The solid-liquid interface reaction between phosphate rock (PR) and metals (Pb, Cu, and Zn) was studied. Phosphate rock has the highest affinity for Pb, followed by Cu and Zn, with sorption capacities of 138, 114, and 83.2 mmol/kg PR, respectively. In the Pb-Cu-Zn ternary system, competitive metal sorption occurred with sorption capacity reduction of 15.2%, 48.3%, and 75.6% for Pb, Cu, and Zn, respectively compared to the mono-metal systems. A fractional factorial design showed the interfering effect in the order of Pb>Cu>Zn. Desorption of Cu and Zn was sensitive to pH change, increasing with pH decline, whereas Pb desorption was decreased with a strongly acidic TCLP extracting solution (pH = 2.93). The greatest stability of Pb retention by PR can be attributed to the formation of insoluble fluoropyromorphite [Pb(10)(PO(4))(6)F(2)], which was primarily responsible for Pb immobilization (up to 78.3%), with less contribution from the surface adsorption or complexation (21.7%), compared to 74.5% for Cu and 95.7% for Zn. Solution pH reduction during metal retention and flow calorimetry analysis both supported the hypothesis of retention of Pb, Cu, and Zn by surface adsorption or complexation. Flow calorimetry indicated that Pb and Cu adsorption onto PR was exothermic, while Zn sorption was endothermic. Our research demonstrated that PR can effectively remove Pb from solutions, even in the presence of other heavy metals (e.g. Cu, Zn).

https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1014&context=ersc_fac

Understanding the Mechanisms of Lead, Copper, and Zinc Retention by Phosphate Rock

Chemical stabilization remediation for heavy metals in contaminated soils on the latest decade: Available stabilizing materials and associated evaluation methods－A critical review

Partitioning, leachability, and speciation of chromium in the size-fractions of soil contaminated by chromate production.

Heavy metal contamination of soil and water is one of the most prominent environmental issues worldwide. Through bioaccumulation and biomagnification of the food chain, heavy metals can be enriched hundreds of times and eventually enter the human body, posing a major threat to human health. Biomineralization has the greatest potential to become an efficient and environmentally friendly heavy metal remediation technology and has received much attention in recent decades. This review summarizes the latest progress of biomineralization technology on carbonate precipitation and phosphate precipitation in heavy metal remediation. Both microorganisms (including bacteria and fungi) and enzymes can induce carbonate and phosphate precipitation, converting the free heavy metal ions into insoluble salts. However, the mechanisms of the heavy metal remediation are significant different. For example, urea hydrolysis, which occurs intracellularly when urease-producing bacteria (UPB) are used, is the most commonly used mechanism for carbonate precipitation based bioremediation. In contrast, phosphate solubilization by either enzymes or organic acids secreted by phosphate solubilizing bacteria (PSB) is extracellular, and both soluble and insoluble phosphorus can be decomposed by PSB. Moreover, some influencing factors such as the different species of microorganism, heavy metals and some environmental conditions that may affect the bioremediation of heavy metals were also summarized in this paper. The challenges of biomineralization based heavy metal remediation are also discussed. Based on the reviews of previous studies, a comprehensive understanding of heavy metal removal through microorganism can be increased, and thus promotes the applications of biomineralization technology in the treatment of large-scale heavy metal contaminated sites. 

Mechanisms and Influencing Factors of Biomineralization Based Heavy Metal Remediation: a Review

Comparing the effects of humic acid and oxalic acid on Pb(II) immobilization by a green synthesized nanocrystalline hydroxyapatite.

Phosphate amendments, especially phosphate rock (PR), are one of the most commonly used materials to stabilize heavy metals in contaminated soils. However, most of PR reserve consists of low-grade ore, which limits the efficiency of PR for stabilizing heavy metals. This study was to enhance the stabilization of heavy metals through improving the available phosphorous (P) release of PR by oxalic acid activation. Raw PR and activated PR (APR) were characterized by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), X-ray powder diffraction (XRD), Brunauer–Emmett–Teller (BET) surface analysis, and laser diffraction to determine the changes of structure and composition of APR. The stabilization effectiveness of lead (Pb), zinc (Zn), and cadmium (Cd) in soils by APR was investigated through toxicity leaching test and speciation analysis. The results indicated that after treatment by oxalic acid, (1) the crystallinity of the fluorapatite phase of PR transformed into the weddellite phase; (2) the surface area of PR increased by 37%; (3) the particle size of PR became homogenized (20–70 μm); and (4) the available P content in PR increased by 22 times. These changes of physicochemical characteristics of PR induced that APR was more effective to transform soil heavy metals from the non-residual fraction to the residual fraction and enhance the stabilization efficiency of Pb, Zn, and Cd than PR. These results are significant for the future use of low-grade PR to stabilize heavy metals.

Enhanced stabilization of Pb, Zn, and Cd in contaminated soils using oxalic acid-activated phosphate rocks

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Enhanced stabilization of Pb, Zn, and Cd in contaminated soils using oxalic acid-activated phosphate rocks