A nanofiltration–coagulation integrated system for separation and stabilization of arsenic from groundwater

Abstract

A membrane-integrated hybrid treatment system has been developed for continuous removal of arsenic from contaminated groundwater with simultaneous stabilization of arsenic rejects for safe disposal. Both trivalent and pentavalent arsenic could be removed by cross flow nanofiltration following a chemical pre-oxidation step for conversion of trivalent arsenic into pentavalent form. The very choice of the membrane module and its judicious integration with upstream oxidation and downstream stabilization resulted in continuous removal of more than 98% arsenic from water that contained around 190mgL−1 of total suspended solid, 205mgL−1 of total dissolved solid, 0.18mgL−1 of arsenic and 4.8mgL−1 of iron at a pH of 7.2. The used flat sheet cross flow membrane module yielded a high flux of 144–145Lm−2h−1 at a transmembrane pressure of only 16kgf·cm−2 without the need for frequent replacement of the membranes. Transmembrane pressure, cross flow rate through the membrane module and oxidant dose were found to have pronounced effects on arsenic rejection and pure water flux. For the first time, an effective scheme for protection of the total environment has been ensured in this context where arsenic separated with high degree of efficiency has been stabilized in a solid matrix of iron and calcium under response surface optimized conditions. The study culminated in a total and sustainable solution to the problem of arsenic contamination of groundwater by offering arsenic-free water at a reasonably low price of only1.41$·m−3.