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JECET; March 2014 – May 2014 Vol.3.No.2, 881-892.

Arsenic Removal from Aqueous Solution using Mixed Mineral Systems Injected with Sphalerite under Sulfidic- Anoxic Conditions II. The Role of Solution Composition and Ageing D.E. Egirani, J.E. Andrews, A.R. Baker

Corresponding Author: D.E. Egirani Faculty of Science, Niger Delta University, Wilberforce Island, Nigeria

JECET; March 2014-May 2014; Vol.3.No.2, 881-892.

E-ISSN: 2278–179X

Journal of Environmental Science, Computer Science and Engineering & Technology An International Peer Review E-3 Journal of Sciences and Technology

Available online at www.jecet.org Section A: Environmental Science Research Article

Arsenic Removal from Aqueous Solution using Mixed Mineral Systems Injected with Sphalerite under Sulfidic- Anoxic Conditions II. The Role of Solution Composition and Ageing D.E. Egirani1, J.E. Andrews2, A.R. Baker2 1 2

Faculty of Science, Niger Delta University, Wilberforce Island, Nigeria

School of Environmental Sciences, University of East Anglia, Norwich, UK

Received: 28 March 2014; Revised: 12 April 2014; Accepted: 14 April 2014

Abstract: This study investigates arsenite removal onto binary mixed mineral sorbents in the presence of zinc sulfide under sulfidic-anoxic condition, relevant to streams and groundwater impacted area. Batch mode studies at room temperature reveal kaolinite, montmorillonite, and kaolinite-montmorillonite exhibit a near flattening sorption capacity over the range of pH investigated. All mineral systems exhibit variable sorption capacity as initial arsenic concentration increases. All mineral systems but zinc sulfide exhibit decrease in sorption as Cp increases. The complex behavior of mineral systems over the range of residence time investigated may be attributed to increased hydroxylation of the mineral surface and availability of thiol (≡S-H) and hydroxyl (≡Me-OH) functional groups and reactive sites. Keywords: Zinc sulfide, sulfidic-anoxic, composition, ageing, mixed mineral systems.

INTRODUCTION Arsenic is a minor terrestrial element that occurs primarily in association with sulfur-containing minerals such as realgar (AsS), orpiment (As2S3), or arsenopyrite (FeAsS). The environmental concern on arsenic is related to its anomalous concentration in surface and ground waters and its JECET; March 2014-May 2014; Sec. A, Vol.3.No.2, 881-892.

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availability to living beings1. Arsenic pollutants discharged by anthropogenic and natural could result in degraded surface and ground water chemistry. It is known that arsenic is responsible for the development of liver, bladder, skin, and kidney cancer, and long-term intake of small doses of inorganic arsenic compounds is a factor in many other diseases2. The need to reduce arsenic concentrations in water bodies remains a priority in both developed and most developing countries. Immobilization of arsenic in the environment occurs through precipitation of low-solubility salts and adsorption on soils and sediments. Remediation processes will follow the same principles, and the most common techniques are based on precipitation and adsorption phenomena3. Therefore, removal of arsenic species can be hampered by the absence of reliable sorbents and solution chemistry adequate to understanding metal sorption4-11. Metal arsenites are much more soluble than the corresponding metal arsenates, and the concentrations of arsenic in waters increase with the reduction of arsenate as a result of more-soluble arsenite solid phases and lower extent of adsorption12. An understanding of factors controlling the distribution of arsenic in ground water requires knowledge of arsenic sources and of processes controlling arsenic mobility. The removal of arsenic from contaminated water is controlled by the solution composition namely, pH and solid concentration, besides the residence time (ageing) of the solid phase in the water13-18. pH is considered a master variable in contaminants removal in aqueous environments. Its effects on arsenic removal by mixed mineral systems of clays and (hydroxides) injected with zinc sulfide under sulfidic-anoxic condition remains an area of research interest19, 20 . In addition, arsenic adsorption reactions are influenced by changes in pH, occurrence of redox (reduction/oxidation) reactions, presence of competing anions, and solid-phase structural changes at the atomic level. Solid-phase precipitation and dissolution reactions are controlled by solution chemistry, including pH, redox state, and chemical composition1. Solution pH controls (a) the solubilities of arsenic species; (b) hydrolysis behavior of arsenic ions; and (c) surface charge of the sorbent21. Adsorption may decrease as particle concentration increases (outer sphere complexation) or not be significantly affected as particle concentration increases (inner sphere complexation) 22-25. Increase in adsorption as particle concentration increases (promotive particle concentration effects) for organic and inorganic substances sorbed on colloidal clay and oxide particles still remains an area of research interest in conventional surface complexation theory26, 27. The solid concentration effect is an anomalous adsorption phenomenon (i.e., the adsorption isotherm declines as particle concentration increases). Although the cause of this phenomenon remains unclear, the nature of ionic species formed in solution is affected by changes in the mineral/ solution ratio28, 29. Prolonging the residence time of solid mineral phase in the absence of a sorbate could results in much mineral surface reorganization. This is due to the fact that high and new reactive sites are formed. However, this phenomenon on its own is not known to linearly affect arsenic sorption30-32. Studies on arsenic sorption exist: Adsorption of As (III) on clays and (hydroxides) 33, activated alumina grains34 and hydrated manganese, aluminum, iron (III) oxides and others have been studied35-38. The influence of pH, size, and structure of the sorptive particles and the competition of other ions and dissolved organic matter on the extent of adsorption, has been investigated39-42. The mechanism of adsorption has also been studied, and the most amorphous hydrated oxides seem to be the most effective in arsenate adsorption processes36, 41. Application of sulfides in water treatment as detailed in companion paper 1 is largely dependent on understanding of fundamental studies into metal sulphide precipitation and sorption mechanism on sulfide43, 44. In addition, understanding of groundwater chemistry in a chemically reducing environment is focused on mechanisms of the reactivity, removal kinetics, contact time of solution and solution composition. There are some advantages to sulphide, including the lower solubility of metal sulphide precipitates, potential for JECET; March 2014-May 2014; Sec. A, Vol.3.No.2, 881-892.

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selective metal removal and fast reaction rates, better settling properties and potential for re-use of sulphide precipitates by smelting45-49. Spectroscopic studies have confirmed thiol (≡S-H) and hydroxyl (≡Me-OH) functional groups on surface of metal sulfides.50-52. These amphoteric reactive units are thought to undergo independent protonation and deprotonation reactions to produce reactive sites for sorption. Under acidic conditions, thiol groups are believed to play an important role in the reactivity of sulfides both in initial removal and subsequent surface reactions. Theoretical Models and Isotherms: To addresses the suitability of mixed mineral suspensions of clay and (hydroxides) for arsenite i.e. Arsenic (III) removal, a theory derived from Freundlich isother model is designed to explain the predicted behavior of mineral-arsenite interactions as influenced by extraneous factors of pH, solid concentration and contact time or ageing53, 54 . Detailed system characterization and empirical model involving the distribution coefficient (Kd) as used in previous paper55, is provided in paper I. Distribution coefficient used in calculating arsenic sorbed was derived from the Freundlich model Equation 1,

S  KdC

N

… (1)

Where S is the sorbed concentration (µg/kg), Kd is the distribution coefficient, C is the equilibrium concentration (µg/g), and N = 1 is a chemical-specific coefficient derived from the slope of the plot. The empirical model as provided previously55Egirani et al to address the mineral-arsenic interactions is given in Equation 2: Arsenic sorbed difference = arsenic sorbed-arsenic sorbedtotal Arsenic sorbedtotal =

(S  S  S n ) 1 2 n

… (2)

Where arsenic sorbedtotal is the theoretical sorption for a 1:1 mixed mineral suspension, S1 is the arsenic sorbed on first single mineral suspension, and S2 is the arsenic sorbed on second single mineral suspension, Sn is the arsenic sorbed on n number of mineral suspensions and n is the number of mineral suspensions. Differences between the actual and predicted sorption capacity for the mixed mineral systems are based on the reasoning that: a. Secondary mineral phase is developed by the mixed mineral suspension. b. Components of minerals in the mixed mineral suspension may act as individual networks or chemisorbed species. c. There is mass differential between the single and mixed mineral systems. The difference between the actual sorption and the theoretical sorption was used to clarify the effects of mineral mixing on arsenic sorption. Mineral mixing is said to (a) enhances arsenic removal where the difference is positive; (b) depresses arsenic removal where the difference is negative; and (c) have no effect on arsenic removal where no difference exist between arsenic sorbed and theoretical arsenic sorption. In this paper the sorption relationship between simulated contaminated water containing arsenic, single minerals of zinc sulfide, kaolinite, montmorillonite, goethite and mixed mineral systems of kaolinite/montmorillonite, kaolinite/goethite and montmorillonite/goethite based on different solution composition such as pH, solid concentration and residence time (ageing) under sulfidic-anoxic condition was investigated.

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MATERIALS AND METHODS Preparation of Sulfidic-Anoxic Zinc Sulfide Suspension: Sulfidic-anoxic conditions are characterized by depletion of dissolved oxygen. These conditions will occur if the rate of oxidation is greater than the supply of dissolved oxygen22. In sulfidic-anoxic environment, hydrogen sulfide occurs as a product of sulfate and sulfide reduction. In this study, 1% acidified zinc sulfidesulfidicanoxic suspension was prepared using deoxygenated deionized water. Purified nitrogen gas was bubbled through the zinc sulfide suspension continuously for 24 hours. The content, securely sealed was stored in airtight containers in the anaerobic chamber in dark environment before use. The formation of hydrogen sulfide was prototypically characterized by a “rotten egg” odor. System Characterization: All solutions were prepared using deaerated and deionized water. This water was prepared by bubbling purified nitrogen gas through deionized water for at least 24 hours. Deionized water was obtained from a Millipore Milli-Q system (18 M_). Then the water was purged overnight in an anaerobic chamber containing a mixture of 5% hydrogen and 95% nitrogen gases. Detailed characterizations are provided in companion paper 1. Clays and zinc sulfide used in this study provided by the Richard Baker Harrison Company and Acros Organics Ltd and goethite provided by Iconofile Company Inc were nitrogen flushed and stored in airtight containers in the anaerobic chamber before use to avoid oxidation. Arsenic (III) stock solution was purchased from Merck. The AAS standard solution of 1000 mg/L Arsenic(III) was prepared by transferring the contents of a Titrisol ampule with As2O3 in H2O (Merck, Germany) into a volumetric flask, which was filled up to the mark and stored at 20±2oC according to the instructions by Merck. The working solutions of different concentrations were prepared by diluting the stock solution immediately before starting the batch studies53 . For sorbent characterization, the (a) Coulter laser method was used to determine the particle sizes; (b) % colloid was estimated from the particle size distribution curves; (c) equilibrium pH of the untreated mineral suspensions was determined using the Model 3340 Jenway ion meter; (d) the standard volumetric Brunauer, Emmett, and Teller (BET) method was used to determine the surface areas54, as shown in Table 1, (f) spectral analysis was performed using scanning electron microscopy, energy dispersive spectroscopy and x-ray diffraction to identify the mineral sorbent55-57. Sorption experiments: Batch mode experiments in this study were conducted adding 1% sulfidicanoxic suspension of zinc sulfide was added to 1% single mineral systems of kaolinite, montmo rillonite and goethite. Also, 1% sulfidic-anoxic mineral suspension of zinc sulfide was added to 1:1 mixed mineral system of kaolinite/montmorillonite, kaolinite/goethite and montmorillonite /goethite, used to elucidate the difference in sorption between the single and mixed mineral phases. For batch mode pH investigation, 1% sulfidic-anoxic suspension of zinc sulfide was added to 1% single and 1:1 mixed mineral suspensions made up to 50 ml containing 1% (by mass) mineral suspension, reacted with solution containing 15 ppm of arsenite at zero electrolyte background. Treated suspension was adjusted to the required pH (ranging from pH 4 to 8) using 0.1 M HNO3 and 0.1 M NaOH. The treated suspensions were equilibrated for 24 hours and pH measured using a Model 3340 Jenway ion meter. For batch mode initial metal concentration investigation, 1% sulfidic-anoxic suspension of zinc sulfide was added to 1% single and 1:1 mixed mineral suspensions made up to 50 ml containing 1% (by mass) mineral suspension, reacted with solution containing 10, 15,20 and 40 ppm of arsenite at zero electrolyte background. Treated suspension was adjusted to the required pH (ranging from pH 4 to 8) using 0.1 M HNO3 and 0.1 M NaOH. The treated suspensions were equilibrated for 24 hours and pH measured using a Model 3340 Jenway ion meter. JECET; March 2014-May 2014; Sec. A, Vol.3.No.2, 881-892.

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For batch mode solid or particle concentration investigation, 1% sulfidic-anoxic suspension of zinc sulfide was added to 1% single and 1:1 mixed mineral suspensions made up to 50 ml containing solid concentrations (g/l) of 2 , 4, 6, 8 and 10 were reacted with solution containing15 ppm of arsenite at zero electrolyte background. The treated mineral suspensions were adjusted to pH 4 and equilibrated for 24 hours. Batch mode ageing investigations was carried out from 24 to 720 hours. 1% sulfidicanoxic suspension of zinc sulfide was added to 1% single and 1:1 aged mixed mineral suspensions containing 1% (by mass were reacted with solution containing15 ppm of arsenite at zero electrolyte background. The treated suspensions, adjusted to pH 4 with no added electrolyte, were equilibrated for 24 hours. In all experimental studies samples were stored in the dark at room temperature (23±3 ◦C) for a maximum of 24 hours before analysis58. Supernatant was filtered through a cellulose acetate filter (pore size 0.2 µm) and analyzed for arsenic (III), using a Hitachi Atomic Absorption Spectrophotometer (HG-AAS). RESULTS AND DISCUSSION Mixed Mineral Systems and Ph Effect on Arsenite Sorption: a. b. c. d. e. f. g.

9

Arsenic sorbed (µg/g)

8 7

ZnS K M G KM KG GM

6 5 4 3 2

4

5

6

7

8

pH

Figure 1: Plots of arsenic sorbed versus pH on (a) Zinc sulfide. (b) kaolinite., (c) montmorillonite., (d) goethite., (e) kaolinite/montmorillonite., (f) kaolinite/goethite., (g) goethite/montmorillonite., sulfidic-anoxic mineral systems. In previous study in the absence of sulfidic-anoxic condition55 , arsenic (III) demonstrated a near linear sorption increase with increasing pH for single mineral systems of kaolinite, montmorillonite, and mixed mineral system of kaolinite/montmorillonite. Also, the behavior of goethite and mixed mineral systems containing goethite demonstrated a non-linear behaviour in arsenite removal. In this study under sulfidic-anoxic condition, zinc sulphide exhibit highest sorption capcity. Sorption capacity increases with pH increase as shown in Figure 1. Kaolinite-goethite exhibit lowest sorption capacity, marginally increasing with pH increase. Kaolinite, montmorillonite, and kaolinite-montmorillonite exhibit a near flattening sorption capacity over the range of pH investigated. Goethite exhibit a cross over sorption capacity at pH 5, indicating that sorption capacity for goethite and kaolinite-goethite are similar at the cross-over pH. This complex variability in sorption may be attributed to increased deprotonation of reactive sites as pH was increased and the effect of thiol (≡S-H) and hydroxyl (≡MeOH) functional groups and reactive sites in solution. However, sorption pattern appeared to be controlled by outer sphere complexation, inner sphere complexation and intra-particle diffusion for arsenic sorption on goethite and mixed mineral systems containing goethite. JECET; March 2014-May 2014; Sec. A, Vol.3.No.2, 881-892.

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Mixed Mineral Systems and Initial Metal Concentration Effect on Arsenite Removal: a. b. c. d. e. f. g.

20000

Sorption capacity (µg/kg))

18000 16000 14000

ZnS K M G KM KG GM

12000 10000 8000 6000 4000 2000 0

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Initial arsenic concentration (µg/L) at pH 4

Figure 2: Plots of sorption capacity versus Initial ion concentration for arsenic sorbed on (a) Zinc sulfide, (b) kaolinite; (c) montmorillonite; (d) goethite., (e) kaolinite/montmorillonite., (f) kaolinite/goethite., (g) goethite/montmorillonite, sulfidic-anoxic mineral systems. In previous study55, arsenic (III) demonstrated a near linear sorption increase with increase in initial metal concentration for single mineral system of kaolinite, montmorillonite, and mixed mineral system of kaolinite/montmorillonite55. In this study, all mineral systems exhibit linear sorption increase as initial arsenic concentration increases as shown in Figure 1. In previous study, in the absence of sulfidic-anoxic condition the behavior of goethite and mixed mineral systems containing goethite demonstrated a non-linear behaviour in arsenic (III) removal55 . In this study, zinc sulfide demonstrates lowest sorption and mineral systems containg goethite exhibit higher sorption capacity. This variability in sorption may be attributed to availability of thiol (≡S-H) and hydroxyl (≡Me-OH) functional groups on surface of metal sulfides reactive sites. Mixed Mineral Systems and Cp Effects on Arsenic (III) Removal: In previous study in the absence of sulfidic-anoxic condition55, the behavior of arsenic (III) sorption as particle concentration (Cp) increased was complex being linear for goethite and mixed mineral systems containing goethite. a. b. c. d. e. f. g.

1400 1200 1000

ZnS K M G KM KG GM

Kd (L/Kg)

800 600 400 200 0 0.002

0.004

0.006

0.008

0.010

Particle concentration-Cp (Kg/L)

Figure 3: Plots of actual Kd versus particle concentration for arsenic sorbed on (a) Zinc sulfide., (b)kaolinite., (c) montmorillonite., (d) goethite., (e) kaolinite/montmorillonite.,(f) kaolinite/goethite; (g) goethite/montmorillonite; sulfidic-anoxic mineral systems. JECET; March 2014-May 2014; Sec. A, Vol.3.No.2, 881-892.

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Actual and theoretical Kd differences (L/Kg)

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KM KG GM

600

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0

-200

-400

0.002

0.004

0.006

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Particle concentration-Cp (Kg/L)

. Figure 4: Plots of actual and theoretical Kd differences versus particle concentration-Cp for arsenic sorbed on: (a) kaolinite/montmorillonite; (b) kaolinite/goethite; and (c) goethite/montmorillonite., zinc sulfide sulfidic-anoxic mineral systems. In this study under sulfidic-anoxic condition, goethite-montmorillonite demonstrates lowest sorption flattening out as Cp increases as shown in Figure 2. All but zinc sulfide demonstrate step-wise decrease in sorption as Cp increases. Zinc sulfide exhibit an initial increase in sorption upto 0.08Kg/L, then decreasing over the rest of the Cp. Goethite in mixed suspensions may form separate (discrete) particles or it may form coatings on other mineral surfaces. Coatings of only a few atomic layers thickness are sufficient to influence sorption rates59. This may account for differences in sorption behaviour for arsenite sorbed on mixed suspensions containing goethite. Decrease in arsenite sorption as Cp increases may be attributed to increase in particle size and aggregation of the mineral suspensions. The Cp effect is also related to effective surface area, pressure, and force at the mineral/water interface30. Increase in Cp results in low pressure at the interface and a subsequent decrease in sorbing ion diffusion to reactive sites55. Differences between the actual and predicted arsenite sorption as shown in Figure 3, for kaolinite-goethite is in the positive territory indicating that mineral mixing emhanced sorption throughout the range of Cp investigated. On the contrary, goethite-montmorillonite began on the positive territory ending in the negative, indicating a complex Cp effect over the range investigated. kaolinite/montmorillonite showed negative sorption difference at the onset of Cp increasing into the positive territory at 0.06Kg/L then dipping into the negative again over the rest of Cp investigated This means that mineral mixing reduced arsenite sorption within this range of Cp investigated for kaolinite-montmorillonite. Mixed Mineral System and Ageing Effects on Arsenic (III) Removal: a. b. c. d. e. f. g

Arsenic sorbed (%)

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Residence Time (Hours)

Figure 5: Plots of arsenic sorbed versus residence time for (a) zinc sulfide, (b) kaolinite, (c) montmorillonite, (d) goethite, (e) kaolinite/montmorillonite; (f) goethite/kaolinite; and (g) goethite/montmorillonite. Sulfidic-anoxic mineral systems. JECET; March 2014-May 2014; Sec. A, Vol.3.No.2, 881-892.

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In previous investigation55 in the absence of sulfidic-anoxic condition, all single and mixed mineral systems except kaolinite demonstrated step-wise arsenite sorption. Also, kaolinite/goethite demonstrated a step down arsenite sorption for the first 288 hours, increasing over time for the remaing residence time of the reaction. In this study, under sulfidic anoxic condition, zinc sulfide exhibit increase in sorption upto 288 hours of contact, flattening out for the rest of residence time investigated is shown in Figure 5. Kaolinite exhibit a flat sorption, indicating no change in sorption over the range of contact time investigated. Actuially, zinc sulfides exhibit a cross-over sorption with kaolinite at 288 hourresidencve time, indicating similar sorption for these two mineral systems at that period. Goethite exhibit flat sorption characteristics upto 432-hour contact time, stepping down for the rest of the time investigated.

Actual and theoretical arsenic sorbed differences (%)

Kaolinite-montmorillonite, kaolinite-goethite and goethite-montmorillonite demonsytrate an increase in sorption overtime for all the period of investigation. The complex behavior of these mineral systems during ageing under sulfidic-anoxic condition may be attributed to the presence of thiol (≡SH) and hydroxyl (≡Me-OH) functional groups on surface of metal sulfides reactive sites. Arsenite step-wise sorption probably indicated reaction phases attributed to outer sphere, inner sphere complexation and intra-particle diffusion as reported in paper I. a. b. c.

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Figure 6: Plots of actual and theoretical arsenic sorbed differences versus residence time for (a) kaolinite/montmorillonite; (b) goethite/kaolinite; and (c) goethite/montmorillonite., zinc sulfide sulfidic-anoxic mineral systems In previous investigation55 in the absence of sulfidic –anoxic condition, differences between actual and theoretical arsenite sorption was positive for all mixed mineral systems, indicating increase in arsenite sorption due to mineral mixing. In this report under sulfidic-anoxic condition, differences between actual and theoretical sorption is negative for all mixed mineral systems, indicating attenuation in arsenite sorption due to mineral mixing. Increase in arsenite sorption as residence time increeases may be attributed to increased hydroxylation of the mineral surfaces, resulting in the formation of new reactive sites18. Decrease in arsenite sorption by mineral mixing may be attributed to masking of reactive sites by the component minerals. JECET; March 2014-May 2014; Sec. A, Vol.3.No.2, 881-892.

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CONCLUSIONS The possibilities of using zinc sulfide and mixed smineral systems of kaolinite, montmorillonite, and goethite to remove arsenite from simulated ontaminated surface and ground water under sulfidicanoxic condition has been investigated as a function of solution composition and ageing. Variability in arsenite sorption exist over the range of pH investigated. Zinc sulphide exhibit highest sorption capcity. Sorption capacity increases with pH. Kaolinite-goethite exhibit lowest sorption capacity, marginally increasing with pH increase. Kaolinite, montmorillonite, and kaolinite-montmorillonite exhibit a near flattening sorption capacity over the range of pH investigated. Goethite exhibit a cross over sorption capacity at pH 5, indicating that sorption capacity for goethite and kaolinite-goethite are similar at the cross-over pH. This variability in sorption may be attributed to increased deprotonation of reactive sites as pH was increased and the presence of thiol (≡S-H) and hydroxyl (≡Me-OH) functional groups on surface of metal sulfides reactive sites. Sorption pattern appeared to be controlled by outer sphere complexation, inner sphere complexation and intra-particle diffusion for arsenic sorption on goethite and mixed mineral systems containing goethite. All mineral systems exhibit linear sorption increase as initial arsenic concentration increases. Zinc sulfides demonstrate lowest sorption and mineral systems containg goethite exhibit higher sorption capacity. This variability in sorption may be attributed to availability of thiol (≡S-H) and hydroxyl (≡Me-OH) functional groups on surface of metal sulfides reactive sites. Single and mixed mineral systems exhibited different sorption patterns. All but zinc sulfide demonstrate step-wise decrease in sorption as Cp increases. Zinc sulfide exhibit an initial increase in sorption upto 0.08Kg/L, then decreasing over the rest of the Cp. Goethite in mixed suspensions may form separate (discrete) particles or it may form coatings on other mineral surfaces. Coatings of only a few atomic layers thickness are sufficient to influence sorption rates. While increase in arsenite sorption with increase in Cp may be attributed to increase in specific surface area, a decrease in arsenite sorption of the range of Cp investigated may be attributed to increase in particle size, flocculation and aggregation of mineral suspension. All single and mixed mineral systems except kaolinite demonstrated step-wise arsenite sorption which may be attributed to inner sphere complexation, outer sphere complexation and intra-particle diffusion. The variable and complex behavior of mineral systems over the range of residence time investigated may be attributed to increased hydroxylation of the mineral surface resulting in the formation of new reactive sites. REFERENCES 1. M. Clara F. Magalhães, Arsenic An environmental problem limited by Solubility, Pure Appl. Chem. 2002, 74(10), 1843–1850. 2. Chatterjee, D. Das, B. K. Mandal, T. R. Chowdhury, G. Samanta, D. Chakraborti. Analyst.1995, 120, 643–650. 3. M. Williams. Arsenic in mine waters: an international study. Envi. Geo. 2001, 40, 267–278. 4. T. Altun, & E. Pehlivan, Removal of Cr (VI) from aqueous solutions by modified walnut shells. Food Chemistry, 2012, 132, 693–700. 5. L. Dupont, G. Jolly, & M. Aplincourt, Arsenic adsorption on lignocellulosic substrate loaded with ferric ion. Environmental Chem Letters, 2007, 5, 125–129. 6. J. C. Ng, Environmental contamination of arsenic and its toxicological impact on humans. Environmental Chemistry, 2005, 2, 146–160.

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*Corresponding Author: D.E. Egirani; Faculty of Science, Niger Delta University, Wilberforce Island, Nigeria

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