Chromium(III) Binding to Purified Fulvic and Humic Acids and Their Solid-Solution Partitioning and Speciation in Soil

Abstract

Chromium (Cr) is a metal of high environmental concern, which exists as Cr(III) and Cr(VI) in the environment, with Cr(VI) considered the most toxic and mobile. Humic substances may reduce Cr(VI) to Cr(III) while simultaneously binding Cr(III), reducing its bioavailability and protecting it against (re)oxidation. Nevertheless, binding of Cr(III) to humic substances is insufficiently quantified, as binding constants in ion binding models are based on limited data and consequently uncertain. In this paper, Cr binding experiments to humic and fulvic acids were conducted in conjunction with the Donnan membrane technique to determine the free Cr³⁺ concentration. New NICA-Donnan binding parameters were derived using ORCHESTRA-PEST, exploring scenarios with solely Cr³⁺ binding and additionally considering Cr binding in the form of CrOH²⁺ or Cr₂(OH)₂⁴⁺. Parameters for the scenario of combined Cr³⁺ and Cr₂(OH)₂⁴⁺ binding were deemed most realistic, yet obtained binding affinities were unexpectedly low, potentially related to the previously suggested overestimated electrostatic contribution, particularly for trivalent ions in the NICA-Donnan model. Validating the new parameters showed a significant improvement compared to the generic NICA-Donnan parameters in predicting Cr-HA binding in a synthetic system and in predicting free Cr³⁺ concentrations in soil solution, the most relevant species concerning toxicity and (bio)availability. The new parameters underestimated binding to organic matter, however, resulting in inaccurate predictions of solid-solution partitioning in organic-poor deeper soil layers. The new parameters ultimately better reflect the spectroscopically identified mechanisms of Cr binding to humic substances, yet the found limitations highlight the incorrect description of electrostatic binding by the Donnan model.