The adsorption and reduction mechanism of Cr(VI) by kerogen with different degrees of geochemical alteration using a thermal simulation method

Abstract

Any analysis that fails to consider the existence of kerogen materials in soils and sediments within the subsurface environment may result in serious deviations in quantifying and characterizing the fate of Cr(VI), especially considering that kerogen may be of different degrees of geochemical alteration, and the effect is still unclear. An anaerobic thermal simulation technique was utilized to simulate the diagenesis (T ≤ 350 °C), catagenesis (350 °C < T ≤ 500 °C) and metagenesis (500 °C < T ≤ 700 °C) geochemical processes of kerogen using a typical lignite. The molecular characteristics transition of kerogen from aliphatic to aromatic and aromatic to aliphatic were recorded at 300 and 600 °C respectively with temperature increasing. The adsorption capacity increased as a function of temperature, and Cr(VI) adsorption mechanism by kerogen was controlled by the carboxylic group driven by different forces at diagenesis, catagenesis, and metagenesis stages. Cr(VI) reduction capacities decreased with temperature increasing due to the decreasing of phenolic moieties and hydroxyl groups, which were the main electron donors for Cr(VI) reduction. The complexation mechanism of Cr(III), which is mainly related to the process of Cr(VI) reduction by phenol, was consequently complexed by the oxidized phenol sites, rather than to be released into aqueous phase and then be complexed by carboxylic groups. This study highlighted that within the subsurface environment, the highly physicochemically heterogeneous kerogen have varied functionalities, aliphaticity and aromaticity, which could play various roles in Cr(VI) adsorption, reduction and complexation, and hence should not be considered as a single organic phase.