Mechanochemical remediation of soil contaminated with heavy metals and persistent organic pollutants by ball milling with nZVI-CaO-Ca3(PO4)2 additives

Mechanochemical method was proposed for the remediation of soil contaminated with heavy metals (HMs, Cu, Pb, and Cd) and persistent organic pollutants (POPs, PBDEs and PCBs) through ball milling with nanoscale Zero-valent Iron (nZVI)-CaO-Ca3(PO4)2 (NCC). The mechanical energy supplied by ball milling and NCC considerably enhanced the stabilization of HMs (Cu 99.22%, Pb 100.00%, and Cd 98.42%) as well as the efficiency of POP degradation (BDE209 99.57%, PCB3 94.55%, PCB4 94.07%, PCB5 86.96%). Upon an increase in the rotation speed from 0 to 650 rpm, the weak acid extractable fraction (F1) of Cu, Pb, and Cd gradually transformed into relatively stable (F2 + F3) and residual forms (F4) and gradually reached a steady state. Moreover, under the influence of SiO2 and excited-state PO43- in the soil, HMs also transformed from their originally mobile form to other stable forms. The C-O and C-Br bonds of BDE209 were cleaved during mechano-chemical reactions, yielding perbromobenzene, 2,3,4,5,6-pentabromophenol, and other low-brominated phenols such as 1,2,3,4,5-pentabromo-6-(2,3-dibromophenoxy)benzene. The C-Cl bonds of PCBs underwent selective cleavage, resulting in the formation of low-chlorinated biphenyls and hydrocarbons. Furthermore, the stability of C-Cl bonds increased with a higher number of chlorine atoms in PCBs, making them more difficult to degrade. This study offers an environmentally friendly and highly effective remediation approach for POPs-HMs contaminated soil.