Freundlich

Many physicochemical methods have been proposed for their removal from industrial effluents, such as electro-chemical precipitation, ultrafiltration, ion exchange and reverse osmosis. A major drawback with precipitation is sludge production. Ion exchange is considered a better alternative technique for such a purpose (Kazemipour et al., 2008). Biosorption can be an efficient low-cost process to remove toxic heavy metals from wastewater. Schiewer and Balaria (2009) investigated the uptake of Pb2+ by processed orange peels, a pectin-rich by-product of the fruit juice industry.

12.2 mg/g

Potentiometric titrations showed a significantly higher negative surface charge of protonated peels compared to original peels, with acidic groups around pH 4, 6, and 10. FTIR spectra of peels were similar to those of pectin. The carboxylic group peak shifted from 1636 to 1645 cm−1 after Pb2+ binding indicated the involvement of carboxyl groups. Depending on the particle size, equilibrium was achieved in 30 min to 2 h. The first-order model was inferior to second- or third-order models. The obtained rate constants were much higher for smaller particles, while the capacity was jsimilar for all sizes. Low pH, increased ionic 359

Hlihor and Gavrilescu/Environmental Engineering and Management Journal 8 (2009), 2, 353-372

strength, or competing co-ions reduced Pb2+ binding at low sorbent dosages, but at high sorbent dosages removal remained above 90%. The Pb2+ uptake at 300ppm was 2 mmol/g (40% dry weight). Due to high uptake, favorable kinetics and good stability, citrus peel biosorbents hold high promise for industrial applications. Overall, this study suggests that biosorption of Pb2+ ions by orange peels can be an inexpensive and effective way of metal ion treatment and should be investigated further for its practical application. The removal of poisonous Pb (II) from wastewater by different low-cost abundant adsorbents (rice husks, maize cobs and sawdust) was investigated by Abdel-Ghani et al. (2007) at different adsorbent/metal ion ratios, pH, contact time, metal concentration, adsorbent concentration. The adsorption efficiencies were found to be pH dependent, increasing by increasing the solution pH in the range from 2.5 to 6.5. The equilibrium time was attained after 120 min and the maximum removal percentage was achieved at an adsorbent loading weight of 1.5 gm. The equilibrium adsorption capacity of adsorbents used for lead were measured and extrapolated using linear Freundlich, Langmuir and Temkin isotherms and the experimental data were found to fit the Temkin isotherm model (Abdel-Ghani et al., 2007). Meunier et al. (2002) examined the efficiency with which some natural adsorbents (cocoa shells, cedar bark, pine bark, spruce bark, vermiculite and volcanic rocks) remove heavy metals, especially lead, from very acidic leachate produced during soil decontamination by a chemical leaching process using hydrochloric acid. Cocoa shells were the most efficient sorbent with a maximal capacity of fixation (qmax) of 2.60 mg Pb/g measured during assays conducted with an acidic soil leachate (initial pH=1.59 and [Pb]i=45.4 mg/L). Cedar bark can also be used for metal removal in very acidic solutions but are less efficient than cocoa shells. Kinetic measurements of lead removal by cocoa shells have revealed that sorption equilibrium was obtained after approximately 4 h of contact (Meunier et al., 2002). With the goal of identifying innovative, lowcost adsorbents, Rashed (2006) evaluated the adsorption technologies, by determining the suitable conditions for the use of peach and apricot stones, produced from food industries as solid waste, as adsorbents for the removal of lead from aqueous solution. Chemical stability of adsorbents, effect of pH, adsorbents dose, adsorption time and equilibrium concentration were studied. The results reveal that adsorption of lead ions onto peach stone was stronger than onto apricot stone up to 3.36% at 3 h adsorption time. Suitable equilibrium time for the adsorption was 3–5 h (% Pb adsorption 93% for apricot and 97.64% for peach). The effective adsorption range for pH in the range was 7–8. Application of Langmuir and Freundlich isotherm models show high adsorption maximum and binding energies for using these adsorbents for the removal of lead ions from 360

contaminated water and wastewater. The results of this study will be highly useful in designing costeffective treatment plants for the removal of Pb(II)ions from wastewater. Table 3 presents some works reported in the literature for the removal of lead by using agricultural waste materials. 2.4. Removal of Copper Copper (Cu) is one of toxic metals. In particularly, excessive intake of copper over 1.0 mg/L from drinking results in hemochromatosis and gastrointestinal catarrh diseases because it is accumulated in the livers of human and animals. In recent years, various adsorbents have been used for removal of Cu(II) (Sari et al., 2007a). Since copper is an essential metal in a number of enzymes for all forms of life, problems arise when it is deficient or in excess. However, the carcinogenic character of copper is accepted and epidemiological evidence, such as the higher incidence of cancer among coppersmiths, suggests a primary carcinogenic role for copper. In addition, copper is phytotoxic and, indeed, has been used as an algicide to control algal blooms. It can, therefore, cause plant damage if, for example, it is present at too high a concentration in sewage sludge that is applied to agricultural land. A principal source of copper in industrial waste streams is metal cleaning and plating baths, and rinses, as brass, boiler pipe, cooking utensils, fertilizers, and from copper metal working, which requires periodic oxide removal by immersing the metal in strong acid baths (Ho and McKay, 2004). Cotton boll was used as an adsorbent by Duygu Ozsoy and Kumburfor (2006) with the aim of removing of the Cu(II) ions from the aqueous solutions. The adsorption process was carried out in a batch process and the effects of contact time (2–24 h), adsorbent concentration (1–20 gL−1), initial pH (2.0– 6.0), initial metal ion concentration (20–160 mgL−1) and temperature (20–45 C) on the adsorption were investigated. Experimental results showed that the maximum adsorption capacity was determined at pH 5.0 and adsorbed Cu(II) ion concentration was increased with increasing adsorbent concentration and contact time. The isothermal data of cotton boll could be well described by the Langmuir equations and the Langmuir monolayer capacity had a mean value of 11.40 mg g−1. Experimental results indicated that the pseudo-second order reaction model provided the best description of the data with a correlation coefficient 0.99 for different initial metal concentrations and therefore it was explained that chemical sorption was the basic mechanism in this system. FT-IR results showed that oxygen and nitrogen atoms in structure of cotton boll were involved in Cu(II) ions adsorption. The study of Yazici et al. (2008) was aimed at determining the effect of chemical pretreatment on copper (II) biosorption by Marrubium globosum subsp. Globosum leaves.

Removal of some environmentally relevant heavy metals using low-cost natural sorbents

Table 3. Summary of work done by various researchers using variety of agricultural waste materials for the removal of Lead and other heavy metals Agricultural waste

Metal ion Pb(II)

Adsorption model -

Adsorption capacity 130-175 mg/g

Removal capacity >80%

Pb(II) Pb(II)

Langmuir Langmuir Freundlich

44.67 mg/g -

90% >95%

Fruit stones Pomegranate peel

Pb(II) Pb(II) Cu(II)

Langmuir Langmuir, Freundlich Temkin

13,600 mg/kg -

>90% >90%

Carbon produced from nutshells of walnut, hazelnut, pistachio, almond, and apricot stone Sawdust activated carbon

Cu(II) Zn(II) Pb(II) Cd(II)

-

-

>90%

Kazemipour et al., 2008

Pb(II)

0.223 mmol/g

88.6%

Sreejalekshmi et al., 2009

Pecan nutshell

Cu(II) Mn(II) Pb(II)

1.35 mmol/g 1.78 mmol/g 0.946mmol/g

>90%

Vaghetti et al., 2009

Bog iron ores

Pb(II) Cu(II) Zn(II) Cr(III) Pb(II)

Langmuir, Freundlich Redlich-Peterson Langmuir Freundlich Sips Redlich– Peterson Langmuir, Freundlich

97.0 mg/g 25.2 mg/g 25.5 mg/g 55.0 mg/g 40 mg/g

>90%

Rzepa et al., 2009

-

Ho et al., 2004

16.95 mg/g 5.66 mg/g -

>90% 70% >40% 70%

Qaiser et al., 2007

Euphorbia echinus Launea arborescens Senecio anthophorbium Carpobrotus edulis Cotton waste Maize leaf

Tree fern Biomass prepared from Ficus religiosa leaves Unmodified oil palm fruit fibre (UOPF) Modified oil palm fruit fibre (MOPF) Palm kernel fiber

Olive stone

Casuarina glauca tree leaves

Pb(II) Cr(VI) Pb(II) Ni(II) Pb(II) Ni(II) Pb(II)

Pb(II) Ni(II) Cu(II) Cd(II) Pb(II)

Langmuir Freundlich Redlich-Peterson Langmuir -

References Benhima et al., 2008 Riaz et al., 2009 Adesola Babarinde et al., 2006 Rashed, 2006 El-Ashtoukhy et al., 2008

Abia and Asuoquo, 2006

>45% 72% Langmuir 1 Langmuir 2 Freundlich Redlich-Peterson Langmuir, Freundlich Langmuir Freundlich Temkin Langmuir Freundlich

47.6 mg/g

Ho and Ofomaja, 2006

4.47×10−5 mol/g 3.63×10−5 mol/g 19×10−5 mol/g 6.88×10−5 mol/g -

>85%

Fiol et al., 2006

97.37%

Abdel-Ghani et al., 2008

65 mg/g

90%

Amarasinghe and Williams, 2007 Ahluwalia and Goyal, 2005

Tea waste

Pb(II)

Tea leaves waste

Langmuir Freundlich

-

Barley straws

Pb(II) Fe(II) Zn(II) Ni(II) Pb(II)

Langmuir

23.20 mg/g

96% 91% 72% 58% 80%

Tree fern

Pb(II)

Langmuir Freundlich Redlich-Peterson

39.8 mg/g

-

Pehlivan et al., 2008 Ho et al., 2002

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The uptake capacity of the biomass was increased by chemical pretreatment when compared with the raw biomass. The results of biosorption experiments, carried out at the conditions of 50 mg l−1 initial metal concentration and pH 5.5, showed that pre-treating the biomass with alkali solutions (laundry detergent, sodium hydroxide and sodium bicarbonate, 0.5 M) improved the biosorption capacity of biomass (45.90, 45.78 and 43.91%, respectively) compared with raw biomass. Pretreatment with sulfuric and nitric acid solutions, 0.5 M, increased the Biosorption capacity of biomass by 11.82 and 10.18%, respectively, while there was no considerable change in the biosorption capacity of biomass (0.35%) after pretreatment with formic acid solution, 0.5 M. Furthermore, sodium chloride and calcium chloride, 0.5 M, pretreatments resulted in the improvement in biosorption capacity of biomass (31.38 and 26.69%, respectively). FT-IR analysis revealed that hydroxyl and carboxyl functional groups were mainly responsible for copper (II) biosorption. The litter of natural trembling poplar (Populus tremula) forest (LNTPF) was used for the biosorption of Cu(II) ions in a batch adsorption experiments by Dundar et al. (2008). The sorption capacity of LNTPF was investigated as a function of pH, particle size, agitating speed, initial Cu(II) concentration, adsorbent concentration and temperature. The efficiency of copper uptake by the used LNTPF increases with a rise of solution pH, adsorbent concentration, agitating speed, temperature, and with a decline of particle size and initial Cu(II) concentration. The biosorption process was very fast; 94% of Cu(II) removal occurred within 5 min and equilibrium was reached at

around 30 min. These results demonstrate the LNTPF is great potential and low-cost heavy metal adsorbents. The LNTPF could be used to as an effective, cheap and abundant adsorbent for the treatment of Cu(II) containing wastewaters. Table 4 synthesizes the work reported in literature for the removal of copper by using agricultural waste materials. 2.5. Removal of other metal ions Other metal ions such as zinc, arsenic, mercury and cobalt present in various industrial effluents are of environmental concern due to their toxicity even in low concentrations (Sud et al., 2008). Arsenic poisoning is a serious health concern worldwide. Arsenic concentration above permissible limits is reported from many countries. Batch isotherms for arsenic sorption on Vindhyan shales were compared by Paikaray et al. (2005) with arsenic sorption on black cotton soil. High sorption was observed on pyrite-rich shales and the Freundlich capacity constant KF yielded a good correlation with sediment pyrite content. Shales with high organic carbon sorbed more arsenic; however, the organic carbon-rich soil demonstrated significantly lower sorption. This difference may be due to the condensed nature of organic carbon in shale, which may have facilitated formation of organo-arsenic complexes. The pyrite content was also strongly correlated with the organic carbon content, possibly due to microbial synthesis during shale diagenesis.

Table 4. Summary of work done by various researchers using variety of agricultural waste materials for the removal of Copper and other heavy metals Agricultural waste

Adsorption model Langmuir Freundlich

Adsorption capacity

Tea waste

Metal ion Cu(II)

48 mg/g

Removal capacity 90%

Woody hazelnut shell

Cu(II)

Langmuir Freundlich

0.03 nmol/g

-

Demirbas et al., 2008

Barley straws

Cu(II)

Langmuir

4.64 mg/g

70%

Pehlivan et al., 2008

Unmodified maize cob

Cu(II) Co(II) Fe(II) Cu(II) Co(II) Fe(II) Cu(II) Zn(II)

-

526.3 mg/g 1000 mg/g 909.1 mg/g 1000 mg/g 1000 mg/g 1000 mg/g 10.6 mg/g 7.58 mg/g

50% 100% 90% 100% 100% 100% -

Igwe and Abia, 2007

-

97.8% 66.8% 89-98% 48-79%

Saeed et al., 2005

Modified maize cob

Tree fern Papaya wood Coffee husks (CH)

362

Cu(II) Zn(II) Cu(II) Zn(II)

Langmuir Freundlich Redlich-Peterson Langmuir Langmuir Freundlich

7.5 mg/g 5.6 mg/g

References Amarasinghe and Williams, 2007

Ho et al., 2002

Oliveira et al., 2008

Removal of some environmentally relevant heavy metals using low-cost natural sorbents

Arsenic sorption by commercially available carbons and other low-cost adsorbents are surveyed and critically reviewed by Mohan and Pittman (2007) and their sorption efficiencies are compared. An extensive table summarizes the sorption capacities of various adsorbents. Some low-cost adsorbents are superior including treated slags, carbons developed from agricultural waste (char carbons and coconut husk carbons), biosorbents (immobilized biomass, orange juice residue), goethite and some commercial adsorbents, which include resins, gels, silica, treated silica tested for arsenic removal come out to be superior. Immobilized biomass adsorbents offered outstanding performances. Desorption of arsenic followed by regeneration of sorbents has been discussed. Strong acids and bases seem to be the best desorbing agents to produce arsenic concentrates. Arsenic concentrate treatment and disposal obtained is briefly addressed. This issue is very important but much less discussed. This review should help in initially screening various sorbent media for setting up the treatment plants based on the community level or household levels in developed, developing and underdeveloped countries. The removal of Zn(II) from aqueous solution by different adsorbents was also investigated. Clarified sludge (a steel industry waste material), rice husk ash, neem bark and a chemical adsorbent activated alumina were used for the adsorption studies. The adsorption of Zn(II) increased with increased concentration of the adsorbents and reached maximum uptake at 10 g/L and pH between 5 and 7. The equilibrium time was achieved after 1 h for clarified sludge, 3 h for rice husk ash and 4 h for activated alumina and neem bark, respectively. The best adsorbent for the Zn(II) removal is the clarified sludge (Bhattacharya et al., 2006). Pilot tests have shown that coir (fibres from Coco nucifera) is suitable as a metal ion sorbent (Conrad and Hansen, 2007). In the range tested, more than 80% of the metal was sorbed within the first 10 min and the total amount of metal sorbed was 91– 97% of the metal initially added. In addition to high affinity and capacity, the two most interesting properties of the coir biosorption are the low pH optima for sorption (pH 4.5 for Zn and 2.5 for Pb) and the low desorption (less than 13% for Zn and 1% for Pb), the low pH optima making it possible to use coir directly in cleaning acidic waste water without a prior pH increase. The low degree of desorption ensures that the metal ions will not desorb in situations with lowering of the metal concentration in the solution, e.g. caused by a periodic drop in the metal concentration level of the waste water. Before applying coir for water cleaning purposes, further investigations are needed, the most important being sorption studied in flow experiments, desorption at low pH and competition between ions. In conclusion, coir has a promising potential for being a metal ion sorbent.

In the paper of Chen and Wang (2008) the removal of four metal ions, Pb2+, Ag+, Sr2+ and Cs+ by waste biomass of brewery was studied. The experimental results showed that metal uptake is a rapid process, which can be described by pseudosecond order kinetic model. The maximum biosorption capacities for four metal ions were 0.413 mmol Pb2+/g, 0.396 mmol Ag+/g, 0.091 mmol Sr2+/g and 0.076 mmol Cs+/g, respectively. The binding of metals was also discussed in term of several factors. The order of accumulated metal ions at equilibrium state on the molar basis was as follows: Pb2+ >Ag+ >Sr2+ >Cs+. Eucalyptus camaldulensis bark, a forest solid waste, is proposed as a novel material for the removal of mercury (II) from aqueous phase. The maximum sorption capacity was 33.11 mg g−1 at 20 C and the negative value of free energy change indicated the spontaneous nature of sorption. These results demonstrate that eucalyptus bark is very effective in the removal of Hg(II) from aqueous solutions (Ghodbane and Hamdaoui, 2008). A novel sorbent, Carica papaya, was evaluated for sorption of Hg(II) from aqueous solution. Maximum removal was observed at pH 6.5, 70.8 mg g-1. The equilibrium data followed Langmuir isotherm confirming the monolayer coverage of Hg(II) onto papaya wood particles. This work illustrated an alternative solution for the management of unwanted biological material like C. papaya, witch is a waste from papaya tree, when completes its fruit bearing life. Therefore the use of C. papaya for the removal of heavy metals from contaminated waters may be a novel and cost-effective alternative (Basha et al., 2009). Fluted pumpkin (Telfairia occidentalis) is a creeping vegetative shrub that spread low across the ground with larged lobed leaves, and long twisted tendrils. In the West African sub-region and therefore creates one of the major agro-waste problems in Nigeria. The purpose of Horsfall Jnr. and Spiff (2005) was to use the waste of fluted pumpkin as adsorbent for metal (Al3+, Co2+ and Ag+) removal from aqueous system. The experimental results were analyzed in terms of five two-prameter adsorption isotherm equations – the Langmuir, Frendlich, Temkin, Dubinin–Radushkevich and Flory–Higgins isotherms. According to the evaluation using Langmuir equation, the momolayer sorption capacity obtained was 16.98 mg/g, 10.43 mg/g and 8.03 mg/g for Al3+, Co2+ and Ag+ respectively. Sorption capacity increases with increase in smaller ionic radius metal ion. The result showed that fluted pumpkin waste could be used for the removal pf Al3+, Co2+ and Ag+ from wastewater. The fluted pumpkin is abundantly available but is scarcely useful. Agricultural by-products and in some cases appropriately modified have shown to have a high capacity for heavy metal adsorption. Toxic heavy metals such as Pb(II), Cd(II), Hg(II), Cu(II), Ni(II), Cr(III), and Cr(VI) have been successfully removed

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from contaminated industrial and municipal waste waters using different agro-waste materials. 3. Other natural materials as sorbents for heavy metals 3.1. Clay Clay is a natural, earthy, fine-grained material composed largely of a group of crystalline minerals. Clays have been used for thousands of years and they still keep their position among the most important industrial material (Tuzen et al., 2006). Clay minerals are widespread and abundant in aquatic and terrestrial environments and, being finely divided, have large surface areas per unit of mass. The large surface area of the natural clays, helped by edges and faces of clay particles, accounts for the excellent capacity of the clay minerals to adsorb heavy metals (Bhattacharyya and Gupta, 2008; Abollino et al., 2008). There are three types of clay: montmorillonite, bentonite and kaolinite. Clay mineral has high cation exchange capacity (CEC) in solution (Kurniawan et al., 2006), so they are good adsorbents for metal ions from aqueous solutions, and have the advantage of being abundant and inexpensive; therefore they can find application as low-cost, effective materials for the removal of metal ions from various effluents, such as industrial and processing waters and wastewaters, or extracts resulting from the treatment of contaminated soils by soil washing (Abollino et al., 2008). Adebowale et al. (2006) reported that modification of kaolinite clay mineral with orthophosphate (p-modified sample) enhanced adsorption of Pb and Cd ions from aqueous solutions of the metal ions. Increasing pH of solutions of metal ions, increasing adsorbent dose and increasing concentration of metal ion, increased the adsorption of metal ions. Adsorption of both metal ions simultaneously on both unmodified and p-modified samples indicates that adsorption of one metal ion is suppressed to some degree by the other. Modeling data obtained with Langmuir, Freundlich, Toth and Langmuir–Freundlich isotherms, indicated that very good fits were produced with the Freundlich model at high metal ion concentration. Poor fit was however obtained with Toth and Langmuir–Freundlich models. This implies that the sites on both modified and unmodified kaolinite clay adsorbents available for the adsorption of high concentrations of Pb2+ and Cd2+ may be heterogeneous in nature. Sari et al. (2007a) examined the adsorption of Pb(II) onto Turkish (Bandirma region) kaolinite clay in aqueous solution with respect to the pH, adsorbent dosage, contact time, and temperature. The monolayer adsorption capacity was obtained 31.75 mg/g at optimal experimental conditions. Gupta and Bhattacharyya (2008) investigated the immobilization of Pb(II), Cd(II) and Ni(II) on clays (kaolinite and montmorillonite) in aqueous medium through the process of adsorption under a set of variables (concentration of metal ion, amount of 364

clay, pH, time and temperature of interaction). Both kaolinite and montmorillonite are capable of removing metal ions (Pb(II), Cd(II) and Ni(II)) from aqueous solution. The adsorption capacity of montmorillonite is much more than kaolinite for all the three metal ions. Adsorption increases with pH till the metal ions are precipitated out as metal hydroxides. Bentonite, which has a large specific surface area and high ion-exchange capacity, is considered as main candidate in the decontamination and treatment of heavy metal ions. Bentonite is also considered as the best backfill material in the disposal of radioactive nuclear waste, and has been studied intensively The adsorption of Pb(II) was studied by Xu et al. (2008) from aqueous solution with MX-80 bentonite using batch technique under ambient conditions. Removal percent (>50%) and distribution coefficient (Kd) were determined as a function of shaking time, pH, ionic strength and temperature. The results showed that the adsorption behavior of Pb(II) on bentonite was strongly dependent on pH and ionic strength and suggest that bentonite is suitable as a sorbent material for the recovery and adsorption of Pb(II) from aqueous solution. Table 5 includes the work for the removal of different heavy metals by using clay minerals reported in literature. 3.2. Minerals In recent years minerals like vermiculite, zeolite, pumice have been studied with respect to their ability to remove various pollutants and thus their potential use in remediation methods (Erdem et al., 2004; Panuccio et al., 2008). In Table 6 there are shown some chemical and physical properties of zeolite, vermiculite, and pumice. Zeolites are natural silicate minerals; their capability to exchange cations is one of their most useful properties and determines their ability to remove heavy metals from industrial wastewater. Kocaoba et al. (2007) investigated the removal of heavy metal ions from aqueous solution using natural clinoptilolite, obtained from the Biga-Canakkale region of Turkey under different experimental conditions was investigated. The efficiency of zeolite as an adsorbent for the removal of heavy metals such as Cd (II), Cu (II), Ni (II) from aqueous solutions has been determined at the different initial concentration, zeolite amount, agitation speed and pH. The selectivity of the studied metals was determined as Cd (II)>Ni (II)>Cu (II) Studies on the rate of uptake of heavy metal ions by the zeolite indicated that the process was quite rapid and maximum adsorption occurred within the first one hour of contact. This initial rapid adsorption subsequently gave way to a very slow approach to equilibrium, and saturation was reached in 20 and 30 minutes.

Removal of some environmentally relevant heavy metals using low-cost natural sorbents

Table 5. Removal of different heavy metals by using clay minerals Clay mineral Celtek clay

Phosphatic clay Montmorillonite

Stevensite Kaolinite Montmorillonite Organic-modified clays Bentonite

Na+ rich-Montmorillonite

Aluminum-pillared montmorillonite

Metal ion Cu(II) Cd(II) Pb(II) Cr(III) Ni(II) Co(II) Pb(II)

Adsorption model -

Adsorption capacity

Langmuir

38 mg/g

Cd(II) Pb(II) Zn(II) Mn(II), Cu(II) Ni(II) Cd(II) Pb(II)

-

Cu(II) Pb(II) Cd(II) Zn(II) Cu(II) Co(II) Ni(II) Zn(II) Ag(I)

Cd(II)

Temkin, Langmuir Freundlich Langmuir

5.20 mg/g 9.58 mg/g 3.61 mg/g 3.22 mg/g 3.04 mg/g 3.63 mg/g 0.931 mmol/g 0.9905 mmol/g

100%

-

>21 mg/g

References Tuzen et al., 2006

Singh et al., 2006 Abollino et al., 2008

Mouzdahir et al., 2007 Bhattacharyya and Gupta, 2006 Stathi et al., 2007

-

-

>80%

-

-

Ayala et al., 2008

Langmuir Freundlich LangmuirFreundlich Temkin DubininRadushkeivich Freundlich Langmuir

0.5 mequiv/g

99.5% 99.0% 100% 99.4% -

≈ 28 mg/g

70%

Yu et al., 2008

Table 6. Chemical and physical properties of zeolite, vermiculite, and pumice (Panuccio et al., 2009) SiO2 (% wt) MgO (% wt) Al2O3 (% wt) Fe2O3 (% wt) K2O (% wt) CaO (% wt) Na2O (% wt) TiO2 (% wt) pH C.E.C. (meq/100 g) BET surface area (m2/g) Apparent density (g/cm3)

-

Removal capacity 98% 96% 97% 99% 98% 100% >80%

Zeolite 46.5 2.3 15 3 6 10 0.6 0.3 7 170

Vermiculite 39 20 12 8 4 3 7 100

Pumice 72 0.1 11.9 2.1 5.1 0.6 0.1 7 30

700

790

5-10

2.1

2.6

0.7

These studies showed that clinoptilolite is an effective and inexpensive adsorbent for Cd (II), Cu (II) and Ni(II) removal from aqueous solutions (Kocaoba et al., 2007).

Praus et al., 2008

The utilization of clinoptilolite was investigated by Gedik and Imamoglu (2008). Cd(II) removal from aqueous solutions via clinoptilolite was investigated in terms of the effect of pretreatment and regeneration. Four different chemicals (NaCl, KCl, CaCl2 and HCl) were tested for this purpose. This study emphasizes the potential of clinoptilolite to be a part of sustainable wastewater treatment technologies, enabling the recovery of both the sorbent and the metal, via demonstration of effective cadmium removal and clinoptilolite recovery, besides successful concentration of metal in the regenerant solution (Gedik and Imamoglu, 2008). The adsorption behavior of natural (clinoptilolite) zeolites with respect to Co2+, Cu2+, Zn2+, and Mn2+ has been studied by Erdem et al. (2004) in order to consider its application to purity metal finishing wastewaters. The batch method has been employed, using metal concentrations in solution ranging from 100 to 400 mg/L. The percentage adsorption and distribution coefficients (Kd) were determined for the adsorption system as a function of sorbate concentration. In the ion exchange evaluation part of the study, it is determined that in every concentration range, adsorption ratios of 365

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clinoptilolite metal cations match to Langmuir, Freundlich, and Dubinin–Kaganer–Radushkevich adsorption isotherm data, adding to that every cation exchange capacity metals has been calculated. It was found that the adsorption phenomena depend on charge density and hydrated ion diameter. According to the equilibrium studies, the selectivity sequence can be given as Co2+ > Cu2+ > Zn2+ > Mn2+. These results show that natural zeolites hold great potential to remove cationic heavy metal species from industrial wastewater (Erdem et al., 2004). A natural material from Cuba, containing zeolite, has been used by Cabrera et al. (2005) for the removal of several metal ions (Cu2+, Zn2+, and Ni2+) as well as to evaluate its potential use as a low-cost adsorbent. Results suggested that this natural zeolite has a high potential for heavy metal retention. The selectivity of the studied metals was determined as Cu2+ Zn2+ > Ni2+, related to the first hydrolysis equilibrium constant. The metal removal efficacy was strongly dependent on pH, and to a lesser extent on metal/zeolite ratio (Cabrera et al., 2005). Vermiculite is classified as 2:1 clay mineral type and it has been explored with the aim of evaluating structural changes, adsorptive, catalytic and electrochemical properties. Among other advantages, vermiculite is widely available, easily handled, low-costly adsorbent, and quite selective over other solids. Fonseca et al. (2006) showed the application of Brazilian vermiculite in the removal of specific toxic metal as zinc, cadmium, chromium and manganese from aqueous solution. The adsorbent showed good sorption potential for these cations. The experimental data was analyzed by Langmuir isotherm model showing reasonable adjustment. The quantity of adsorbed cations was 0.50, 0.52, 0.60, and 0.48 mmol g−1 of Cd2+, Mn2+, Zn2+, and Cr3+, respectively. This study indicated that vermiculite could be used as an effective adsorbent for the sequestration of heavy ions in aqueous solution. Sari and Tuzen (2008) focused on the removal of Cr(VI) using Turkish vermiculite from aqueous solution. The operating parameters, pH of solution, adsorbent dosage, contact time, and temperature, were effective on the removal efficiency of Cr(VI). The maximum removal efficiency was found as 85% at the conditions of 10 gL-1 adsorbent dosage, pH 1.5, 120 contact time, and 20 C. Adsorption equilibrium was better described by the Langmuir isotherm model than the Freundlich model. The monolayer adsorption capacity of Turkish vermiculite for Cr(VI) was found to be 87.7 mg/g ions. Based on all results, the researchers concluded that the Turkish vermiculite is an effective and alternative adsorbent for the removal of Cr(VI) ions from aqueous solution because of its considerable adsorption capacity, being of its natural and low-cost (Sari and Tuzen, 2008). The use of pumice as an adsorbent to remove metals from wastewater treatment at low cost is a well-established process. Pumice is a light, porous igneous volcanic rock. It has a porous structure and a large surface area and it can be processed easily. The 366

large proportion of free silica sites at the grain surface results in a negatively charged surface. The structure contains open channels that allow water and ions to travel into and out of the crystal structure. It is a valuable scouring, scrubbing, and polishing material both in powdered form and as pumice stone. Pumice has a skeleton structure that allows ions and molecules to reside and move within the overall framework (Ghebremichael, 2004; Yavuz et al., 2008; http://www.minerals-n-more.com/Mes-Nat Sco_Info.html; http://www.galleries.com/minerals/Silicate/stilbite/stil bite.htm). The adsorption of Cu2+ and Cr3+ onto pumice (Pmc) and polyacrylonitrile/pumice (PAn/Pmc) composite has been investigated and their adsorption properties were compared by Yavuz et al. (2008). More than 80% of studied cations were removed by Pmc and 87% by (PAn/Pmc) respectively, from aqueous solutions in single step. Effective removal of metal ions was demonstrated at pH values of 8.0. The mechanism for cations removal by the Pmc and (PAn/Pmc) composite includes complexation and sorption. The process is very efficient especially in case of low concentrations of pollutants in aqueous solutions, where common methods are either economically unfavorable or technically complicated (Yavuz et al. 2008). Natural Jordanian sorbent (consisting of primary minerals, i.e., quartz and aluminosilicates and secondary minerals, i.e., calcite and dolomite) was shown to be effective for removing Zn(II), Pb(II) and Co(II) from aqueous solution. The major mineral constitutions of the sorbent are calcite and quartz. Dolomite was present as minor mineral and palygorskite was present as trace mineral. The equilibrium sorption capacities of the metals were: 2.860, 0.320, 0.076mmolcation g-1 for Zn(II), Pb(II) and Co(II) at pH 6.5, 4.5 and 7.0, respectively. Acid treatment of the sorbent reduces the sorption capacity with this attributed to elimination of carbonate minerals (calcite and dolomite) from the adsorbent. The researchers found that the mechanism of metal sorption is mainly precipitation as metal carbonate complexes. Furthermore, the natural sorbent is especially suited to retaining Zn, as Zn is generally considered to be more mobile than Pb (Al-Degs et al., 2006). Summary of research work done for removal of some heavy metals with minerals is presented on Table 7. 4. Conclusion Toxic heavy metal release into the environment has been increasing continuously as a result of man’s industrial activities and technological development. Due to magnitude of the problem of heavy metal pollution, research into new and cheap methods of metal removal has been on the increase recently.

Removal of some environmentally relevant heavy metals using low-cost natural sorbents

Table 7. Summary of research work done for removal of some heavy metals with minerals Mineral

Metal ion

NaY zeolite

Fe(III) Zn(II) Mn(II) Ni(II) Zn(II) Cd(II) Cu(II) Pb(II) Cd(II)

Vermiculite

Vermiculite Zeolite Pumice Manganese oxide coated zeolite Clinoptilolite

Expanded perlite

Cu(II) Pb(II) Ni(II) Cu(II) Pb(II) Cd(II) Cu(II) Pb(II)

Adsorption model Freundlich Langmuir -

Adsorption capacity

Langmuir Freundlich

118 μmol/g 143 μmol/g 47 μmol/g 0.135 mmol/g 0.363 mmol/g 13.03 mg/g 25.76 mg/g 27.7 mg/g 4.22 mg/g 8.62 mg/g 13.39 mg/g

Langmuir Freundlich Langmuir Freundlich DubininRadushkeivich

Most of this work has shown that natural products can be good sorbents for heavy metals. Several workers have reported the potential use of agricultural by-products as good substrates for the removal of heavy metal from wastewaters. This process attempts to put into use the principal of using waste to treat waste. This review focuses on the toxic metal ion sorption with inexpensive and efficient natural sorbents from agricultural waste materials and natural one such as clay and minerals. Literature revealed that in some cases the modification of the adsorbent increases the removal efficiency. Therefore, low-cost adsorbents and at the same time natural sorbents can be viable alternatives for the treatment of metals-contaminated wastewater. This aspect needs to be investigated further in order to promote large-scale use of non-conventional adsorbents. In spite of the scarcity of consistent cost information, the widespread uses of low-cost adsorbents in industries for wastewater treatment applications today are strongly recommended due to their local availability, technical feasibility, engineering applicability, and cost effectiveness. Acknowledgement This paper was elaborated with the support of BRAIN project Doctoral scholarships as an investment in intelligence - ID 6681, financed by the European Social Found and Romanian Government and ID_595 Project within the National Program for Research, Development and Innovation, PN-II.

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2.93 meq/g 2.83 meq/g -

Removal capacity >30% >70% 100% >80% 30% 100% 58% 70% 30-40% -

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80% 95%

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