Coagulation-Flocculation Technologies for Arsenic Removal-A Review

This review article presents in brief an overview of various technologies available based on coagulationflocculation methods. Coagulation and flocculation are among the most employed and documented techniques for arsenic removal from water. Arsenic is a human carcinogen in drinking water, having harmful effects on both human health and environment, even at low concentration. A limit of 0.01ppm (10 μg/L) has been set by WHO as the safe limit in drinking water, above which it shows health hazards. In coagulation method the, positively charged ion of coagulants {e.g., aluminum sulphate (Al₂(SO₄)₃), ferric chloride (FeCl₃)} neutralize or reduce the negative charge of colloids, thereby causing coagulation of particles. On the other hand in Flocculation the addition of an anionic flocculants leads to charge neutralization between the formed larger particles which ultimately change in to big flocs. As a result the dissolved arsenic is transformed into an insoluble solid. The precipitation or co precipitation with certain metal hydroxides can be done to remove arsenic from drinking water through sedimentation or filtration. The pH of the medium plays an important role in deciding the suitability of a coagulant for the removal of Arsenic. Below pH 7.5, Al₂(SO₄)₃, and FeCl₃ are equally effective in removing arsenic from water Between the two inorganic arsenic species, most researchers suggested that arsenate is more efficiently removed compared to arsenite and that FeCl₃ is a better coagulant than Al₂(SO₄)₃ at pH higher than 7.5.There is no effective treatment for arsenic toxicity. Removal of arsenic from water is the one and only method of preventing its toxicity. A great deal of research over recent decades has been done to lower the concentration of arsenic in drinking water and still there is a need to develop low cost viable techniques. Existing major arsenic removal technologies include oxidation, adsorption, precipitation, coagulation and membrane separation. This paper presents the review of current status of research in the area of arsenic removal from contaminated water with coagulation and flocculation methods.