Modeling As(III) oxidation and removal with iron electrocoagulation in groundwater.

Understanding the chemical kinetics of arsenic during electrocoagulation (EC) treatment is essential for a deeper understanding of arsenic removal using EC under a variety of operating conditions and solution compositions. We describe a highly constrained, simple chemical dynamic model of As(III) oxidation and As(III,V), Si, and P sorption for the EC system using model parameters extracted from some of our experimental results and previous studies. Our model predictions agree well with both data extracted from previous studies and our observed experimental data over a broad range of operating conditions (charge dosage rate) and solution chemistry (pH, co-occurring ions) without free model parameters. Our model provides insights into why higher pH and lower charge dosage rate (Coulombs/L/min) facilitate As(III) removal by EC and sheds light on the debate in the recent published literature regarding the mechanism of As(III) oxidation during EC. Our model also provides practically useful estimates of the minimum amount of iron required to remove 500 µg/L As(III) to <50 µg/L. Parameters measured in this work include the ratio of rate constants for Fe(II) and As(III) reactions with Fe(IV) in synthetic groundwater (k(1)/k(2) = 1.07) and the apparent rate constant of Fe(II) oxidation with dissolved oxygen at pH 7 (k(app) = 10(0.22) M(-1)s(-1)).

Removing arsenic from synthetic groundwater with iron electrocoagulation: an Fe and As K-edge EXAFS study.

Electrocoagulation (EC) using iron electrodes is a promising arsenic removal strategy for Bangladesh groundwater drinking supplies. EC is based on the rapid in situ dissolution of a sacrificial Fe(0) anode to generate iron precipitates with a high arsenic sorption affinity. We used X-ray absorption spectroscopy (XAS) to investigate the local coordination environment (<4.0 Å) of Fe and As in EC precipitates generated in synthetic Bangladesh groundwater (SBGW). Fe and As K-edge EXAFS spectra were found to be similar between samples regardless of the large range of current density (0.02, 1.1, 5.0, 100 mA/cm(2)) used to generate samples. Shell-by-shell fits of the Fe K-edge EXAFS spectra indicated that EC precipitates consist of primarily edge-sharing FeO(6) octahedra. The absence of corner-sharing FeO(6) octahedra implies that EC precipitates resemble nanoscale clusters (polymers) of edge-sharing octahedra that efficiently bind arsenic. Shell-by-shell fits of As K-edge EXAFS spectra show that arsenic, initially present as a mixture of As(III) and As(V), forms primarily binuclear, corner-sharing As(V) surface complexes on EC precipitates. This specific coordination geometry prevents the formation of FeO(6) corner-sharing linkages. Phosphate and silicate, abundant in SBGW, likely influence the structure of EC precipitates in a similar way by preventing FeO(6) corner-sharing linkages. This study provides a better understanding of the structure, reactivity, and colloidal stability of EC precipitates and the behavior of arsenic during EC. The results also offer useful constraints for predicting arsenic remobilization during the long-term disposal of EC sludge.

Arsenic remediation of drinking water using iron-oxide coated coal bottom ash.

We describe laboratory and field results of a novel arsenic removal adsorbent called 'Arsenic Removal Using Bottom Ash' (ARUBA). ARUBA is prepared by coating particles of coal bottom ash, a waste material from coal fired power plants, with iron (hydr)oxide. The coating process is simple and conducted at room temperature and atmospheric pressure. Material costs for ARUBA are estimated to be low (approximately $0.08 per kg) and arsenic remediation with ARUBA has the potential to be affordable to resource-constrained communities. ARUBA is used for removing arsenic via a dispersal-and-removal process, and we envision that ARUBA would be used in community-scale water treatment centers. We show that ARUBA is able to reduce arsenic concentrations in contaminated Bangladesh groundwater to below the Bangladesh standard of 50 ppb. Using the Langmuir isotherm (R(2) = 0.77) ARUBA's adsorption capacity in treating real groundwater is 2.6 x 10(- 6)mol/g (0.20 mg/g). Time-to-90% (defined as the time interval for ARUBA to remove 90% of the total amount of arsenic that is removed at equilibrium) is less than 1 hour. Reaction rates (pseudo-second-order kinetic model, R(2) > or = 0.99) increase from 2.4 x 10(5) to 7.2 x 10(5) g mol(- 1)min(- 1) as the groundwater arsenic concentration decreases from 560 to 170 ppb. We show that ARUBA's arsenic adsorption density (AAD), defined as the milligrams of arsenic removed at equilibrium per gram of ARUBA added, is linearly dependent on the initial arsenic concentration of the groundwater sample, for initial arsenic concentrations of up to 1600 ppb and an ARUBA dose of 4.0 g/L. This makes it easy to determine the amount of ARUBA required to treat a groundwater source when its arsenic concentration is known and less than 1600 ppb. Storing contaminated groundwater for two to three days before treatment is seen to significantly increase ARUBA's AAD. ARUBA can be separated from treated water by coagulation and clarification, which is expected to be less expensive than filtration of micron-scale particles, further contributing to the affordability of a community-scale water treatment center.