WITS WATER FILTER, Arsenic Removal Matrix

A Simple, Low Cost, And Effective Arsenic Removal Filter

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Preliminary
(The Useful Bit)

Introduction
(The Technical Bit)

Chemistry and general considerations
(The Science Bit)

Tests and performance
(The Boring Bit)

Operation and maintenance
(The Interesting Bit)

Future outlook
(The Last Bit)

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Chemistry and general considerations (The Science Bit)

In groundwater (pH = 6.5–7.5) arsenic is present in two oxidation states (As(III) in H3AsO3 and As(V) in H2AsO-4 and HAsO2-4 ). It is known that in most groundwater in Argentina more than 50% of total arsenic is present as the neutral H3AsO3 at groundwater pH. The other 50% is divided equally in two As(V) species H2AsO-4 and HAsO2-4 .

An ideal filter must remove all three species without chemical pre-treatment, without regeneration, and without producing toxic wastes. The unit WITS filter had satisfied these requirements. The WITS filtration systems also passed the ETVAM tests and a similarly made system was tested in Nepal by a MIT group with an arsenic removal capacity of 20 µg/L. The primary active material in the WITS filter is the Arsenic Removal Matrix (ARM), made through a proprietary process to maintain active for a minimum 2 of years.

The ARM removes the inorganic arsenic species quantitatively through all possible reactions. Infrared spectroscopy (IRS) and extended X-ray absorption fine structure (EXAFS) show that arsenate and arsenite formbidentate, binuclear surface complexes with =FeOH (or =FeOOH or hydrous ferric oxide, HFO) as the predominant species tightly immobilized on the ARM surface. Also, inorganic As(III) species are oxidized to As(V) species by the active O-2 , which is produced by the oxidation of soluble Fe(II) with dissolved oxygen. Manganese (1–2% by wt) in ARM catalyzes oxidation of As(III) to As(V). Therefore, the process does not require pre-treatment of water with external oxidizing agents such as hypochlorite or potassium permanganate.

In addition to arsenic species, =FeOH is also known to remove many other toxic species. The primary reactions are: =FeOH + H2AsO-4 ?=FeHAsO-4 + H2O (K = 1024) and =FeOH + HAsO2- 4 ?=FeAsO2-4 + H2O (K= 1029). These intrinsic equilibrium constants indicate very strong complexation and immobilization of inorganic arsenic species. It is known that excess Fe2+, Fe3+, and Ca2+ in groundwater enhance positive charge density of the inner Helmholtz plane of the electrical double layer and specifically binds anionic arsenates to form surface complexes. We found that As(III) and As(V) removal process was independent of the input arsenic concentration i.e., a zero-order reaction. Detailed thermodynamics and kinetics of these reactions are still under investigation.