PRINCIPAL INVESTIGATORS:
Gautam Samanta and Dennis A. Clifford
OBJECTIVES:
The objective of the research was to determine the feasibility of using filter beds of ferrous carbonate (siderite) in combination with low levels (0−2 ppm) of chlorine to oxidize As(III) to As(V) and generate 0-3 ppm Fe(OH)3(s) as Fe for in-situ coagulation and removal of As(V). Batch and column adsorption tests using six different siderite materials in comparison with iron oxide (FeO, wustite) were conducted meet the objectives.
BACKGROUND:
Arsenic is commonly removed from small system groundwater supplies using packed beds of granular iron media such as granular ferric hydroxide (GFH) and granular ferric oxide (GFO). These systems are simple in design and operation but are more costly than complicated iron-coagulation filtration (C-F) processes that meter a coagulant such as ferric chloride into the feed water to a filter system. An ideal solution would be a packed bed of iron-bearing material that releases enough iron to coagulate and remove the arsenic without having to carefully meter corrosive ferric chloride into the feed water.
APPROACH:
Samples of siderite from six different mineral deposits were obtained from two different vendors for screening tests to establish the best mineral for the arsenic removal tests. For the proof-of-concept, attempts were made to prepare ferrous carbonate in the laboratory. The small-scale column tests (SSCTs) were conducted using the best performing siderite to remove arsenic in the presence of 1−3 mg/L Cl2. Pretreatment of the siderite bed was done using alkaline or acidic Cl2 solution to increase the solubility of siderite. SSCTs were conducted with other Fe(II) media including wustite and PEL Technology Fe(II) media. Finally, an electrolytic dissolution system was constructed, and SSCTs with siderite were performed using a stainless steel anode and a graphite-rod cathode embedded in siderite.
HIGHLIGHTS:
• Although siderite is a very common mineral, it was not readily commercially available, presumably because of lack of current markets for it as a raw material.
• Results of SSCTs with chlorinated feed water containing representative cations and anions showed that Fe(II) was not dissolving as expected, thus little to no ferric hydroxide was produced to coat the siderite media. Based on the experimental results, it was concluded that under the experimental conditions, the solubility of siderite was much lower than the calculated theoretical value, possibly due to the presence of silica in the representative challenge feed water.
• Attempted electrolytic generation of iron from siderite in a column with stainless steel cathode and graphite/siderite anode also failed to produce ferric iron for hydrolysis and precipitation of Fe(OH)3(s) for arsenic sorption.
RESULTS/FINDINGS:
The objective of the proposed research was to determine the feasibility of using filter beds of ferrous carbonate (siderite) in combination with low levels (0-2 ppm) of chlorine to oxidize As(III) to As(V) and generate 0-3 ppm Fe(OH)3(s) as Fe for in-situ coagulation and removal of As(V). Based on the SSCT results, the first sample of siderite mineral (obtained from Ward's Natural Science) showed very poor arsenic-adsorption performance. The media was saturated at 1000 BV and 10 ppb arsenic breakthrough occurred only at 40 bed volumes throughput with a 3 cm3 siderite bed in the presence of 1 mg/L Cl2 at pH 7.5. Attempts were made to pre-dissolve the siderite using high and low pH conditions, but the resulting run lengths of the pretreated siderite columns were only 1600 bed volumes, i.e., far shorter than what is considered practical for arsenic treatment in small systems.
Because the laboratory preparation FeCO3(s) was not successful, due to its very rapid oxidation in the atmosphere, screening tests were performed using six siderite samples obtained from around the world to select the most efficient siderite material to remove arsenic. The results showed that even at lower pH (pH 5.0), the performance of the siderite materials was not promising in the representative challenge water.
Minicolumn tests with untreated wustite (FeO) demonstrated longer arsenic run length (3200 BV), but with the disadvantage of particulate iron in the column effluent. The particulate iron, which took up to 2400 BV to clear up, did not contain significant arsenic but would have to be filtered during a post treatment step in a drinking water treatment system. Reuse of wustite after washing was not possible due to its poor performance compared to fresh media.
IMPACT:
Based on the experimental results, it is recommended that siderite not be considered further as a source of iron hydroxide for arsenic removal from drinking water. Electrolysis of siderite also proved to be ineffective as a means of arsenic removal and should not be studied further unless different conditions, justified from theory, are utilized. Also, wustite (FeO) did not prove to be a promising source of iron for packed bed sorption of arsenic.
RESEARCH PARTNER:
This study was jointly funded by the Water Research Foundation and the U.S. Department of Energy through the Arsenic Water Technology Partnership. The report will also be published by WERC (a Consortium for Environmental Education and Technology Development at New Mexico State University).
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