Antimony (Sb) is a metalloid of growing environmental concern due to the increased use in various products globally (e.g., flame retardants, alloys, semiconductors, plastics, batteries). Antimony compounds are released into the environment through mining, smelting activities, combustion of fossil fuels and spent ammunition. Mobilization of Sb due to mining activities is often associated with highly acidic condition. Environmental fate of Sb in water depends on different factors such as pH, redox potentials and other environmental conditions. Two predominant forms of Sb present in water as oxyanions of which tri-valent Sb (Sb(III)) tends to bind more strongly in neutral and alkaline soils than pentavalent Sb (Sb(V)). However, the sorption of Sb(V) from aqueous solution can be significant at low pH. Biochar, a solid by-product of bio-refineries, is an emerging low-cost sorbent for environmental contaminants. In this study, three biochar products were investigated for their efficacy to sequester Sb(V), including biosolid (BS), cow manure (CM) and horse manure (HM) biochar. Slow pyrolysis (300 °C, 30 min) was employed at a 7 °C min^–1 heating rate for biochar production. Among the three biochars, the biosolid biochar was modified with zirconyl chloride solution (biochar: solution = 1:10) as it was hypothesized that the modification will enhance the sorption capacity.

The sorption behavior of Sb(V) on BS, CM and HM biochar was investigated using batch sorption techniques (sorbent dosage 2.5 g/L, temperature 25 °C). The sorption isotherms and pH edges indicated that the affinity of Sb(V) toward the BS biochar was strongly dependent on solution pH and biochar characteristics. Sorption of Sb(V) was favored at pH 1.0–2.0 as a consequence of the high affinity between the positively charged biochar surface and the predominant Sb species (Sb(OH)₆^– and Sb(OH)₅⁰). Increasing pH (4.0–8.0) resulted in a dramatic decrease in the sorption (0–5% sorption of Sb(V)) efficiency. The Langmuir isotherm model described experimental data well for Sb(V) (R² = 0.97). The maximum sorption capacity (Q_m) of BS, CM and HM biochars at pH 2.0 were 10.15, 4.06 and 4.94 mg/g, respectively.

Zirconium (IV) modified biosolid biochar (ZMBS) enhanced Sb(V) sorption by 3 fold greater than the unmodified biochar. In this investigation, ZMBS performed well at pH 3.0–5.0 (maximum Sb removal efficiency found at pH 4.0). ZMBS was not effective below pH 3.0. Thus, the unmodified BS biochar shows that it has a great potential in removing Sb(V) from acidic aqueous environments such as groundwater and acid mine drainage, whilst ZMBS would be more suitable for less acidic waters, the more commonly encountered environment. Further research on the chemical modification of BS biochar is required to enhance the Sb(V) removal efficiency.