Immobilization of manganese in solution and soil contaminated with trivalent chromium using biochars

Mn (manganese) exists in various oxidation states in soil, and Mn 2+ is the most mobile species of Mn, which is toxic to plants and restricts their growth. When soil is contaminated with trivalent chromium (Cr 3+ ), Mn oxides in the soil are reduced to Mn 2+ by oxidizing Cr 3+ while oxidized Cr is subsequently reduced back to Cr 3+ by organic matter in soil, leaving Mn 2+ and Cr 3+ in the soil. Therefore, the objective of this study was to immobilize Mn 2+ without altering the Cr species in the soil and to evaluate the effectiveness of biochar treatment in immobilizing both Mn 2+ and Cr³⁺ in Cr³⁺-contaminated soil. Biochars derived from different sources including rice bran (RB), chicken manure (CM) and cow manure (WM) were tested for Mn adsorption and the chicken manure derived-biochar showed the highest removal efficiency (100%) for Mn in Mn solution. Moreover, 100% of both Mn²⁺ and Cr³⁺ were removed in Mn²⁺ and Cr³⁺ mixed solution without oxidizing Cr 3+ . In Mn 2+ and Cr 3+ mixed solution, initially 1.7% of Cr³⁺ was oxidized to Cr⁶⁺ by Mn, which was subsequently reduced back to Cr³⁺ by biochar, leading to its complete adsorption. In Cr 3+ spiked soils treated with 5% and 10% CM biochar, bioavailable Mn and Cr concentrations were significantly reduced. Therefore, biochar is a promising amendment for reducing the bioavailability of Mn and Cr limiting Cr 3+ oxidation in Cr 3+ contaminated soils.