Co-pyrolysis of corn stover with industrial coal ash for in situ efficient remediation of heavy metals in multi-polluted soil

Highlights: • A cost-efficient composite was prepared by a facile co-pyrolysis method. • The composite was superior than pristine biochar for heavy metal immobilization. • The composite promoted the paddy rice growth and biomass production. • The composite exhibited promising potential as alternative soil amendment. Coal ash incorporated biochar (CA/BC) composite was prepared by co-pyrolysis of agricultural residue and industrial coal ash and applied for remediation of soils polluted by lead (Pb) and cadmium (Cd). The results showed that immobilization efficiency of CA/BC for heavy metals (HMs) was significantly enhanced by 77.1 % (Pb) and 42.7 % (Cd) compared to pristine biochar (BC), and this was mainly due to the increased pH value, surface functionality and surface negative charge. By the introduction of 5 % CA/BC, the polluted soils showed the highest reduction of leaching toxicity by 67.9 % (Pb) and 49.7 % (Cd), respectively. The chemical speciation of Pb and Cd in soils was changed remarkably and the reduced bioavailable Pb and Cd were mainly transformed from acid-soluble fraction into the most stable form of residual fraction. The mechanism study showed that surface precipitation, complexation, cation exchange and cation-π interaction of CA/BC mainly contributed to heavy metals (HMs) immobilization. The pot experiments further confirmed that incorporation of 5 % CA/BC effectively reduced plant Pb and Cd accumulation by 81 % and 62.5 % respectively, and significantly promoted the plant growth of paddy rice by 3.1, 2.2 and 2.0 times in terms of root, stem length and dry mass parameters. The present study offered a cost-effective and green method to prepare soil amendment with great potential for remediation of soils polluted by HMs and realized the value-added utilization of waste agricultural residue and industrial coal ash.