Factors influencing the sorption of polycyclic aromatic hydrocarbons, cadmium and mercury to biochars – a surface response methodology approach

Biochar is a versatile soil additive and CO₂ neutral or negative “green” sorbent. Biochar can improve the soil quality, water retention capacity, and is effective in binding contaminants. Previous studies showed that biochar is a suitable remediation option at sites with residual contamination and that it is particularly favorable in immobilizing organic polycyclic aromatic hydrocarbons (PAHs) and cadmium. To achieve remediation goals, however, careful assessment of the soil geochemistry (pH, background ions, organic carbon content), the nature of the contaminant, and the application rate need to be carried out. We have screened the sorption affinity of 11 different types of biochars and 2 activated carbons for selected heavy metals and ionizable and non-ionizable PAHs. The biochars differed in the starting feedstock material (rice husk, wheat straw pellets, mixed softwood pellets, oil seed rape straw, miscanthus straw pellets), temperature of production (550°C, 700°C), and elemental composition. The target contaminants included acenaphthene, dibenzofuran, carbazole, dibenzothiophene, 2-hydroxybiphenyl, cadmium, and mercury. The sorption of the organic contaminants was driven by the carbon content, specific surface area, and aromatic nature of the biochars. Sorption of cadmium was higher onto biochars with greater inorganic fractions, whereas for mercury no specific trends could be observed. Based on the results of the initial screening, a novel approach to statistically design experiments using the Box-Behnken model was employed for selected biochars. Using this method, experiments were conducted to systematically investigate the influence of four factors (pH, dissolved organic carbon, ionic strength and contaminant concentration) on sorption at three different levels (-1, 0, +1). The results from the surface response modeling approach provides fundamental new insight into the applicability of these biochars at contaminated sites and can help identify scenarios favorable for remediation with biochar. Our results will help in the development of a remediation strategy with an overall low environmental footprint for contaminated soils.