Response surface methodology (RSM) was applied to evaluate the effect of the main operational variables, including initial pH, initial chromium ion concentration, bulk density of GAC and time on the removal of hexavalent chromium Cr(VI) from contaminated groundwater by permeable reactive barriers (PRB) with acid-modified granular activated carbon (GAC) as an adsorbent material. The removal rates of Cr(VI) under different values of these parameters were investigated and results indicated high adsorption capacity at low pH and low initial metal ion concentration of Cr(VI), but the bulk density of GAC slightly influenced the process efficiency. According to the ANOVA (analysis of variance) results, the model presents high R 2 values of 94.35% for Cr(VI) removal efficiency, which indicates that the accuracy of the polynomial models was good. Also, quadratic regression models with estimated coefficients were developed to describe the pollutant removals.

Hexavalent chromium Cr(VI) is of particular environmental concern due to its toxicity, mobility, and challenging removal from industrial wastewater. It is a strong oxidizing agent that is carcinogenic and mutagenic and diffuses quickly through soil and aquatic environments. Moreover, it does not form insoluble compounds in aqueous solutions; therefore, separation by precipitation is not feasible. While Cr(VI) oxyanions are very mobile and toxic in the environment, trivalent Cr(III) cations are the opposite and, like many metal cations, Cr(III) forms insoluble precipitates. Thus, reducing Cr(VI)-Cr(III) simplifies its removal from effluent and also reduces its toxicity and mobility. Permeable reactive barriers (PRBs) with zero-valent iron (ZVI) have been used to remediate contaminated groundwater with metals, but using ZVI in remediation of contaminated groundwater or wastewater is limited due to its lack of stability, easy aggregation, and difficulty in separation of iron from the treated solution. Thus, the technology used in the present study is developed to address these problems by placing a layer of bentonite after the PRB layer to remove iron from the treated water. The removal rates of Cr(VI) under different values of pH were investigated, and the results indicated the highest adsorption capacity at low pH.

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