Contrasting abiotic As(III) immobilization by undissolved and dissolved fractions of biochar in Ca²⁺-rich groundwater under anoxic conditions

Abstract

Engineered black carbon (biochar) can be introduced into groundwater through its extensive engineered applications (e.g., in-situ remediation of groundwater/soils), which can participate in geochemical processes that may alter the fate of trace contaminants such as arsenic (As(III)). Here we examined the impacts of the undissolved and dissolved fractions of reduced biochar (hereafter denoted as rUBC and rDBC, respectively) on the As(III) immobilization in the absence/presence of Ca²⁺ (50 mM) at pH 11.5 under anoxic conditions. While neither rUBC nor rDBC alone was capable of immobilizing As(III), the apparent As(III) immobilization by rUBC and rDBC synergistically occurred in the presence of Ca²⁺, with an efficiency of 73.1% and 89.6% within 24 h, respectively. In the rUBC/Ca²⁺/As(III) system, rUBC enabled full oxidation of As(III) to As(V) by its residual redox-active moieties such as quinoid C[dbnd]O and persistent free radicals, thereby facilitating precipitation of the newly generated As(V) with Ca²⁺ adsorbed onto the rUBC's surface. In contrast, rDBC induced in-situ local enrichment of Ca²⁺ in the nascent rDBC-derived flocs with predominant non-oxidative and slight oxidative precipitation of As(III) via ternary rDBC-Ca-As complexation. This ternary complex was created by Ca²⁺-bridging interactions between As species and oxygen-containing functional groups of rDBC, as evidenced by the FTIR results and the Ca²⁺-impeded As(III) oxidation. The generation of the flocs physically trapped a small amount of As species particularly As(III). Both the increases in Ca²⁺ concentration (0–100 mM) and solution pH (10.0–12.5) enhanced the apparent As(III) immobilization. This study provides new insights into the environmental impacts of two reduced biochar fractions released into typical Ca²⁺-rich aquifers on the fate and transport of As species.