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

Mn (manganese) exists in various oxidation states in soil, and Mn²⁺ is the most mobile species of Mn, which is toxic to plants and restricts their growth. When soil is contaminated with trivalent chromium (Cr³⁺), Mn oxides in the soil are reduced to Mn²⁺ by oxidizing Cr³⁺ while oxidized Cr is subsequently reduced back to Cr³⁺ by organic matter in soil, leaving Mn²⁺ and Cr³⁺ in the soil. Therefore, the objective of this study was to immobilize Mn²⁺ without altering the Cr species in the soil and to evaluate the effectiveness of biochar treatment in immobilizing both Mn²⁺ 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 Cr³⁺. In Mn²⁺ and Cr³⁺ 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 Cr³⁺ 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 Cr³⁺ oxidation in Cr³⁺ contaminated soils.