Chromium(VI) bioreduction by cysteine over clay particle edges: Catalytic mechanisms and influencing factors

Abstract

Bioremediation is effective for scavenging Cr(VI), one of the eight most toxic chemicals according to the world health organization. This study presents a comprehensive understanding of Cr(VI) bioreduction by cysteine (Cys) over clay minerals that serve as supports, and the central roles of Cys during Cr(VI) bioremediation are verified and unraveled at the molecular level. Impacts of Cys coverage, active sites, and the reducing agent are further discussed. Cr(VI) bioreduction proceeds via competitive mechanisms: Oxidative dehydrogenation (ODH) driven by π-conjugation (C=S) development and SS dimerization (SDM) driven by SS bond formation. The optimal reaction paths for ODH at the Al(III) site, SDM at the Al(III) site, ODH at the Mg(II) site, and SDM at the Mg(II) site have the apparent energy barriers of 124.6, 115.9, 117.3, and 71.9 kJ/mol, respectively. Cr(IV) represents the major product except for SDM at Mg(II) site where Cr(II) dominates. Mechanisms and product distribution are affected by various factors: 1) Increase of Cys coverage triggers Cr(VI) reduction to occur sequentially via Stage 1 of ODH, Stage 2 of ODH, Stage 1 of SDM, and Stage 2 of SDM; 2) Cr(VI) reduction occurs preferentially at the Mg(II) versus Al(III) site, and the conformation transition of CrIV structure promotes reaction and appears for the most preferred paths of the AlIII rather than MgII site; 3) Mechanisms are decided by the Fe(II) content for Fe(II) while by π-conjugation development for alcohols and Cys (ODH): Ethanol is preferred kinetically, while high Cys coverage adopts SDM that lacks for ethanol. Results provide significant new insights for Cr(VI) bioremediation and accelerate catalyst designs for Cr(VI) reduction which conduce to Cr(VI) management and rational utilization of less toxic products.