This paper provides an updated review on the subjects, the available alternative to produce biochar from biomass, quantification and characterization of biochar, the adsorptive capacity for the adsorption of contaminants, and the effect of biochar addition to agricultural soils on contaminant bioavailability. The property of biochar produced is much dependent upon the composition and type of biomass and the conditions at which biomass is carbonized. The physical and chemical characterizations are necessary to identify the basic structure and property of biochar and to predict its potential in various environmental application. Biochar is a promising alternative to remedy the soils contaminated with heavy metals and organic compounds through adsorption and immobilization due to its large surface area, charged surface, and functional groups. Overall, the bioavailability of heavy metals and organic compounds decreases when biochar is amended into soils.

The contents and release of the nutrient elements N, P and K in biochars have been investigated. Sulfur is an indispensable element for plants, but its content and release in biochar are still unclear. The effect of pyrolysis temperature (300, 500 and 700 °C) on the sulfur content, extractable fraction and release of sulfate in corn straw biochars (CS300, CS500 and CS700) was investigated. The biochars were characterized using element analysis, BET, FTIR, and XRD. It was shown that the contents of sulfur in biochars decreased significantly with increasing pyrolysis temperature. The extraction results indicated that the percentages of water extractable-sulfate (W–SO₄ ²⁻) and organosulfur in biochars decreased while those of HCl- and NaH₂PO₄-extractable sulfate (HCl–SO₄ ²⁻, NaH₂PO₄–SO₄ ²⁻) increased with pyrolysis temperature. Batch release experiments were conducted to test the effect of contact time and addition of Hoagland nutrient solution (HNS) on the release of sulfate from biochars. The release kinetics fitted well with a pseudo-second-order model. Approximately 10.7 mg g⁻¹ of sulfate was released from CS300 during the initial 2 h, whereas 6.32 and 3.93 mg g⁻¹ were released from CS500 and CS700, respectively. Increasing the amounts of HNS led to negative effects on sulfate release. The results indicate that low-temperatures might be optimal for producing biochar from corn straw to improve the sulfur fertilization.