Surface Complexation Modeling for the Stabilization of ... - ASCE Library

Geo-Congress 2014 Technical Papers, GSP 234 © ASCE 2014

Surface complexation modeling for stabilization of an industrial sludge by alternate materials Arif Ali Baig Moghal1, Syed Abu Sayeed Mohammed2, B Munwar Basha3 and Mosleh Ali Al-Shamrani4 1 Assistant Professor, Department of Civil Engineering, College of Engineering, King Saud University, Riyadh – 11421, Saudi Arabia. Email: [email protected] 2 Professor, Faculty of Civil Engineering, HKBK College of Engineering, #22/1, Nagawara, Bangalore 560045, India. Email: [email protected] 3 Assistant Professor, Department of Civil Engineering, Indian Institute of Technology Hyderabad, Yeddumailaram 502205, India. Email: [email protected] 4 Professor, Department of Civil Engineering, College of Engineering, King Saud University, Riyadh – 11421, Saudi Arabia. Email: [email protected] ABSTRACT: The main aim of this paper is to model the behavior of a mixture of 30% fly ash, 60% black cotton soil, 30% heavy metal laced industrial sludge and 15% cement for sorption of chromium and copper at different pH values. It was found that the mixture of 30% fly ash, 60% soil, 30% sludge and 15% cement was the ideal combination found from extensive leaching experiments, for heavy metals such as Cr+6 and Cu 2+. Owing to practical difficulties in carrying out leaching tests over wide range of pH values, Visual minteq was used to simulate these conditions. It is found that the model predicts the retention behavior accurately and was further confirmed with the output of experimental work done earlier. A series of visual minteq simulations revealed that surface complexation and reduction play an important role in the sorption process. It was concluded that the hazardous sludge laced with heavy metals can be stabilized. The study would benefit the design engineers in understanding and in finding alternate means for treating industrial sludge wastes.

INTRODUCTION Solidification and stabilization (S/S) has emerged as an efficient method for the treatment of sites contaminated with potentially toxic metals and metalloids (PTMs). S/S processes involve mixing binders, such as ordinary Portland cement, calcium aluminate, furnace slag, lime etc. into the soil, in order to transform soils into a solid material with low leachability of contaminants. The main objective of S/S is therefore to develop a recipe (binder formulation) that produces a stable and sustainable end product, which will pose the minimum threat to the environment. The high strength, low permeability and relatively high durability of products make cement the most adaptable binder currently available for the immobilization of PTMs in soil. It also has the advantage that it is a consistent manufactured material with known hydration characteristics. Cement treated soils and other waste products encapsulate PTMs by reducing the soil surface/leachant contact and by forming a stable pH environment in

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which many PTMs remain insoluble (George and Vaclavikova, 2008). In addition to physical encapsulation, various contaminated soil-binder interactions occur to immobilize contaminants in the product chemically, further reducing the potential for pollutant transfer into the environment (Moghal and Sivapullaiah, 2012). The results of these interactions are cement-stabilized soil products that are non-hazardous, or significantly less hazardous than the original soil. Despite incomplete information regarding the long-term durability of cement-stabilized soil monolith products, necessity, and the lack of other cost-effective and practical remediation methods, is driving these types of technologies to become widely used in many industrial countries with an increasing number of abandoned industrial sites. Cementitious S/S is today recognized as the “best demonstrated available technology” by the US Environmental Protection Agency for land disposal of toxic wastes. Solidification/ stabilization of hazardous waste by using various materials is a technology which has been applied to many types of wastes and industrial effluents particularly sludge containing heavy metal ions. In the present study an attempt has been made to stabilize chromium sludge using soil, fly ash mixtures along with the small percentage of additives like lime and cement. Leaching tests were carried out for stabilized samples at the end of different curing periods in order to assess the leaching behavior of heavy metal ions such as chromium and copper. MATERIALS AND METHODS Heavy metal laden sludge rich in chromium was collected from an electroplating industry, located in Bangalore, India. The properties of industrial sludge are shown in Table 1. Table 1. Properties of industrial sludge Parameter pH Water content (%) Chromium (mg/kg) Nickel (mg/kg) Copper (mg/kg) Aluminum (mg/kg)

Value 2.58 9.5 91.5 7.31 23.95 83.5

Fly ash was obtained from the Raichur thermal power station (RTPS), Raichur, Karnataka; the fly ash has been classified as F type. Black cotton soil was obtained from Belgaum, Karnataka. The black cotton soil was preferred because it contains good amount of clay and silt which imparts good plastic properties for the stabilization of sludge. Approximately 10 Kg soil samples were drawn below 15cm from the ground surface. The soil was air dried and sieved to 2mm sub samples of air dried soil were ground and sieved to obtain aggregates less than 0.1mm to ensure uniformity of the material. The physical properties of the soil along with compaction characteristics are presented in the Table 2.


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Table 2. Properties of black cotton soil Property Specific gravity Grain size analysis

Consistency limits

Proctor’s compaction characteristics

Clay % Silt % Sand % Gravel % Liquid limit % Plastic limit % Plasticity index % Shrinkage limit % Optimum moisture content %

Value 2.58 58.15 17 22.04 2.11 52.5 22.5 22.5 9.17 24

Maximum dry density gm/cm³

2.1

Sample Preparation In the present work soil and fly ash were mixed in different proportions, namely soil in the range of 70 to 90 % and fly ash in the range of 10 to 30% in different series. In this study, the heavy metal sludge was air dried until the moisture was removed. The soil and flyash was oven dried for 24 hrs at 105° c and sieved with 4.75 mm to remove the coarse material. Solidified/stabilized treatment was performed by mixing sludge, fly ash, soil, lime and cement in different combinations. The samples in the study were classified into five series: 1. In the first series 90% black cotton soil was mixed with 10% of fly ash followed by different percentage of sludge designated as “A”. 2. In the second series 80% black cotton soil was mixed with 20% of fly ash followed by different percentage of sludge designated as “B”. 3. In the third series 70% black cotton soil was mixed with 30% of fly ash followed by different percentage of sludge designated as “C”. 4. In the fourth series 40% black cotton soil, 30% of fly ash, 30% sludge mixed with different percentage of lime and was designated as “D”. 5. In the fifth series 40% black cotton soil, 30% of fly ash, 30% sludge mixed with different percentage of cement and was designated as “E”. Leaching test The leaching test was conducted according to ASTM D3987-12, water leach test to evaluate the leaching behavior of heavy metal in the stabilized matrices. The water leach test defined in ASTM D3987-12 is intended as a rapid means of obtaining an aqueous extract from a solid waste. Site-specific leaching conditions were not simulated in this test. The method is only appropriate for inorganic compounds. In the


present study, the material to be tested was mixed homogeneously and then a representative 20-g sample of the material was added to 400 mL of deionized water in a sealed 500 ml container. So as to maintain a liquid to solid ratio of 20 ( i.e liquid/solid=20). The mixture was agitated continuously for 18 hours at a rate of 30 rotations per min at room temperature. The mixture was then allowed to settle for 5 min, and the aqueous phase was separated by decantation. The decant was filtered through filter paper and was subjected to chemical analysis. For all the samples the concentrations of contaminants were determined using Atomic absorption spectrophotometer Perkin Elmer Analyst 400 flame type to measure the metal ion concentrations in the leachate. Parameters for surface complexation model Due to brevity only mixtures which gave highest sorption from the experimental data was selected for surface complexation modeling. The hydrous ferric oxide (HFO) content of 30% fly ash, 60% soil, 30% sludge and 15% cement mixture was measured by ascorbate extraction method and the amorphous aluminum hydroxide content of the 30% fly ash, 60% soil, 30% sludge and 15% cement mixture was measured by oxalate extraction. Prior to chemical extractions the 30% fly ash, 60% soil, 30% sludge and 15% cement mixture was mixed with water until L/S = 2.5L/kg. These suspensions were subsequently equilibrated for 24 hr at four pH values of 4, 6.5, 8.5 and 10.5. The equilibrated suspensions were filtered over 0.2μm membrane filters, and the remaining solid material was extracted. HFO was extracted from 15g of 30% fly ash, 60% soil, 30% sludge and 15% cement mixture with 300ml of ascorbic acid solution (20g/l) according to the method of Ferdelman described in Kostka and Luther. The extractions were performed at pH 8 and took 24 hr at room temperature. Amorphous aluminum hydroxide was extracted from 3g of 30% fly ash, 60% soil, 30% sludge and 12% cement mixture with 300 ml of 0.2M of ammonium oxalate at pH 3 for 4 hr in the dark. All Fe and Al extracts were analysed by ICP – AES to obtain the concentrations of Fe and Al). The model development is based on the assumption that HFO is the primary sorbent mineral in soil, and second on the assumption that amorphous aluminum hydroxide also plays a role in the sorption process. For modeling purposes HFO was taken as surrogate sorbent for amorphous aluminum hydroxide. It can be justified that the use of HFO as a surrogate for aluminum hydroxide for the following reasons. The most reactive aluminum and iron hydroxides are Fe(OH)3 (ferrihydrite /HFO) and Al(OH)3. Iron(III) and Al(III) are known to substitute for each other in natural metal hydroxide. With respect to the input of sorbent mineral concentrations 1 mol of Al was assumed to be representative of 1mol of Fe. The molecular weight of 89g of HFO/mol of Fe recommended by Dzombak and Morel was used to calculate the concentration of HFO from the extracted Fe and Al. The resulting sorbent mineral concentrations are given in Table 3. Specific surface area of HFO the general value of 600m2/g recommended by Dzombak and Morel has been incorporated in the version 3.0 of visual minteq.

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Table 3. Summary of sorbent and sorbate concentrations Sl. Name of sorbent Fe concentration in Al concentration in No. g/l Molal 1 30% fly ash, 60% soil, 30% 13.89 7.8293E-04 sludge and 15% cement 2 Soil 10.34 4.8347E-04 3 Sorbate / site concentrations of chromium 1.9232E-03 Molal 4 Sorbate / site concentrations of copper 1.5736E-03 Molal

RESULTS AND DISCUSSIONS Quantifying the environmental impact of stabilized/solidified materials in real environmental scenario is crucial for selecting proper disposal and reuse alternatives and certification of immobilization technologies. The performance of solidified/stabilized treated waste is generally measured in terms of leaching tests. Although numerous leaching tests are available to evaluate the solidified/solidified treatment, no single test can describe the complex leaching behavior of the treated materials. The leachability of heavy metals at the end of 1, 7, 14, 28 days are summarized in Table 4. It was observed that as the percentage addition of sludge increased the leachability of heavy metal ions also increased. The specimens of A series achieved the leaching requirement of 5mg/L at 28 days of curing, whereas specimen of B series achieved the value at 14 days of curing. And specimens of C series achieved the minimum requirement at 7 days of curing this was due to the high percentage of fly ash in C series than in A and B series. It was also observed that as the curing period increased the leachability of heavy metals decreased considerably. The decrease in leaching with increase in the fly ash content was because fly ash retained some amount of copper. This might be mainly due to precipitation of ions by the calcium present in fly ash mixtures. For an additive like cement in the range of 1% to16%, leaching was reduced indicating the higher retention of copper, which might be due to hydrolysis and formation of a series of chemical complexes over a curing time. Addition of cement in to sludge constituting a cement sludge matrix played a positive role in fixing of copper ions. It was observed that as the percentage of addition of lime increased the leachability of chromium decreased and it was also observed that as the curing period increased the leachability decreased. This decrease in leachability of chromium with increase in percentage of lime was due to the addition of higher percentage of lime which might have given rise to formation of chromium hydroxide precipitate due to which some retention of chromium has occurred. Generally the mode of retention of chromium is by adsorption, surface complexation and precipitation. It was also observed that as the percentage of cement increased the leachability of chromium decreased considerably. And it can also be observed that as the curing period

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increased the leachability of the chromium also decreased. From extensive experimental works the following combination of mixtures were found to be most efficient, as seen from Table 4 Table 4. Chromium & Copper leaching for different mixtures

Specimen 30%flyash+60% Soil+30% Sludge+5% Cement+ Copper 30%Flyash+60%Soil+30%Sludge +5%Lime+ Copper 30%Flyash+60%Soil+30%Sludge +15%Cement+ Chromium 30%Flyash+60%Soil+30%Sludge +15%Lime+ Chromium

1-Day Leaching (mg/L)




0.96

0.34

0

0

0.58

0.34

0

0

19

15

5

1

29

18

6

3

Due to brevity only combinations of mixtures which sorbed maximum amount of contaminants were considered for surface complexation modeling using Visual Minteq. It was resolved to use only one combination which took up maximum amount of sludge and also its sorption efficiency to be highest. It was found that a mixture of 30% fly ash, 60% soil, 30% sludge and 15% cement was the ideal combination found from extensive leaching experiments for heavy metals such as Cr(VI) and Cu 2+. From the experimental data it can be seen that it is very difficult to determine the behavior of these mixtures at different range of pH, hence Visual minteq helps us to simulate these conditions and gives us a clear picture of processes taking place at different pH. In this paper 30% fly ash, 60% soil, 30% sludge and 15% cement mixtures were considered, since the database of visual minteq has only HFO and other database of aluminum hydroxide is not available. Hence for each mixture Fe and Fe with Al were taken as input data into the model and an ionic strength of 0.001 M was set for all the calculations. Behavior of 30% fly ash, 60% soil, 30% sludge and 15% cement From Fig. 1, it can be seen that the speciation of hydrous ferric oxide over different pH is as shown the SOH represents the solid oxide/ hydroxide – water interface. FeO represents ferrous oxide and Fe(OH)2 ferrous hydroxide, FeCrO-4 ferrous chromate and iron chromium complex. The solids hydroxide water interface represents the physical adsorption taking place it can be seen that this phase is predominant at pH 3 and 10. The interaction of FeO and Fe(OH )2 is quite opposite and the point of zero surface charge is at a pH of 9, below this pH Fe(OH)2 is predominant and above pH 9 FeO is dominant. Fe(OH)2 forms a stable precipitate at a pH of 9 and remains constant this is in confirmation with the speciation of Cr6+ as given in literature. It can also be seen that the complexation of chromium takes place from pH 4 to 8. Comparing the sorption of chromium for a mixture of 30% fly ash, 60% soil, 30%


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sludge and 15% cement, it can be seen clearly that there is a jump in sorption of chromium with fly ash, cement and sludge than only soil. It might be due to the presence of iron and alumina in excess quantities in fly ash than soil. Since Fe is a good reducing agent and chromium is a highly reactive element, the reduction of Cr takes place effectively. Protonation of H+ ions takes place therefore Fe(OH)2 is dominant at acidic pH and after reaching neutral pH FeO becomes dominant, which can be seen in Fig’s 1 and 2 respectively. During this process many compounds are formed and it can be seen that FeCrO-4 is one such compound formed and remains constant through a wide range of pH. 0

2

pH

4

6

8

10

-0.002 0.000

Site concentration in moles

0.002 0.004 0.006 SOH of Black cotton soil FeCrO4of Black cotton soil FeO of Black cotton soil Fe(OH)2 of Black cotton soil Iron chromate complex of Black cotton soil SOH of Black cotton soil with flyash FeCrO4of Black cotton soil with flyash FeO of Black cotton soil with flyash Fe(OH)2 of Black cotton soil with flyash Iron chromate complex of Black cotton soil with flyash

0.008 0.010 0.012 0.014 0.016 0.018 -2

0

2

4

6

8

10

12

14

16

Figure 1. Speciation of 30% fly ash, 60% black cotton soil, 30% sludge and 15% cement with Cr 6+ Behavior of chromate at different pH As shown in Fig. 2, it can be seen that a graph of concentration of chromate at different pH is plotted. It can be seen that three oxides of chromate have been considered they are chromate, dichromate and per chromate. This was done to understand the reduction reactions taking place. It can be observed that concentration of per chromate is highest at acidic pH and remains constant from 2 to 5, whereas at the same pH range the concentration of chromate is lowest and slowly increases from pH 5. At the neutral pH of 7 there is point of zero charge and a transition of HCrO4- to CrO4-2 takes place. De-protonation of HCrO4- is taking place and is being converted to CrO4-2. This speciation of chromate proves that redox reaction is very prominent in sorption of Cr6+. The concentration of dichromate is constant through all the pH ranges and proves it is very stable. In order to observe the interaction of Fe and Al, it was proposed to model the behavior of Al for mixtures of black cotton soil, fly ash, cement and sludge. The concentration of Al present was put into the model and visual minteq was run. The data obtained was plotted as shown in Fig. 3. It can be observed that there are marked differences in the way the model has projected the system. It was found that Al is a very reactive element and its interaction with chromium in reducing it into a stable compound is much higher than only with Fe as was done in our earlier discussions above. Al acts as a competing ion and hence the values are much lower, also it was


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found that the model failed to predict a number of parameters with combination of Fe and Al the reason being there is no data base for Al in visual minteq and it is assumed that Al acts as a surrogate to Fe in modeling and we invariably use the same database of Fe. But even then this gives us a rough estimate of the sorption process taking place in presence of Al. It can be observed from Fig. 3 that SOH has slightly shifted into a purely alkaline environment for sorption; this might be due to the presence of a competing ion like Al which increases the magnitude of ions getting reduced than complexed. The behavior of Fe(OH)2 and FeO is similar to what was observed in our study above. 0

2

pH

4

6

8

10


0.0000

0.0005 -2

Conc. of Chromate CrO4

-2

Conc. of dichromate Cr2O7

0.0010

-

Conc. of perchromate HCrO4 0.0015

0.0020

-2

0

2

4

6

8

10

12

14

16

Figure 2. Behavior of chromate at different pH Comparison of speciation with 30% fly ash, 60% soil, 30% sludge and 15% cement From Fig. 4, it can be seen that a comparison of speciation of 30% fly ash, 60% soil, 30% sludge and 15% cement with only soil is made. The behavior of soil cement mixture is similar to that of soil but the magnitude of sorption is more than a factor of 10. This shows that by the addition of cement the sorption of copper has increased. Bonding and complexation of copper takes place which forms organic and inorganic ligands in the soil solution phase and thus adsorb better than soil. Cement under goes hydrolysis and many cementitious compounds are formed like silicates, aluminates etc, which play an important role in the retention of copper. These cementitious compounds form complexes with copper which leads to stabilization of copper onto its cement matrices and hence retention takes place effectively, and these compounds are active at a particular pH range from neutral to alkaline (Mohammed, 2013).


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pH 0

2

4

6

8

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0.0

-20

5.0x10

-19

1.0x10

SOH of BCS FA FE & Al FeCrO4 of BCS FA FE & Al FeO of BCS FA FE & Al Fe(OH)2of BCS FA FE & Al Iron chromate complex of BCS FA FE & Al

-19

1.5x10

-19

2.0x10

-2

0

2

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Figure 3. Speciation of Cr6+ for Fe and Al This is more a realistic value as based on literature survey it was found that in the pH range of 3 to 5 copper predominantly exists as Cu(OH)+ (copper mono hydroxide). From pH of 5 to 5.5, it exists as a complex of Cu(OH)1.5(SO4)0.25 which is non stoichiometric compound. From pH 5.5 to 7.7 and above, it forms Cu(OH)2 (cupric hydroxide) precipitate, which is a stable compound and these hydrolyzed species are strongly adsorbed to soil surfaces (Meima and Comans, 1998). By observing Fig. 5, it can be seen that at pH 9 there is a steep fall in the concentration, complexation and precipitation reaction has occurred forming a stable cupric hydroxide precipitate. One of the drawback of Visual Minteq is it cannot model surface precipitation where as surface complexation reactions are modeled satisfactorily, and this can be observed through Fig. 5. 0

2

4

pH

6

8

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Site concentration in Moles

0.0

-6

1.0x10

-6

2.0x10

-6

3.0x10

-6

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-2

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8

SOH of Fe FeO Fe(OH)2 FeOCu SOH of Cement FeOCu cement Fe(OH)2 cement 10

12

14

16

Figure 4. Comparison of speciation with 30% fly ash, 60% black cotton soil, 30% sludge and 15% cement with only black cotton soil both laced with copper


0

2

pH

4

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2244

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Site concentration in Moles

0.0

-8

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-7

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1.5x10

Cu + Fe Cu + Fe + Al -7

2.0x10

-2 -1

0

1

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7

8

9 10 11 12 13 14 15 16

Figure 5. Variation of surface complexation model CONCLUSIONS Based on extensive leaching tests to a number of combinations of mixtures it was found that a mixture of 30% fly ash, 60% soil, 30% sludge and 15% cement was the ideal combination to retain chromium and copper. Also in this paper an attempt has been made to use the latest version of Visual Minteq to model the sorption behavior of soil and soil fly ash cement mixture with chromium and copper. It has been found that the model accurately predicts the behavior of these mixtures, it can be concluded that soil flyash cement mixture is a better material than soil. The advantage of using this model was to study the behavior over a range of pH which was not possible experimentally. This simulation helped to find ways and means of stabilizing hazardous waste sludge at different ranges of pH. REFERENCES ASTM (2012). “Standard Practice for Shake Extraction of Solid Waste with Water.” D3987. West Conshohocken, PA. Moghal, A.A.B., and Sivapullaiah, P.V. (2012). “Retention Characteristics of Cu2+, Pb2+ and Zn2+ from Aqueous Solutions by Two Types of Low Lime Fly Ashes”, Toxicological and Environmental Chemistry, 94(10), 1941-1953. George, P. and Vaclavikova, G. M. (2008). “Removal of Chromium (VI) from Water Streams: a Thermodynamic Study”, Environmental Chemical Letters, 6, 235–240. Meima, J.A., and Comans, R.N.J. (1998). “Application of surface complexation/ precipitation modeling to contaminant leaching from weathered municipal solid waste incinerator bottom ash”, J. of Environ. Sci. and Tech., 32, 688-69. Mohammed, S.A.S. (2013). “Potential of surface complexation and redox modeling for chromium (VI) adsorption on local materials as liners for waste containment facilities”, Turkish J. of Enrgr. and Environ. Sci., 37 (1), 100- 108.