Microscopic evidence for humic acid induced changes in lead immobilization by phosphate in a counterdiffusion system

Abatement of lead (Pb) contamination in soil via chemical immobilization can reduce potential risks but is influenced by soil organic matter. The aim of this study was to observe and understand the influence of organic matter on Pb immobilization by phosphate. For this purpose, humic acid (HA) was introduced into a counterdiffusion system to mimic ionic reactions of the mineralization processes between the pollutant (Pb) and amendment agent (phosphate) in soil system, and were characterized jointly by in situ optical microscopy and ex situ XRD, SEM, TEM, and LSCM. The results indicate that lead immobilization in the counterdiffusion system involves a time-dependent crystallization process and that supersaturation occurs at nearly central region of the reaction zone. Entrapped HA had influence on crystal growth and size, causing more fragmented crystal morphology with increasing HA content, which can be explained by HA wrapping of the nucleation products and subsequent inhibition of reactions and crystal growth, as indicated by TEM and LSCM images. Mineral conversion from secondary lead orthophosphates to pyromorphite implies the promotion of more stable minerals. This approach provides evidence for a more intuitive understanding of the effects of HA on the immobilization of lead by phosphates.