Co-Thermal Conversion, Atmosphere, and Blend Type Controls Over Heavy Metals in Biochars and Bottom Slags of Textile Dyeing Sludge and Durian Shell

Abstract

The co-thermal conversions (co-pyrolysis versus co-combustion) of heavy metal (HM)-rich wastes with second-generation residues may be turned into a feasible and green technology to valorize energy and products. This study aimed to quantify the migrations and distributions of HMs in biochars and bottom slags and their environmental risks in response to the (co-)pyrolysis and (co-)combustion of durian shell (DS) and textile dyeing sludge (CTDS and PDTS, respectively), atmosphere type, blend ratio, and temperature. The Cd, Cr, Mn, and Ni contents of the bottom slags of the (co-)combustions of CTDS and 50% DS and 50% TDS (CDT11) in the 80% N2/20% O2 atmosphere (80N220O2) rose with the increased temperature between 800 and 1000 °C. The co-combustion interaction raised the residual HM contents of the bottom slag. The potential environmental risks of HMs in the (co-)pyrolytic biochars were lower in the CO2 atmosphere. Cr, Zn, and Cu in the CTDS bottom slag at 900 °C in 80N220O2 had the highest leaching toxicity concentrations which first rose and then fell with the increased temperature. At 800 °C regardless of the atmosphere type, the leaching toxicity concentrations of HMs were Cr > Cu > Zn > Mn in TDS and Cr > Zn > Cu in CDT11. According to the thermodynamic simulations, the O2 concentration and atmosphere type did not significantly affect the HM morphology and transformation. K in DS increased the temperature of converting solid-phase Ni to slag-phase NiO and affected the transformation temperature and strength of Ni, Zn, and Pb in CTDS and CDT11 at the high temperature. The combined results of all the three optimizations of HM contents, forms, and risks pointed to 50% DS and 50% PTDS in the N2 atmosphere and 50% DS and 50% CTDS in 50% N2 and 80% CO2 atmospheres as the optimal co-pyrolysis/combustion settings, respectively.

Keywords: Textile dyeing sludge, Heavy metals, interaction effects, thermochemical conversion dynamics