Arsenic removal by iron based co-precipitation: Mechanisms in groundwater treatment

Arsenic (As) contamination of groundwater with is a wide-scale problem, affecting health of people around the world. The World Health Organization (WHO) guideline for As in drinking water is currently set at 10 µg/L, however recent studies suggest that As can cause a considerable damage to human health even at concentrations lower than the WHO guideline. As a result, several drinking water companies are making efforts to reduce As concentrations in drinking water to very low concentrations, below 1 µg/L. 

Co-precipitation of As with iron(III)(oxyhydr)oxides [Fe(III)(oxyhydr)oxides] is a widely used As removal method in groundwater treatment. Fe(III)(oxyhydr)oxides are produced in an As contaminated water, typically by oxidation of naturally occurring ferrous iron [Fe(II)] in groundwater using oxygen (O2) and/or dosing a ferric iron [Fe(III)] coagulant such as Ferric Chloride (FeCl3). Arsenic strongly adsorbs to the surface of freshly formed Fe(III) precipitates and subsequently the As bearing Fe(III) precipitates are removed by filtration to produce As-safe water. The adsorption efficiency of As onto Fe(III) precipitates and the size of As bearing Fe(III) particles is governed by several interdependent factors such as the conditions of Fe(III)(oxyhydr)oxide generation in water, oxidation state of As, solution pH and the concentration of As and co-occurring ions with respect to Fe in the initial solution. The objective of this thesis is to discern mechanistic understanding of As removal by co-precipitation with Fe(III)(oxyhydr)oxides under different redox, ion composition and filtration conditions and to investigate routes to reduce As in drinking water to very low levels, below 1 µg/L.

We carried out sampling campaigns at water treatment plants in the Netherlands to gain understanding of the pertinent As removal mechanisms during groundwater treatment. It was found that rapid sand filtration is the most important treatment step for oxidation and removal of As during groundwater treatment. The removal of As is tightly coupled to Fe removal in rapid sand filters and mainly attributed to co-precipitation of As with Fe(III)(oxyhydr)oxides, which are generated by oxidation and subsequent hydrolysis of Fe(II). The As co-precipitation efficiency with Fe(III)(oxyhydr)oxides is much higher in rapid sand filter beds compared to aeration and supernatant storage. This is ascribed to oxidation of arsenite [As(III)] to arsenate [As(V)] in the rapid sand filter beds, potentially executed by manganese oxides (MnO2) and/or As(III) oxidizing bacteria, as both are observed in the coating of rapid sand filter media grains. In the pH range of most groundwaters, As(V) adsorbs to Fe(III)(oxyhydr)oxides much more effectively than As(III).

Typical aeration techniques such as cascades are inefficient in oxidizing As(III) to As(V) before rapid sand filters at water treatment plants, resulting in inefficient As co-precipitation with Fe(III)(oxyhydr)oxides. Nevertheless, dosing a strong oxidant such as potassium permanganate (KMnO4) rapidly accomplishes As(III) oxidation to As(V) and drastically improves As co-precipitation efficiency with Fe(III)(oxyhydr)oxides, resulting in As reduction to very low levels, below 1 µg/L. While no negative effect on the removal efficiency of Fe(II), Mn(II) and ammonium (NH4+) in rapid sand filters is observed due to KMnO4 dosing, the pre-established Fe(II) and Mn(II) removal mechanisms in rapid sand filters are altered due to KMnO4 dosing, generating a need for rapid sand filter media replacement. We also found that dosing of strong oxidants during groundwater treatment impacts the composition and structure of the formed Fe and Mn bearing precipitates. For example, in the absence of competing ions, O2 produces Mn(III)-incorporated moderately crystalline lepidocrocite, sodium hypochlorite (NaOCl) produces Mn(III)-incorporated poorly-ordered hydrous ferric oxide, and KMnO4 produces poorly-ordered MnO2 and poorly-ordered hydrous ferric oxide phases. This diversity of formed precipitates under different redox conditions should be considered in As removal during groundwater treatment.

In this thesis we show that As levels below 1 µg/L can alternatively be achieved by dosing a small amount of FeCl3 in the effluent of rapid sand filter at groundwater treatment plants. The effluent of rapid sand filter predominantly contains arsenate [As(V)] which is much more effectively adsorbed to Fe(III)(oxyhydr)oxides than As(III). In this way use of KMnO4 or other strong oxidants can be avoided at groundwater treatment plants. Nevertheless, the ionic composition of water strongly controls As(V) removal by iron based co-precipitation, by affecting the adsorption efficiency of As(V) with Fe(III)(oxyhydr)oxides and the size of As bearing Fe(III) particles. We show that silicate (SiO4-4) and phosphate (PO4-3) reduce As(V) removal, mainly due to competition with As(V) adsorption to Fe(III) precipitates. Though SiO4-4 en PO4-3 oxyanions are known to strongly reduce Fe(III) precipitate growth, we show that presence of high calcium (Ca) concentrations in groundwater (common in the Netherlands and several other parts of the world) counteracts the negative effects of both SiO4-4 en PO4-3 and promote coagulation of Fe(III) precipitates to form large particles which are easily separated from water by gravitation settling and rapid sand filtration. Despite presence of high Ca concentrations, Natural Organic Matter (NOM) reduces As(V) removal quite drastically, attributed largely to the formation of soluble and colloidal Fe(III)–NOM complexes which are not easily separated by conventional filtration.

In-line dosing of a small amount of FeCl3 in the feed water of ultrafiltration (UF) step (typically used for final polishing and disinfection) is shown to be effective for As reduction to <1 µg/L at water treatment plants which use artificially recharged water as source. In this process, As(V) co-precipitation with Fe(III)(oxyhydr)oxides rapidly reaches equilibrium, thus little contact time before the membranes is required. Moreover, when As bearing Fe(III) precipitates grow to sizes larger than the pore size of UF membranes (expected for most Ca bearing groundwaters) the Fe(III) particles foul the membranes mainly by forming a cake-layer on the surface which is effectively removed with a hydraulic backwash. Thus, we conclude that sustainable long term operation of UF membranes with in-line FeCl3 dosing for As removal is highly viable.

Based on the present work, three groundwater treatment plants in the Netherlands have received an upgrade with KMnO4 dosing for reducing As to below 1 µg/L. Another treatment plant, which makes use of artificially recharged groundwater, will receive an upgrade with FeCl3 dosing before the polishing UF step.