Co-pyrolysis of agricultural and industrial wastes changes the composition and stability of biochars and can improve their agricultural and environmental benefits

Abstract Co-pyrolysis is an alternative process for waste management and pollution elimination, producing a solid material (biochar) useful for energy generation and soil improvement. The objective of this study was to assess the changes in the physicochemical characteristics, mineral composition, and functional groups of biochars derived from the co-pyrolysis of agricultural and industrial wastes produced by different pyrolysis temperatures and feedstocks. Ten different types of biochars were produced by co-pyrolysis of two agricultural wastes (poultry litter - PL and swine manure - SM) and three industrial wastes (construction wood - CW, tire - TR and PVC plastic - PVC) combined in proportions of 1:1 (w:w) at five temperatures (300, 400, 500, 600 and 700 °C). With the increase of pyrolysis temperatures from 300 to 500 °C, there were drastic reductions in biochar yield (79-48%) and volatile matter (43-17%) and increases in ash content (21-33%), fixed carbon (30-45%), cation exchange capacity (CEC) (16-68 cmolc kg-1) and concentration of nutrients and heavy metals. Between 500 and 700 °C these changes were much less pronounced. The biochars prepared with mixtures from PL and SM had high ash contents (44%), relatively high CEC (37 cmolc kg-1), water holding capacity (WHC) (41%), and alkalinity (10.0) and can enhance the nutrient supply and CEC in soils. All biochars had low H:C (0.06) and O:C (0.30) molar ratios, suggesting a potential for carbon sequestration in soils. Our study shows that the interaction or synergy between agricultural and industrial materials in co-pyrolysis improved certain properties of biochars fundamental to soil quality, such as reduction of electrical conductivity (EC), increase in WHC, neutralizing power, and stability, and enabling the release or concentration of macro and micronutrients. Therefore, co-pyrolysis of agricultural and industrial wastes to produce biochars would have both agricultural and environmental benefits.