Biochar Derived from Rice by-Products for Arsenic and Chromium Removal by Adsorption: A Review

Environmental pollution by arsenic (As) and hexavalent chromium (Cr(VI)) has been one of the most serious environmental problems in recent years around the world. Their presence in water is a result of both natural and anthropogenic activities, and poses serious risks to human health due to their high toxicity. Adsorption is a leading method used to remove arsenic and chromium, with biochar, a carbonaceous pyrolytic product made from various types of biomass, under low oxygen conditions, being one of the most common adsorbents due to its high surface area. Although biochar's ability to immobilize and remove As and Cr(VI) is high, in order to increase the adsorption capacity and nutrient release potential of rice husk biochar, it is essential to select an appropriate pyrolysis and biochar modification technique. Physical or biological activation, steam/gas activation, UV irradiation, magnetization, alkali/acid treatment, and nano-modification are the main modification methods that will be discussed in this review. These modifications have led to multi-fold enhancement in adsorption/reduction capacity of As and Cr(VI), compared with plain biochar. This review provides a recent literature overview of the different biochar modification methods, as well as the factors that influence their capacity to successfully remove As and Cr(VI), along with regeneration potentials.

Automated summary

Environmental pollution from arsenic (As) and hexavalent chromium (Cr(VI)) has become a major global concern, caused by both natural and human activities. Adsorption using biochar, a carbon product derived from biomass under low oxygen conditions, has emerged as a common method to remove As and Cr(VI) due to its high surface area. Despite biochar's high immobilization and removal capacity, selecting an appropriate pyrolysis and biochar modification technique is vital to enhance its adsorption capacity and nutrient release potential. This review discusses various biochar modification methods such as physical or biological activation, steam/gas activation, UV irradiation, magnetization, alkali/acid treatment, and nano-modification, which have shown significant improvement in As and Cr(VI) adsorption/reduction capacity compared to plain biochar. It provides an overview of recent literature on biochar modification methods, factors influencing their removal capacity, and regeneration potentials.