The study, published in Frontiers in Environmental Chemistry, evaluated a poly(hydroxamic acid) (PHA) ligand synthesized from pine cone biomass through Fenton-assisted oxidation. This modification introduces abundant hydroxamic acid functional groups, each containing carbonyl and hydroxyl oxygen atoms capable of forming strong coordination complexes with metal ions. The research was conducted at the Biosorption and Water Research Laboratory of Vaal University of Technology in Vanderbijlpark, and it represents one of the first attempts to thoroughly explain lead selectivity on hydroxamate-functionalized biomass by simultaneously integrating Hard-Soft Acid-Base (HSAB) theory, competitive multi-metal adsorption, and validation in actual acid mine drainage systems.

Under optimal conditions, the PHA adsorbent achieved a maximum lead adsorption capacity of 86 milligrams per gram at pH 6, with equilibrium reached after 180 minutes. Kinetic analysis revealed that the pseudo-second-order model best described the process, with a coefficient of determination of exactly 1.000, indicating that chemisorption governed the rate-limiting step rather than diffusion-controlled physisorption. Equilibrium data exhibited excellent agreement with the Langmuir isotherm, suggesting predominantly monolayer adsorption onto energetically equivalent sites. However, the experimentally observed capacity exceeded the Langmuir-predicted maximum, pointing to strong coordination-driven lead-oxygen interactions beyond ideal monolayer assumptions.

Thermodynamic analysis confirmed that adsorption was endothermic and spontaneous, with positive enthalpy values and negative Gibbs free energy values across the temperature range studied. The adsorption capacity increased consistently as temperature rose from 299 to 314 kelvin, reflecting improved ion mobility and activation of coordination sites within the polymeric matrix. Higher temperatures also promoted stronger coordination interactions between lead ions and oxygen donor atoms, while decreased solution viscosity enhanced mass transfer between the solution and the adsorbent surface.

The mechanistic pathway proposed by the researchers proceeds through a sequential process involving interfacial hydration, electrostatic enrichment, ligand activation, inner-sphere chelation, and secondary coordination interactions. Upon contact with aqueous solution, the modified matrix hydrates, forming a structured interfacial water layer around hydroxyl, carboxylate, and hydroxamic acid groups that enhances ion diffusion toward reactive domains. Hydrated lead species are then attracted to negatively charged sites created by pH-mediated deprotonation of surface functional groups. As pH increases from 2 to 6, adsorption capacity rises from approximately 64 to 73 milligrams per gram, reflecting reduced proton competition and enhanced donor strength of the deprotonated hydroxamate anions.

During formal coordination, lead ions first form outer-sphere surface complexes before progressing to inner-sphere chelation. The hydroxamate functional groups coordinate metal ions through adjacent carbonyl and deprotonated hydroxyl oxygen atoms, creating stable five-membered chelate rings. Lead, with its high polarizability and borderline-soft Lewis acid character, is particularly suited to this coordination mode. Its stereochemically active 6s² lone pair produces hemidirected geometries and flexible coordination numbers between 4 and 6, enabling various binding modes on the PHA surface. At low surface coverage, mononuclear bidentate chelation dominates, while multidentate and bridging configurations develop as surface coverage rises, consistent with Langmuir-type monolayer adsorption on energetically equivalent locations.

The Hard-Soft Acid-Base theory provides a powerful framework for understanding why the PHA ligand selectively captures lead in the presence of competing metals. Lead is classified as a borderline acid with considerable attraction for oxygen donor ligands of intermediate polarizability, such as hydroxamate groups. Competing hard acids like manganese ions and hydrolyzed ferric species under circumneutral conditions compete less effectively. In the sulfate-rich acid mine drainage environment, lead speciation further favors ligand exchange processes that generate more stable inner-sphere hydroxamate complexes. Notably, the acid mine drainage matrix exhibited marginally higher adsorption capacity and removal efficiency than the single-metal system, suggesting that multivalent cations and elevated ionic strength enhance surface activation and electrostatic stabilization of reactive functional groups.

Competitive adsorption experiments in real acid mine drainage collected from a chrome mine in Witbank, Mpumalanga Province, South Africa, confirmed the material’s selectivity. The field samples exhibited pH values ranging from 2.8 to 4.6, electrical conductivity between 2.69 and 5.18 millisiemens per centimeter, and redox potentials from 572 to 728 millivolts. Among the metals tested, copper demonstrated the highest removal effectiveness, exceeding 73.5 milligrams per gram at elevated concentrations, while ferric ions and lead showed nearly identical efficiencies around 71.7 and 74.2 milligrams per gram respectively. Manganese exhibited the lowest removal, attributed to its larger ionic radius and reduced tendency to form strong inner-sphere complexes with oxygen donor ligands. At high metal loadings, removal efficiencies converged, indicating progressive site saturation characteristic of capacity-limited behavior in multi-metal systems.

Comparative evaluation against conventional and advanced adsorbents situates the PHA ligand within the upper range of modified mineral-based materials such as zeolites, and well above unmodified biochar at 14.7 milligrams per gram and activated bentonite at 61.77 milligrams per gram. While graphene oxide and activated carbon have reported higher capacities, these materials are typically evaluated under idealized single-metal conditions and rely predominantly on surface area-driven adsorption. In contrast, the PHA adsorbent maintained relatively high lead uptake under mild pH and ambient temperature conditions representative of real acid mine drainage systems, highlighting the role of selective, functional group-mediated interactions rather than nonspecific adsorption.

Desorption and regeneration studies confirmed the practical viability of the material. Mineral acid consistently produced the best regeneration performance across multiple cycles, followed by water, weak acid, and alkaline solution, confirming that adsorbed metals were mainly bound by reversible coordination rather than weak physical forces. The PHA ligand retained more than 80 percent of its initial removal efficiency after five adsorption-desorption cycles, demonstrating excellent structural stability and operational recyclability. The use of mineral acid as an efficient desorbing agent implies that lead can be recovered in concentrated form, offering possibilities for resource recovery and metal valorization. The researchers note that partial site saturation and potential structural alterations indicated by the gradual capacity decline across repeated cycles may restrict long-term reuse, and they recommend that spent adsorbent be stabilized before disposal to prevent metal leaching. Future work will investigate fixed-bed column performance under continuous flow conditions to assess scale-up potential for practical wastewater treatment applications.

Subject of Research: Environmental Chemistry

Article Title: Selective adsorption of Pb2+ from acid mine drainage through a poly (hydroxamic acid) (PHA) ligand: an HSAB-guided coordination mechanism

Article References: Mzinyane, N. N. (2026). Selective adsorption of Pb2+ from acid mine drainage through a poly (hydroxamic acid) (PHA) ligand: an HSAB-guided coordination mechanism. Frontiers in Environmental Chemistry, 7, Article 1835320. https://doi.org/10.3389/fenvc.2026.1835320

Image Credits: AI Generated

DOI: 10.3389/fenvc.2026.1835320

Keywords: acid mine drainage, lead removal, poly(hydroxamic acid), HSAB theory, adsorption, chemisorption, water treatment, pine cone biomass, selective recovery, Frontiers in Environmental Chemistry, Selective, acid