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«Frederick Kenneh Partey New Mexico Tech Department of Earth and Environmental Science MECHANISM OF ARSENIC SORPTION ONTO LATERITE IRON CONCRETIONS By ...»

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MECHANISM OF

ARSENIC SORPTION

ONTO LATERITE

CONCRETIONS

Frederick Kenneh Partey

New Mexico Tech

Department of Earth and

Environmental Science

MECHANISM OF ARSENIC

SORPTION ONTO LATERITE IRON

CONCRETIONS

By

Frederick Kenneh Partey Submitted to the faculty of New Mexico Tech in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Earth and Environmental Science with Dissertation in Geochemistry New Mexico Institute of Mining and Technology Department of Earth and Environmental Science Socorro, New Mexico January 2008 Advisory Committee Advisor______________________________

Dr. Dave Norman Member 1________________________________

Dr. Robert Bowman Member 2________________________________

Dr. Jan Hendrickx Member 3________________________________

Dr. Malcolm Siegel Member 4________________________________

Dr. Virgil Lueth Dedication This Thesis is dedicated in loving memory of Mr. and Mrs. Henry Teye Partey and to my beloved wife Cynthia Partey not forgetting Deborah, Daisy, Dave and Diandra. Your support and encouragement has brought me this far.

Such is life Men’s evil deeds are written on brass the good ones on water when I am right no one remembers when I am wrong no one forgets.

ABSTRACT

The objective of this study is to understand the sorption mechanisms and to quantify sorption of arsenic on laterite concretions (LC). Laterite concretions are known to sorb arsenic. I investigated As (III) and As (V) sorption onto Prestea and Awaso laterite concretions (LC) to test its suitability for use in low-tech treatment of arsenic-bearing drinking water. The two Fe-Al oxide-hydroxide concretions were selected for the study because they represent compositional end members, Al-rich (Awaso) and Fe-rich (Prestea), of lateritic soil concretions. The ultimate goal of this project is to demonstrate how and why LC can be used as an effective and inexpensive means of water purification system for communities that cost less and is easy to maintain, and produced drinking water of high quality.

Attenuated Total Reflection Fourier transform infrared (ATR-FTIR) spectroscopic methods were combined with sorption experiments, electrophoretic mobility measurements, and surface complexation modeling to study the interaction of As (III) and As (V) with laterite concretion surfaces. Arsenic sorption on Prestea and Awaso laterite concretions was also investigated as a function of solution pH.

The sorption capacity was determined for both concretions through batch experiments on crushed samples. Prestea LC was studied at different temperatures to evaluate the effect of temperature on the media. Competitive sorption experiments were also conducted in the presence of phosphate and sulfate, as this represents the case of greatest threat to arsenic remediation in most ground waters and sulfide mining waste waters from stock piles.

Experiments of Prestea LC show that sorption capacity for both arsenite and arsenate increases with temperature. The equilibrium sorption capacity for As (III) is larger than that for As (V) over temperatures ranging from 25° to 60°C. A Langmuir model satisfactorily fits the arsenite and arsenate sorption isotherm data for both Prestea and Awaso LC. Both As (III) and As (V) sorbed well for the pH range of natural waters with little change.

Arsenic (III) sorption on both Prestea and Awaso LC exhibits decreasing sorption with increasing ionic strength, indicating an outer-sphere sorption mechanism. Arsenic (V) sorption on both Prestea and Awaso LC shows slight ionic strength dependence with increasing solution pH, and an increase in sorption with increasing solution ionic strength. These behaviors are indicative of an inner-sphere sorption mechanism for As (V) on both studied types of LC.

The results of the electrophoretic measurements (EM) indicate that both As (III) and As (V) form inner-sphere complexes on Prestea LC. Arsenic (III) forms outer-sphere sorption mechanisms on Awaso LC because there is no shift in pHzpc even with an increase in As (III) concentration. Arsenic (V) however, forms inner sphere complexes on Awaso LC due to shifts in pHpzc and reversals of EM with increasing ion concentration.

The ATR-FTIR analysis shows an increase in peak intensities and band shift to lower wavelengths for both As (III) and As (V) on Prestea and Awaso LC. The presences of the peaks in the treated LC spectra that are not present in the untreated sample are an indication of chemical bonding between the arsenic species and the surface of the Prestea LC. The peak shift and the change in peak intensity may be indicative of an inner-sphere sorption mechanism. The peak positions of the arsenic treated samples (sorbed samples for both Prestea and Awaso LC) are significantly different from those of the dissolved arsenic species and can be attributed to sorption of the arsenic species. In general, the spectra of both As (III) and As (V) sorbed onto the Prestea and Awaso LC are very different from those of arsenic aqueous solutions.





This difference and the lack of pH dependence on the positions of the vibrational modes indicate that these modes are “protected” from changes in pH and indicate that these groups are involved in direct complexation to the surface. Another line of evidence for the mechanism of sorption that is converse to the ATR-FTIR spectra for dissolved arsenic species is that a shift in band position was not observed in As (V) and As (III) adsorbed spectra with changing pH. The lack of change in band position at various pH values suggests that arsenic formed the same inner-sphere surface complexes on both Prestea and Awaso LC.

Surface complexation models successfully constrained both macroscopic and microscopic measurements. The effect of changes in ionic strength on sorption of As (III) and As (V) on Prestea and Awaso was modeled using both diffuse and triple layer models. Arsenic (V) sorption, which is slightly affected by ionic strength, was modeled with both the diffuse layer and the triple layer model, although the triple layer model shows a better fit at higher pH’s than the diffuse layer model.

Arsenic (III) sorption, which is markedly reduced by increasing ionic strength, is best modeled using the triple layer model.

The presence of phosphate and sulfate reduces the amount (mg) of As (III) sorbed per gram of Prestea and Awaso LC. However, an aqueous solution of As (V) spiked with sulfate did not reduce As (V) sorption rather it increased the sorption.

The increase was more prominent on Awaso than Prestea LC.

The negative “Gibbs free energy (∆Go)” values for arsenite and arsenate sorption on Prestea LC agree with spontaneous reaction between the species and the medium. Positive “entropy (∆So)” values suggest the affinity of LC for the arsenic species in solution.

The sorption capacity value indicates that significant sorption sites are available for specific sorption of both arsenic species. The development of low-cost arsenic filters using LC is therefore practical. The Prestea and Awaso LC both treated approximately 5000 bed volumes of 42 µL As (V) Socorro water to the maximum contamination limit of 10 ppb. Analysis of the arsenic sorption data suggests that LC can be used for a low-tech natural-materials arsenic water treatment and has a number of advantages over commercial materials for this use including the ability to remove arsenic from waters with a wide range in pH, to sorb both common arsenic aqueous species equally well, and cost less. The positive sorption temperature dependence of LC will enhance sorption in tropical climates, and more especially in areas where groundwater sources are related to geothermal springs.

The media has potential in remediating other toxic trace elements to very low concentrations. A TCLP leaching test also reveals that the used adsorbent is not toxic and can be disposed of without the need for confinement. Investigations of arsenic sorption onto these two end members show that, all other laterites whose mineralogical compositions fall within these two end members should filter arsenic from drinking water.

This dissertation is accepted on behalf of the faculty of the institute by the following committee.

–  –  –

I release this document to the New Mexico Institute of Mining and Technology.

____________________________________________________________________

–  –  –

I wish to express my deepest gratitude first to the almighty God who gave me the strength to undertake this project. I am very grateful to my committee members Dr. Dave Norman, Dr. Robert Bowman, Dr Jan Hendrickx, and Dr. Malcolm Siegel whose useful suggestions and encouragement led to the successful completion of this work. Further more I would like to thank Dr. Christa Hochensmith who helped me with all the ATR-FTIR work.

I am also grateful to Dr. Samuel Ndur and Matt Earthman for helping me with laboratory work. To all members of the faculty, staff, graduate students, and undergraduate students here in New Mexico Tech, I say thank you for your love and care. Lastly to all and sundry, I say thank you and God bless you.

This project was funded by the Sandia National Laboratories, the Geological Society of America, the Office of Advancement and Research at New Mexico Tech, and the Graduate Student Association at New Mexico Tech. I appreciate scholarships from these organizations: the American Federation of Mineralogical Societies, the Budding Geosciences Research Award, the Women's Auxiliary to the American Institute of Mining, and the Metallurgical and Petroleum Engineers for feeding

–  –  –

Fig. 9. Arsenic (III) sorption on Prestea LC as a function of pH and ionic strength 39 Fig. 10. Arsenic (V) sorption on Prestea LC as a function of pH and ionic strength 39 Fig. 11. Arsenic (III) sorption on Awaso LC as a function of pH and ionic strength 40 Fig. 12. Arsenic (V) sorption on Awaso LC as a function of pH and ionic strength 40 Fig. 13 Comparing sorption of As (III) on Prestea LIC in phosphate-free and phosphate-bearing solutions as a function of pH and ionic strength 41 Fig. 14 Comparing sorption of As (V) on Prestea LIC in phosphate-free and phosphate-bearing solutions as a function of pH and ionic strength 42 Fig. 15 Comparing sorption of As (III) on Prestea LIC in sulfate-free and phosphate-bearing solutions as a function of pH and ionic strength 43 Fig. 16 Comparing sorption of As (V) on Prestea LIC in sulfate-free and phosphate-bearing solutions as a function of pH and ionic strength 43 Fig. 17 Comparing sorption of As (III) on Awaso LIC in phosphate-free and phosphate-bearing solutions as a function of pH and ionic strength 44 Fig. 18 Comparing sorption of As (V) on Awaso LIC in phosphate-free and phosphate-bearing solutions as a function of pH and ionic strength 45 Fig. 19 Comparing sorption of As (III) on Awaso LIC in sulfate-free and phosphate-bearing solutions as a function of pH and ionic strength 46 Fig. 20 Comparing sorption of As (V) on Awaso LIC in sulfate-free and phosphate-bearing solutions as a function of pH and ionic strength 46 Fig. 21 Electrophoretic mobility of Prestea laterite iron concretion as a function of pH and total As (III) concentration in 0.01 M NaCl solution 48

–  –  –

Fig. 26 ATR-FTIR spectra of aqueous suspension of Prestea LC, 0.1M As (III)-treated LC and 0.1 M As (III) solution for the region 1400-600 cm−1 52 Fig. 27 ATR-FTIR spectra of aqueous suspension of Prestea LC, 0.1M As (V)-treated LC and 0.1 M As (V) solution for the region 1400-600 cm−1 53 Fig. 28 ATR-FTIR spectra of aqueous suspension of Awaso LC, 0.1M As (III)-treated LC and 0.1 M As (III) solution for the region 1400-600 cm−1 54 Fig. 29 ATR-FTIR spectra of aqueous suspension of Awaso LC, 0.1M As (V)-treated LC and 0.1 M As (V) solution for the region 1400-600 cm−1 55 Fig. 30 Diffuse layer modeled calculations for As (III) sorption on Prestea LC 61 Fig. 31. Diffuse layer modeled calculations for As (V) sorption on Prestea 62 Fig. 32. Diffuse layer modeled calculations for As (V) sorption on Awaso LC 63 Fig. 33. Diffuse layer modeled calculations for As (V) sorption on Awaso LC 64 Fig. 34. Triple layer modeled calculations for As (III) sorption on Prestea LC 65 Fig. 35. Triple layer modeled calculations for As (V) sorption on Prestea LC 66 Fig. 36. Triple layer modeled calculations for As (V) sorption on Awaso LC 67 Fig. 37. Triple layer modeled calculations for As (V) sorption on Awaso LC 68

–  –  –

Fig A-6. Socorro water chemistry for trace elements before going through Prestea LC and effluent water chemistry after filtering arsenic 127

–  –  –

Fig A-9. Socorro water chemistry for trace elements before going through Awaso LC and effluent water chemistry after filtering arsenic 130

–  –  –

Table A-5. Summary of description of media tested and Pilot Demonstration Results 136 Table A-6 (As (III)) sorption onto Prestea laterite iron concretion as a function of equilibrium concentration at various temperatures 137

–  –  –

Table A-18 Competitive sorption of As (III) and phosphate on Prestea LC 148 Table A-19 Competitive sorption of As (III) and sulfate on Prestea LC 148 Table A-20 Competitive sorption of As (V) and phosphate on Prestea LC 149

–  –  –

Table A-37. Chemical composition of laterite concretions found elsewhere in the world 155

LIST OF SYMBOLS AND ABBREVIATIONS



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