Solution for crop burning? Rice husk biochar found to efficiently remove groundwater fluoride

India has an agro-based economy and generates substantial amounts of agricultural residues (500-690 tonnes annually) from different crops. The country produces 20 per cent of the world’s rice, making it the second-largest producer after China. Thus, 20-22 per cent of rice husks are a byproduct of rice milling. 

Agricultural crop residue management is a significant concern in India as it results in mindless stubble burning. Due to a lack of eco-friendly management, safe disposal or mismanagement, crop residues are set on fire openly. This causes massive air pollution in the Indo-Gangetic Plains. 

Besides, in-situ burning of crop residues has significantly impacted soil biodiversity and releases particulate matter & greenhouse gases (GHG), oxides of nitrogen & sulphur as well as volatile compounds. 

Compared to biomass disposal and stubble burning, sustainable crop residue management was adopted to reduce air pollution and GHG emissions by converting surplus biomass into value-added products like biochar. 

Biochar is a carbon-rich renewable material obtained by pyrolysing biomass at certain temperatures under zero or limited oxygen supply. Pyloriasation is the thermal decomposition of materials at elevated temperatures in an inert atmosphere.

Rice husk biomass and biochar with their magnified image. Author provided

Numerous studies have reported that biochar has been implemented in the remediation of organic and inorganic pollutants at the lab scale, and few have demonstrated in-situ applications in treating polluted groundwater. 

Biochar can also be used as a soil conditioner by improving soil fertility, enhancing nutrient retention and increasing soil organic matter. It can also reduce soil acidity and improve water retention, which can be particularly beneficial in areas of the Indo-Gangetic plains that face drought or water scarcity.

Raw / modified biochar synthesised from rice husk showed significant adsorption of fluoride ions from contaminated surface and groundwater, especially with the help of modified biochar. thermal decomposition of materials at elevated temperatures in an inert atmosphere. This was reported in the recently published work on fluoride remediation using porous rice husk biochar in the Journal of Environmental Management.

Modified biochar showed fluoride adsorption at neutral pH and reduced concentrations less than the WHO permissible limit for drinking purposes. Biochar can also be applied in varying saline environments, like coastal areas, to reduce fluoride concentrations from groundwater, depending on pH and salt strengths. Biochar-based adsorbents showed more potential for defluoridation than chemical-based sorbents. 

The authors of the new report conducted batch sorption experiments in Rajgir. 

An impediment of piloting biochar production in field-scale applications is the limited research done on biochar’s potential to be reused, so that the method becomes economical. 

Fluoride levels in groundwater at the study site in Rajgir, Bihar were found to be above the permissible limit (<1.5 milligram / litre) for drinking purposes. The fluoride removal efficiency (98 per cent for 10mg/l fluoride at pH7) of activated magnetic biochar was significantly higher than raw rice husk biochar. 

Salt strength up to 50 mM represents salinity equivalent to the freshwater in alluvial plains. The cost of fluoride adsorption using biosorbents provides insights into the economic viability of the adsorption technique, as cost-benefit analysis is a decision-making tool for choosing a treatment method for removing pollutants. 

Synthesis and treatment costs for contaminated surface and groundwater were revealed as an affordable treatment approach. Synthesis costs of one gram of chemically modified activated magnetic biochar and activated biochar are $0.0111 and $0.0092 respectively, higher than raw biochar ($0.0063) for biochar derived from rice husk.  

Broadly, biochar is implemented in greenhouse gas mitigation, heavy metals immobilisation and removal of other toxic pollutants from contaminated soil and water. Biochar pyrolysed from crop residue helps in sustainable crop residue management and climate change mitigation, achieving circular economy and sustainable development goals. In this study, raw and modified rice husk biochars were synthesised, characterised and performed for fluoride adsorption from contaminated and spiked natural surface and groundwater and recommended future studies. 

This study established that biochar is a potential adsorbent for solving real-world problems by treating surface water and groundwater. The research also recommended replacing biochar with zero-valent ion barriers in water filtration systems, sand-filtration systems at commercial scale and column studies to mimic remediation from polluted soils and water ecosystems. 

Several challenges need to be addressed to make the use of biochar feasible and effective. These include ensuring the sustainable and ethical sourcing of biomass for biochar production, understanding how biochar interacts with different soil types, and figuring out how to distribute and apply biochar to benefit local communities effectively.

Engagement with local communities, governments and organisations will be crucial in deploying biochar technology in agriculture production and water purification. The benefits of biochar could be maximised through policies that support sustainable agriculture, protect water resources and promote renewable energy.

Education and training programmes could also help farmers and other community members understand how to use biochar effectively.

Rakesh Kumar is with the department of biosystems engineering, Auburn University, Auburn, United States. Prabhakar Sharma works with Lok Prerna, Deoghar, Jharkhand.

Views expressed are the author’s own and don’t necessarily reflect those of Down To Earth.