Phosphorus-based remediation is a widely used strategy for cleaning up lead-contaminated soils. The treatment converts toxic, mobile lead into chloropyromorphite (CPY)—a highly stable mineral with extremely low solubility. However, recent studies have detected CPY as colloidal particles (1 nm–1 μm) in environmental matrices. These tiny particles can remain suspended in water and potentially migrate beyond the original contamination site, raising concerns about the long-term effectiveness of this remediation approach.
A new study published in the journal Environmental Surfaces and Interfaces now reveals that the mobility of CPY is critically governed by its interactions with two ubiquitous natural soil minerals—montmorillonite, a negatively charged clay, and goethite, a positively charged iron oxide.
The researchers from Nanjing Normal University investigated how these minerals affect CPY aggregation under varying pH, salt concentration, and cation types. They found that montmorillonite significantly inhibited CPY aggregation by enhancing electrostatic repulsion—increasing the negative surface charge and raising the energy barrier that prevents particles from sticking together.
In contrast, goethite promoted rapid aggregation through charge neutralisation, even reversing CPY’s surface charge from negative to positive, leading to fast settling.
“Environmental fate of chloropyromorphite is not simply a matter of its low solubility, but is strongly influenced by the surrounding mineral matrix and water chemistry,” says Professor Wei Wei, corresponding author of the study. “In goethite-rich or hard-water environments, CPY tends to aggregate and settle, which may help keep it immobilised at the source. But in montmorillonite-dominated or acidic, low-salt conditions, CPY can remain highly mobile and may pose a risk of off-site migration.”
The researchers also found that divalent calcium ions—common in natural waters—strongly promoted aggregation through charge screening and cation bridging, accelerating CPY settling. “Using DLVO theory calculations, we confirmed that montmorillonite increases interaction energy barriers while goethite effectively eliminates them, explaining the contrasting aggregation behaviours,” Wei shares.
The findings underscore the need for risk assessments that account for soil type, water hardness, and pH when evaluating the durability of lead immobilisation strategies.
“When phosphorus-based materials are applied to immobilise lead in contaminated soils, the long-term environmental risk depends not only on the chemistry of the treatment itself, but also on the site-specific mineralogical composition.
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Contact author details: Wei Wei, Jiangsu Engineering Lab of Water and Soil Eco‑remediation, School of Environment, Nanjing Normal University, Nanjing 210023, China. Email: wwei@njnu.edu.cn
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Environmental Surfaces and Interfaces
Experimental study
Not applicable
Surface charge‑mediated heteroaggregation of chloropyromorphite with natural inorganic colloids: Mechanisms and remediation implications
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.