Abstract
Heavy metals and metalloids are common cooccurrence in contaminated soils, making their behaviors more complex than their individual presences. Adsorption to soil minerals and organic components determines the solubility and mobility of heavy metals. However, little information is available regarding coadsorbing metals (e.g., Cd) and metalloids (e.g., Sb) to soil components, and whether there is a universal coadsorption rule needs to be illuminated. This study investigated the coadsorption behaviors of Cd(II) and Sb(V) to goethite, kaolinite, and bacteria ( Bacillus cereus) at both acidic (pH 4.5) and alkaline pH (pH 8.5). Equilibrium adsorption experiments, coupled with scanning electron microscopy- (SEM-) energy-dispersive X-ray spectrum (EDS) and X-ray photoelectron spectroscopy (XPS), were applied to determine the batch adsorption phenomena and possible mechanisms. Batch results showed that Cd(II) adsorption was greater at pH 8.5 whereas Sb(V) adsorption was greater at pH 4.5. The presence of Cd or Sb promoted each other’s adsorption to goethite, kaolinite, and bacteria, but slight differences were that Sb(V) preferred to enhance Cd(II) adsorption at acidic pH, whereas Cd(II) was more able to increase Sb(V) adsorption at alkaline pH. SEM-EDS analyses further showed that the distribution of Cd and Sb was colocalized. The surface FeOH, AlOH, and COOH groups participated in the binding of Cd(II) and Sb(V), probably through the formation of inner-sphere complexes. Two possible ternary complexes, i.e., sorbent-Cd2+-Sb(OH)6 – and sorbent-Sb(OH)6 –-Cd2+, were possibly formed. Both the charge effect and the formation of ternary complexes were responsible for the collaborative coadsorbing of Cd-Sb. The universal synergistic rule obtained suggests that current models for predicting Cd(II) or Sb(V) sequestration based on single systems may underestimate their solid-to-liquid distribution ratio in a coexistence situation. The results obtained have important implications for understanding the chemical behavior of Sb and Cd in contaminated soils.
Highlights
Soils contaminated with both elevated concentrations of heavy metals (e.g., Cd and Pb) and metalloids (e.g., As and Sb) are common in mining and smelting-impacted regions
Our results show that the promotion effects are dependent on pH; i.e., Sb(V) enhances Cd(II) adsorption more pronouncedly at acidic pH, whereas Cd(II) prominently facilities Sb(V) adsorption at alkaline pH
This study provides the first systematic evidence for coadsorbing Cd(II)-Sb(V) to typical mineral and organic components
Summary
Soils contaminated with both elevated concentrations of heavy metals (e.g., Cd and Pb) and metalloids (e.g., As and Sb) are common in mining and smelting-impacted regions. Favorable adsorption is commonly observed on organic colloids such as humus, microbes, and extracellular secretions at environmentally relevant pH [13,14,15,16,17,18] These organic constituents exhibit negative surface charge and abundant functional groups including carboxyl, phosphate, amino, phenolic hydroxyl, and Adsorption Science & Technology sulfhydryl [19,20,21,22], showing a strong ability to complex metal cations to form stable inner-sphere-type complexes. As for metalloids, they are inclined to bind to metal (oxyhydr)oxides [23,24,25,26] rather than the organic constituents. Several studies demonstrated that pH is a dominant influencing factor; i.e., lower pH favors metalloid adsorption whereas higher pH favors metal cation adsorption
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