Techno-Economic Evaluation of Iron and Aluminum Coagulants on Se(IV) Removal

Research on selenium pollution in natural waters is continuous and discouraging. In this study, coagulation/precipitation was applied with the use of Fe(II), Fe(III), and poly-aluminum chloride (PACl) salts for Se(IV) removal at concentration range 10-100 mu g Se(IV)/L that is commonly found in drinking waters. Prehydrolyzed Fe(III)-FeCl3 delivered the best uptake capacity (Q(10) = 8.9 mg Se(IV)/g Fe(III) at pH 6) at the residual concentration equal to the drinking water regulation limit of 10 mu g/L. This was much higher than the efficiencies achieved when applying the other coagulants: i.e., Q(10) = 7.3 mg Se(IV)/g Fe3+-FeClSO4, Q(10) = 6.4 mg Se(IV)/g prehydrolyzed Fe(III)-Fe-2(SO4)(3) and 0.7 mg Se(IV)/g Al-PACl at pH 6, and Q(10) = 0.45 mg Se(IV)/g Fe(II) at pH 7.2. Comparing the different sources of Fe(III), it is apparent that Se(IV) uptake capacity is inhibited by the presence of SO42- in crystal structure of prehydrolyzed Fe-2(SO4)(3), while prehydrolyzed FeCl3 favors Se(IV) uptake. Temperature effect data showed that coagulation/precipitation is exothermic. In techno-economic terms, the optimal conditions for Se(IV) removal are coagulation/precipitation at pH values lower than 7 using prehydrolyzed Fe(III)-FeCl3, which provides a combination of minimum sludge production and lower operating cost.