Biochar aging in contaminated soil promotes Zn immobilization due to changes in biochar surface structural and chemical properties.

Adding biochar to Zn-contaminated soil can immobilize excess Zn and promote plant biomass growth. This was seen previously over the course of a 180-day planted pot trial involving two types of biochar (cattle manure, CM, and grain husk, GH) in a Zn-contaminated soil. Both biochars alleviated Zn-induced phytotoxicity to Ficus by immobilizing Zn and reducing its uptake by the plant, but to different extents. The aim of the current study was to delve into the in-soil mechanisms involved in biochar-mediated Zn immobilization. Biochar particles were excavated from the pot soils. Fresh and aged biochar particles were examined by high-resolution scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), field-emission electron probe micro-analyzer (EPMA), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). The physical and chemical properties of the biochars had changed over the 180 days. SEM-EDS and EPMA indicated that organo-mineral micro-agglomerates had formed on biochar surfaces and in pores. Some of the Zn immobilized by the biochars was bound in the organo-mineral complexes of these agglomerates. XPS and FTIR showed that the complexes had a high concentration of oxygenated functional groups which facilitated Zn binding and encapsulation. The micro-agglomerates were similar in structure and composition to those observed on biochars having resided for much longer times in soils, or having been subjected to accelerated aging. Overall, Zn immobilization by the CM biochar was greater than by the GH biochar, due to its higher alkalinity, higher concentration of available negatively charged groups, and greater accretion of organo-mineral layers. These findings are suggestive that biochar-assisted phytorestoration of heavy metal-contaminated soils can be optimized through selection of biochar having such traits. It is hypothesized that metals may be continually taken up in such micro-agglomerates, since they continue to form over the lifetime of the biochar in the soil.