Multicomponent adsorption of heavy metals onto biogenic hydroxyapatite: Surface functional groups and inorganic mineral facilitating stable adsorption of Pb(Ⅱ)

Due to the coexistence of various heavy metals in the contaminated environment, it is essential to comprehensively study the multicomponent adsorption of heavy metals in order to tackle these combined pollutants. Herein, the adsorption processes of Pb(Ⅱ), Cu(Ⅱ) and Cd(Ⅱ) by biogenic hydroxyapatite (BHAp) were investigated in single and multicomponent systems. The maximum adsorption capacity for Pb(Ⅱ), Cu(Ⅱ) and Cd(Ⅱ) by BHAp in single system reached 311.16, 82.05 and 92.54 mg g^-1, respectively, while adsorption capacity for Cu(Ⅱ) and Cd(Ⅱ) in multicomponent system decreased more obviously than that of Pb(Ⅱ). Furthermore, the stability of Cu(Ⅱ) and Cd(Ⅱ) adsorbed on BHAp was indeed influenced in multicomponent system. By means of the characterization analysis, it was found that ion exchange was more instrumental in the adsorption processes of Cu(Ⅱ) and Cd(Ⅱ) in single system than in multicomponent system. Significantly, it was observed that the proportion of generally stable Pb(II) adsorbed on BHAp exceeded 95% in both single and multicomponent systems. This result might be due to the in-site growth of stable crystals of Pb_xCa_10-x(PO₄)₆(OH)₂, which was synergistically induced by surface functional groups and inorganic mineral of BHAp, and was unaffected by the coexistence of Cu(Ⅱ) and Cd(Ⅱ).

Pb(II)-mediated precipitate transformation promotes Cr(VI) immobilization by biogenic hydroxyapatite

In this work, the mechanism of Pb(II)-mediated precipitation transformation to improve the removal of Cr(VI)-oxyanion on biogenic hydroxyapatite (BHAp) were investigated. The Pb(II)-preloading formed pyromorphite [Pb₅(PO₄)₃Cl] precipitate on the BHAp surface (Pb@BHAp), thus causing an increase of 2.2 times in the uptake of Cr(VI) by Pb@BHAp at pH of 2.4. It was primarily due to the dissolution of Pb₅(PO₄)₃Cl accompanied with the release of Pb(II), resulting in the rapid formation of crocoite (PbCrO₄). Although the K_sp of Pb₅(PO₄)₃Cl was approximately 23 orders of magnitude lower than that of PbCrO₄, Pb(II)-mediated precipitation transformation could still occur. XRD and SEM-EDX analyses demonstrated that the process was a time-dependent that included rapid crystal precipitation in the initial 10 min and subsequent precipitate accumulation for several hours. The Pb(II) released from the dissolution of Pb₅(PO₄)₃Cl was immediately immobilized by Cr(VI); therefore, it did not cause any retention risk of Pb(II) in the solution. Furthermore, a small quantity of Cr(VI) could be reduced to Cr(III) by BHAp, and Cr(III) could enter into the BHAp lattice for the exchange of Ca(II). This study provides a new insight into the resource utilization of Pb-bearing BHAp and a potential method for the successive removal of Pb(II) and Cr(VI).

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

Dissolved organic matter (DOM) is one of the most significant parameters to affect the remediation efficiency of Pb(II) by apatites. Numerous studies chose humic substances as a surrogate of DOM to investigate its influence on Pb(II) immobilization. However, the effect of low-molecular-weight organic acids such as oxalic acid (OA), which is ubiquitous in the environment and a primary component of DOM, in immobilizing Pb(II) was still not fully understood. Herein, humic acid (HA) and OA were examined to distinguish their influence on Pb(II) immobilization by a green synthesized nanocrystalline hydroxyapatite (nHAP). Various parameters were considered to evaluate the removal performance of nHAP towards Pb(II) as affected by HA/OA. Results indicated that Pb(II) immobilization was significantly promoted in the coexistence of OA owing to the precipitation of hydroxypyromorphite (HPY) as well as PbC₂O₄, but was independent on the addition sequence and slightly hindered by HA, disclosing that Pb(II) preferred to bind directly with nHAP instead of via HA. Characterization of the Pb(II) loaded solids by multiple technologies revealed that HPY was the predominant precipitate both in the absence and presence of HA, while the formation of PbC₂O₄ was preferred over that of HPY in the existence of OA. X-ray photoelectron spectroscopy indicated that PbC₂O₄ was the prevalent solid phase with the ratio of 62.97% after Pb(II) immobilization by nHAP in the presence of OA. These findings implied that the transformation efficiency of Pb(II) to HPY by apatites can be overestimated in the presence of OA due to the precipitation of PbC₂O₄.

Recycling of biogenic hydroxyapatite (HAP) for cleaning of lead from wastewater: performance and mechanism

In China, the utilization and recycling of chicken waste have become a significant environmental issue. In this study, we investigate the efficacy of biogenic hydroxyapatite (HAP) materials, recycled from chicken waste, for Pb(II) sequestration from wastewater. The results from batch experiments indicate that biogenic HAP could effectively remove Pb(II) from an aqueous solution. The maximum removal efficiency of Pb (more than > 99%) was observed under the following operational conditions: initial pH of 3.0, initial Pb(II) concentrations of 208 mg/L, and 1 g/L of HAP adsorbents. The presence of coexisting divalent ions, including Ca2+, Mg2+, and Mn2+, had no significant influence on Pb(II) removal. Spectroscopic analysis suggests that the dissolution-precipitation mechanism was mainly responsible for Pb(II) sequestration under acidic conditions (pH ≤ 3.0). Our findings indicate that biogenic HAP recycled from biowaste can be used as an efficient adsorbent for cleaning Pb(II) from wastewater.