Behavior Of As(â…¢) And As(â…¤) Removal In FeCl₃ Coagulation Process

Arsenate and arsenite are common arsenic species in naturally contaminated groundwater and surface water in many countries. Also, arsenic has attracted greater regulatory attention in industrial wastewater discharge. Even small amounts of arsenic in drinking water have adverse effects on human health. Sources of arsenic are both natural and anthropogenic.Arsenite is 25⁶0 times more oxic than arsenate and has been reported to be more mobile in the environment. It has been found that the coagulation is much more effective for the removal of As (V) than As (III). In the case when only As (III) is present, oxidation to convert As (III) to As (V) is needed prior to coagulation. Arsenic removal from water achieved by coagulation process depends on pH and initial arsenic concentration in water. For As(V), sorption was greatest at low pH and decreased at higher pH. The pH-dependent behavior of As(III) was inverse to that of As(V), with As(III) sorption being minor at low pH and increasing at high pH. Recent surveys suggest that the actual pH where sorption of As(III) was equivalent to As(V) (the crossover pH) was dependent on the level of arsenic loading, and the results show that As(III) adsorption to oxides is not always less effective than As(V) adsorption. The main purpose of our study is to compare the behavior of As(V) and As(III) removal in FeCl3 coagulation process as functions of As/Fe ratios, pH and initial arsenic concentration.At As/Fe ratio as low as 0.12, the crossover pH was 8.4. Increasing As/Fe ratio from 0.12 to 0.50 progressively lowered the crossover pH. As(III) removal exceeded that of As(V) at pH as low as 7.4 at As/Fe ratio of 0.50.The presence of co-existing ions effected the crossover pH.For the case of As(III) removal, the coagulation isotherm can be divided into two stages at all pH levels examined in this study, The As(III) removed increased gradually with increasing equilibrium As(III) concentration and reached a intermediate flat region before a linear increase in As(III) removed was observed with further increase in equilibrium As(III) concentration. The amount of As(III) removed did not attain a maximum at all pH levels and at initial As concentrations in solution as high as 7.5 mg/L, which suggested that the mechanisms of As(III) coagulates with FeCl3 was not noly adsorption. At pH 5 or 6, the amount of As(V) removed increased significantly at equilibrium As(V) concentration below 0.50 mg/L and it slowly increased with increasing equilibrium As(V) concentration. However, much different phenomena were observed at pH 7 or 8. Comparing the coagulation isotherms of As(V) and As(III), it was revealed that the presence of As(V) affected the precipitation of iron and reduced the surface sites. Singly present calcium ions reduced As(III) removal throughout the pH range 6⁸, yet increased As(V) removal at pH 6⁸. The enhancement in arsenate removal might be ascribed to the decrease of negative surface charge on the precipitates caused by the adsorption of calcium.The influence of a set of anions (PO43-, humic acid (HA), SiO₃^2-, SO₄^2-) upon the process was studied in the pH range of 6 to 8. Results indicated that these anions may reduce arsenic removal by Fe(III) through the following routes: inhibit the precipitation of iron and compete for sites. But the degree of inhibition varied with respect to competitive capability or concentration of the respective anion.The presence of calcium promoted the precipitation of iron, attenuating the detrimental impacts of HA or phosphate on the arsenic removal at pH=8, but the degree of enhancement varied with respect to competitive capability of the respective anion, too.