Abstract/Summary

The use of biochar for soil remediation has gained a lot of interest due to its ability to adsorb containments owing to its properties such as high surface area, oxygen containing functional groups, high aromaticity, high pH, negative charge, and resistance to degradation. Biochars are variable in their ability to adsorb contaminants. Feedstock type and pyrolysis temperature clearly influences biochar properties and the sorption of contaminants. However, the ability to predict the maximum sorption capacity of a biochar based on feedstock properties and pyrolysis temperature is lacking. This study explored the influence of feedstock type and pyrolysis temperature on biochar properties, and the mechanisms behind its ability to adsorb Pb in water and soil. After a narrative review of the literature, which included a case study of Ishiagu mining site in Nigeria, I carried out a quantitative meta-analysis study to better understand the influence of biochar feedstock type, pyrolysis temperature, and other parameters on Pb mobility, and Pb bioavailability in soils. The findings suggested that biochars produced from plant residues will reduce the mobility of Pb more than woody materials and biochars pyrolyzed at temperatures <450C performed better than higher pyrolysis temperature biochars. To examine experimentally how pyrolysis temperature and feedstock type affects biochar properties, eight feedstocks (hay, coco coir, pine bark, wheat straw, sunflower, buckwheat, clover, and radish) were used to make 80 biochars at 10 pyrolysis temperatures between 300°C and 750°C which were characterised to determine their pH, functional groups, and elemental composition. Pyrolysis temperature influenced biochar properties, but feedstock type separated the biochars into two groups comprising biochars made from crop residues (hay, coco coir, pine bark, and wheat straw) and biochars made from cover crops (sunflower, buckwheat, clover, and radish). Pb sorption isotherms were carried out using all 80 biochars produced to determine their maximum Langmuir sorption capacity. Biochars produced from cover crops removed almost 100% of the Pb from solution, even at the highest concentration of 5000 mg/l, and the subsequent isotherm did not fit to the Langmuir model. The maximum sorption capacity of biochars produced from crop residues increased with an increase in the pyrolysis temperature. A novel sigmodal model was fit to the data to explain the relationship between biochar pyrolysis temperature and maximum Langmiur sorption and used to derive the maximum possible sorption capacity obtainable from a feedstock pyrolyzed at optimum temperature. A positive correlation was found between the feedstock nitrogen content and the maximum possible sorption capacity obtainable from the feedstock. This observation emphasizes the role of heterocyclic N structures in the sorption of Pb to biochar and the significance of feedstock nitrogen content in the creation of biochars with high Pb sorption capacity. In general, I can conclude that the feedstock properties and pyrolysis temperature of biochars are vital factors that provide insights into the performance of biochars for Pb sorption in soil and water.