Research On Removal Of Arsenic From Raw Water By Zero-valent Iron

Naturally occurring arsenic in drinking water supplies may affect more than 100 million people worldwide, especially in Bangladesh, China and Mongolia, arsenic concentration that can lead to human acute and chronic arsenic poisoning was found in drinking water source.Arsenic pollution comes mainly from anthropogenic activities and natural processes, industry three wastes is the main source of anthropogenic pollution. In the water environment, arsenic mainly exists in the form of As(Ⅲ) and As(Ⅴ), but many studies have confirmed that the toxicity, solubility, and mobility of As (Ⅲ) is much higher than As (Ⅳ), and As (Ⅲ) normally exsits in molecular form, so all processes on As (Ⅲ) removal rate is much lower than As (Ⅴ). Zero-valent iron (Fe (0)) is easy to obtain, inexpensive and nontoxic, and As (Ⅲ) can be synchronously oxidized and absorbed by Fe (0),so Fe (0) is widely paid attention by scholars at home and abroad.This paper studied on adsorption properties, adsorption mechanism of Fe (0) onto As (Ⅲ),and factors influencing arsenic-removing efficiency from water by using Fe(0). The results show that greater than 90% of the As(Ⅲ) was removed by Fe(0) filings after 240min of mixing when the mass ratio of Fe (0) to As(Ⅲ) was1000:1, initial concentration of As (Ⅲ) was100μg/L, pH5.0. A Langmuir sorption isotherm was able to describe As(Ⅲ) sorption by Fe(0),with theoretical saturation adsorption capacity of 1.9790 mg/g.The process of Fe(0) for As (Ⅲ) oxidative adsorption followed the first order reaction kinetics model. The removal of arsenic by Fe(0) was attributed to adsorption by iron hydroxides generated from the oxic corrosion of Fe(0). There were surface functional groups≡Fe-OH on iron hydroxides surface. Direct surface complexation between Fe-OH and As(Ⅲ), As (Ⅴ) occurred,and consequently caused the specific chemical adsorption of As(Ⅲ) and As (Ⅴ). Arsenic removal was dramatically affected by the DO content and the pH of the solution.The presence of DO and low pH(pH5-7) increased the rate of iron corrosion and arsenic removal. High-concentration of NO3- increased the rate of iron corrosion and arsenic removal whereas high-concentration of H2PO4- decreased the rate of arsenic removal because of competitive adsorption. The presence of Ca2+,Mg2+ increased the rate of arsenic removal,and the promotion of Ca2+ to arsenic removal is more obvious than Mg2+The column results indicated that a Permeable Reactive Barrier consisting of an 600mm length of Fe(0) reaction layer and a 600mm length of sand filter could be used for remediation of water polluted by arsenic in a low pressure condition. The Permeable Reactive Barrier reduced the arsenic concentration to less than 50μg/L,which was below theⅠ～Ⅲtotal arsenic standard of GB3838-2002 Surface Water Environment Quality Standards and reached to irrigation water quality standards for crops cultivated under waterlogged and vegetables according to GB 5084-92 Irrigation Water Quality Standards. The sand filter reduced total iron concentration to less than 3mg/L,which accorded with the Standards for Drinking Water Quality.In order to make the Reactive Barrier have a practical application,and remediate water with low energy consumption, column experiments use gravity filtration methods without pressure-driven. Column experiments emphasis on quartz filter layer clogging and determine quartz filter layer cleaning period.This study can provide references on arsenic polluted water remediation in engineering practical application. Because the reactive barrier has a lot of advantages such us small investment, low cost, good effect, and simple maintenance, the reactive barrier is effective and feasible on arsenic polluted water remediation.