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THE EFFECT OF PH, PHOSPHATE AND OXIDANT ON THE REMOVAL OF ARSENIC FROM DRINKING WATER DURING IRON REMOVAL
Citation:
Frietch*, C M. AND D A. Lytle*. THE EFFECT OF PH, PHOSPHATE AND OXIDANT ON THE REMOVAL OF ARSENIC FROM DRINKING WATER DURING IRON REMOVAL. Presented at WSWRD Peer Review, Cincinnati, OH, September 23 - 27, 2004.
Impact/Purpose:
To inform the public
Description:
Arsenic is a naturally occurring drinking water contaminant that has known adverse human health effects. The recent compilation of new health effects data prompted the U.S. Environmental Protection Agency (USEPA) to reduce the previous arsenic maximum contaminant level (MCL) of 0.05 mg/L to 0.01 mg/L under the new Arsenic Rule. As a result many water systems, but primarily small water systems, will be required to install some form of arsenic treatment. This study had two objectives: the first objective examined the effect of pH and phosphate on the adsorption of arsenic onto freshly precipitated Fe(III) (hydr)oxides. Secondly, the effect of oxidant type used to oxidize both iron and arsenic on the removal of arsenic from water was studied. Bench and field studies were used to meet the objectives. Arsenic treatment recommendations were made based on experimental observations.
Water chemistry greatly affects the amount of arsenic removed during the removal of natural iron from water. pH is the most influential parameter because it impacts both arsenic speciation, oxidation kinetics, and the properties of iron particles. One option to improve arsenic removal in high pH waters or difficult to treat waters is to increase the iron concentration. Anions such as silicate and phosphate compete with arsenic for adsorption sites on iron surfaces. The capacity for arsenic on the iron particles is reduced by approximately 50% when 3 mg PO4/L was introduced to 100 :g/L arsenic and 5 mg Fe/L water. Type of oxidant used and point of its application is an important factor to consider when applying iron removal processes to remove arsenic. As(V) is easier to remove, a system may be able to remove As(III) to below the new MCL without the need for a stronger oxidant especially if enough iron is available.
Iron removal processes are effective at removing co-occurring arsenic from water to below the new MCL of 10 :g/L. Arsenic removal efficiency is affected by water chemistry including pH and the presence of major anions. The type of oxidant used to oxidize iron and arsenic and the sequence of its addition can also affect the amount of arsenic removed during treatment.