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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 2003 AWWA WQTC, Philadelphia, PA, November 02 - 06, 2003.
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 recently reduce the previous arsenic maximum contaminant level (MCL) of 0.05 mg/L to 0.01 mg/L under the new Arsenic Rule (Register 2003). As a result many water systems, particularly small water systems, will be required to install some form of arsenic treatment.
Arsenic removal from drinking water can be achieved in many ways including: conventional coagulation with aluminum or iron salts, anion exchange, membrane technologies and adsorption onto activated carbon. Iron-based treatment processes (iron media, iron removal, and iron coagulation) are especially effective arsenic treatment strategies due to the strong tendency for iron surfaces to adsorb arsenic. Arsenic removal during iron removal processes will most likely be achieved by oxidization followed by filtration. Iron (Fe(II)) found naturally in water is oxidized by oxygen or other oxidant to the relatively insoluble Fe(III) state, which is then removed during filtration. Co-occurring arsenic adsorbs to the Fe(III) (hydr)oxide particles and is filtered from the water. Water chemistry and the manner in which the oxidation step(s) is practiced can influence arsenic removal. For example, pH affects the degree of arsenic adsorption by controlling arsenic and iron chemical speciation, and the surface charge of iron particles. Anions such as phosphate, silicate and sulfate compete for adsorption sites. Although these effects are known, the degree of interference anticipated over a range of pHs and anion concentrations found in waters in the United States is difficult to predict. The type of oxidant(s) used to oxidize arsenic and iron, and in some cases the addition sequence of multiple oxidant systems can also affect arsenic removal effectiveness although these relationships are less known. Important iron particle properties such as surface area can be affected by the type of oxidant used, which can affect arsenic removal.