Characterization, dissolution and solubility of the hydroxypyromorphite-hydroxyapatite solid solution [(PbxCa1-x)5(PO4)3OH] at 25 °C and pH 2-9

Non-Destructive and Non-Invasive Approaches for the Identification of Hydroxy Lead-Calcium Phosphate Solid Solutions ((PbxCa1-x)5(PO4)3OH) in Cultural Heritage Materials

Lead-calcium phosphates are unusual compounds sometimes found in different kinds of cultural heritage objects. Structural and physicochemical properties of this family of materials, which fall into the hydroxypyromorphite-hydroxyapatite solid solution, or (PbxCa1-x)5(PO4)3OH, have received considerable attention during the last few decades for promising applications in different fields of environmental and material sciences, but their diagnostic implications in the cultural heritage context have been poorly explored. This paper aims to provide a clearer understanding of the relationship between compositional and structural properties of the peculiar series of (PbxCa1-x)5(PO4)3OH solid solutions and to determine key markers for their proper non-destructive and non-invasive identification in cultural heritage samples and objects. For this purpose, a systematic study of powders and paint mock-ups made up of commercial and in-house synthesized (PbxCa1-x)5(PO4)3OH compounds with a different Pb2+/Ca2+ ratio was carried out via a multi-technique approach based on scanning electron microscopy, synchrotron radiation-based X-ray techniques, i.e., X-ray powder diffraction and X-ray absorption near edge structure spectroscopy at the Ca K- and P K-edges, and vibrational spectroscopy methods, i.e., micro-Raman and Fourier transform infrared spectroscopy. The spectral modifications observed in the hydroxypyromorphite-hydroxyapatite solid solution series are discussed, by assessing the advantages and disadvantages of the proposed techniques and by providing reference data and optimized approaches for future non-destructive and non-invasive applications to study cultural heritage objects and samples.

A novel porous lignocellulosic standing hierarchical hydroxyapatite for enhanced aqueous copper(II) removal

Potentially toxic metal-polluted water resources are a heavily discussed topic the pollution by potentially toxic metals can cause significant health risks. Nanomaterials are actively developed towards providing high specific surface area and creating active adsorption sites for the treatment and remediation of these polluted waters. In an effort to tackle the limitations of conventional type adsorbents, nano-hydroxyapatite (HAp) was developed in this study by in situ generation onto wood powder, resulting in the formation of uniform hybrid powder (HAp@wood composite) structure consisting of HAp nanoparticles that showed the removal efficiency up to 80 % after 10 min; the maximum adsorption capacity for Cu(II) ions (98.95 mg/g-HAp) was higher compared to agglomerated nano-HAp (72.85 mg/g-HAp). The adsorption capacity of Cu(II) remained stable (89.85-107.66 mg/g-HAp) during the four adsorption-desorption cycles in multi-component system, thereby demonstrating high selectivity for Cu(II). This approach of using nanoparticle is relatively simple yet effective in improving the adsorption of potentially toxic metals and the developed approach can be used to develop advanced nanocomposites in commercial wastewater treatment.

Additive Manufacturing of Iron Carbide Incorporated Bioactive Glass Scaffolds for Bone Tissue Engineering and Drug Delivery Applications

In this study, we have synthesized a bioactive glass with composition 45SiO2-20Na2O-23CaO-6P2O5-2.5B2O3-1ZnO-2MgO-0.5CaF2 (wt %). Further, it has been incorporated with 0.4 wt % iron carbide nanoparticles to prepare magnetic bioactive glass (MBG) with good heat generation capability for potential applications in magnetic field-assisted hyperthermia. The MBG scaffolds have been fabricated using extrusion-based additive manufacturing by mixing MBG powder with 25% Pluronic F-127 solution as the binder. The saturation magnetization of iron carbide nanoparticles in the bioactive glass matrix has been found to be 80 emu/g. The morphological analysis (pore size distribution, porosity, open pore network modeling, tortuosity, and pore interconnectivity) was done using an in-house developed methodology that revealed the suitability of the scaffolds for bone tissue engineering. The compressive strength (14.3 ± 1.6 MPa) of the MBG scaffold was within the range of trabecular bone. The in vitro test using simulated body fluid (SBF) showed the formation of apatite indicating the bioactive nature of scaffolds. Further, the drug delivery behaviors of uncoated and polycaprolactone (PCL) coated MBG scaffolds have been evaluated by loading an anticancer drug (Mitomycin C) onto the scaffolds. While the uncoated scaffold demonstrated the drug's burst release for the initial 80 h, the PCL-coated scaffold showed the gradual release of the drug. These results demonstrate the potential of the proposed MBG for bone tissue engineering and drug delivery applications.

Enhanced Release of Calcium Ions from Hydroxyapatite Nanoparticles with an Increase in Their Specific Surface Area

Synthetic calcium phosphates, e.g., hydroxyapatite (HAP) and tricalcium phosphate (TCP), are the most commonly used bone-graft materials due to their high chemical similarity to the natural hydroxyapatite-the inorganic component of bones. Calcium in the form of a free ion or bound complexes plays a key role in many biological functions, including bone regeneration. This paper explores the possibility of increasing the Ca2+-ion release from HAP nanoparticles (NPs) by reducing their size. Hydroxyapatite nanoparticles were obtained through microwave hydrothermal synthesis. Particles with a specific surface area ranging from 51 m2/g to 240 m2/g and with sizes of 39, 29, 19, 11, 10, and 9 nm were used in the experiment. The structure of the nanomaterial was also studied by means of helium pycnometry, X-ray diffraction (XRD), and transmission-electron microscopy (TEM). The calcium-ion release into phosphate-buffered saline (PBS) was studied. The highest release of Ca2+ ions, i.e., 18 mg/L, was observed in HAP with a specific surface area 240 m2/g and an average nanoparticle size of 9 nm. A significant increase in Ca2+-ion release was also observed with specific surface areas of 183 m2/g and above, and with nanoparticle sizes of 11 nm and below. No substantial size dependence was observed for the larger particle sizes.

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

Early phase effects of silicate and orthophosphate on lead (Pb) corrosion scale development and Pb release

Orthophosphate is widely used to control lead (Pb) release in drinking water distribution systems, but phosphorus addition is not sustainable. Alternative corrosion control treatments are needed, and sodium silicate is one possibility. Here, pre-corroded Pb coupons-with and without free chlorine-were used to examine early-phase corrosion scale development after silicate addition, with orthophosphate as a reference corrosion inhibitor. Scale development was evaluated in terms of total Pb release, phase transformation, electrochemical impedance, morphological changes, Pb dissolution kinetics, and short-term Pb-Cu galvanic corrosion. Elevated Pb release occurred for approximately one month after silicate addition, and total Pb release peaked at 1968.1 μg/L and 1176.9 μg/L from systems with and without free chlorine, respectively. In contrast, orthophosphate-treated coupons exhibited fewer, less pronounced spikes in Pb release. By day 354, the median total Pb release from orthophosphate-treated coupons with and without free chlorine had decreased to 3.7 and 5.0 μg/L, respectively, while the median total Pb release from corresponding silicate-treated coupons was much higher, at 44.9 μg/L and 34.3 μg/L. Calcium lead apatite (Ca_0.56Pb_3.77(PO₄)₃OH_0.67) was identified in orthophosphate-treated scales, with hydroxylpyromorphite (Pb₅(PO₄)₃OH) present in the absence of free chlorine. Plattnerite occurred on coupons in all chlorinated systems. Pb silicate compounds were not detected, but Ca₂SiO₄ and Na₂Ca₂(SiO₃)₃ were identified by X-ray powder diffraction. The charge transfer: film resistance ratio characterizing the orthophosphate-treated coupons decreased slowly while that of the silicate-treated coupons increased after silicate was added. These variations suggest orthophosphate provided better corrosion control than silicate did. Silicate treatment generally caused degradation of the top Pb scale layer, resulting in elevated Pb release, while orthophosphate encouraged the growth of more structured, generally thicker, corrosion scales that were effective in limiting Pb release.

Integration of Micro-Nano-Engineered Hydroxyapatite/Biochars with Optimized Sorption for Heavy Metals and Pharmaceuticals

From the perspective of treating wastes with wastes, bamboo sawdust was integrated with a hydroxyapatite (HAP) precursor to create engineered nano-HAP/micro-biochar composites (HBCs) by optimizing the co-precipitated precursor contents and co-pyrolysis temperature (300, 450, 600 °C). The physicochemical properties of HBCs, including morphologies, porosities, component ratios, crystalline structures, surface elemental chemical states, surface functional groups, and zeta potentials as a function of carbonization temperatures and components of precursors, were studied. Biochar matrix as an efficient carrier with enhanced specific surface area to prevent HAP from aggregation was desired. The sorption behavior of heavy metal (Cu(II), Cd(II), and Pb(II)) and pharmaceuticals (carbamazepine and tetracycline) on HBCs were analyzed given various geochemical conditions, including contact time, pH value, ionic strength, inferencing cations and anions, coexisting humic acid, and ambient temperature. HBCs could capture these pollutants efficiently from both simulated wastewaters and real waters. Combined with spectroscopic techniques, proper multiple dominant sorption mechanisms for each sorbate were elucidated separately. HBCs presented excellent reusability for the removal of these pollutants through six recycles, except for tetracycline. The results of this study provide meaningful insight into the proper integration of biochar-mineral composites for the management of aquatic heavy metals and pharmaceuticals.

Lead immobilization in simulated polluted soil by Douglas fir biochar-supported phosphate

This study compared the lead (Pb²⁺) immobilization efficacy of biochar-supported phosphate to conventional in-situ heavy metal immobilization methods (with lime, neat biochar and phosphate). The biochar-supported phosphate was obtained by treating Douglas fir biochar (BC) with anhydrous calcium chloride and potassium dihydrogen phosphate. The amount of Pb²⁺ immobilized was determined by comparing the concentration of ammonium nitrate extractable Pb²⁺ lead from lead-spiked soil (without amendment) to that of a 30 d incubation with (a) lead-spiked soil plus 5% (wt./wt.) biochar supported-phosphate, (b) lead-spiked soil plus 5% (wt./wt.) untreated Douglas fir biochar, (c) lead-spiked soil plus 5% (w/w) lime and (d) lead-spiked soil plus 5% (wt./wt.) potassium dihydrogen phosphate. The control (lead-spiked soil without amendment) produced the largest quantity (96.08 ± 9.22 mg L^-1) of NH₄NO₃-extractable Pb²⁺, while lead-spiked soil treated with 5% (wt./wt.) biochar-supported phosphate resulted in the lowest quantity of NH₄NO₃ extractable Pb²⁺ (0.3 ± 0.2 mg L^-1). The mechanism for immobilization of Pb²⁺ by BP occurs at pH < 7 through dissolution of hydroxyapatite embedded in BP during modification, followed by precipitation of insoluble Pb₁₀(PO₄)₆(OH)₂. The residual lead fraction in the lead-spiked soil increased by 20.9% following amendment with BP. These results indicate that biochar-supported phosphate is a candidate to reduce lead mobility (bioavailability) in polluted soil. This amendment may lower Pb²⁺ uptake into plants while minimizing the potential for water contamination due to Pb²⁺mobility.

Trace element adsorption from acid mine drainage and mine residues on nanometric hydroxyapatite

Mining Ag, Cu, Pb, and Zn sulfides by flotation produces great volume of residues, which oxidized through time and release acid solutions. Leachates from tailing heaps are a concern due to the risk of surface water pollution. Hydroxyapatite nanoparticles may remove trace elements from acid leachate collected from an oxidized tailing heap (pH ranged 1.69 ± 0.3 to 2.23 ± 0.16; [SO₄^2-] = 58 ± 0.67 to 60.69 ± 0.39 mmol). Based on the batch experiments under standard conditions, the average removal efficiency was 96%, 92%, 86%, and 67% for Cd, Pb, Zn, and Cu, respectively. The Zn adsorption was modeled by the Freundlich equation, but Cd, Cu, and Pb isotherms do not fit to Freundlich nor Lagmuir equations. Adsorption and other mechanisms occur during trace elements removal by hydroxyapatite. In the polymetallic system, trace elements saturate the specific surface of hydroxyapatite in the following order Zn, Cd, Cu, and Pb. The pH values must be higher than 7.5 to adsorb trace elements. The dose of 3.8% of hydroxyapatite to acid mine drainage removed efficiently > 80% of the soluble Fe, Cu, Mn, Zn, Cd, Ni, and Pb: 4020.0, 37.3, 34.8, 432.0, 4.4, 0.7, and 0.11 mg L^-1 from leachate A and 3357.1, 46.6, 27.8, 569.0, 4.7, 0.6, and 1.7 from leachate B, respectively. The application of 0.7% of hydroxyapatite decreased the extractable Pb in unoxidized tailing heaps from 272 to 100 mg kg^-1. It is likely to use hydroxyapatite to control trace element mobility from mine residues to surrounding soils and surface water.

The sorption and short-term immobilization of lead and cadmium by nano-hydroxyapatite/biochar in aqueous solution and soil

In this study, the composite materials using different ratios of biochar (BC) to nano-hydroxyapatite (nHAP) were prepared for the remediation of lead (Pb) and cadmium (Cd) contaminated water and soil. The sorption and the immobilization experiments indicated a higher sorption capacity and immobilization efficiency of Pb compared to those of Cd. The characteristics of XRD, FTIR, SEM, and XPS manifested that dissolution-precipitation, cation exchange, complexation, and cation-π interaction were the main four mechanisms for the sorption of Pb²⁺ and Cd²⁺ using composite material PC1 (nHAP/BC = 1/1). From semi-quantitative analysis, the mineral effect accounted for the majority of the immobilization of Pb and Cd. Due to obvious Pb-precipitates in the sorbed material, dissolution-precipitation primarily affected the sorption of Pb using PC1, while the immobilization of Cd was mainly attributable to cation exchange. Such results corresponded to the stable Pb-precipitates and unstable Cd-compounds in soil, among which the latter was prone to be released into the environment. The sorption capacity in aqueous solutions and the immobilization efficiencies in the soil for both Pb and Cd increased with the addition of nHAP, which were linearly correlated to the nHAP proportion in the composite materials. In future practical applications, the percentages of composite materials can be designed according to the specific pollutant concentration. This study sheds light on the explicit immobilization mechanisms for Pb and Cd in aqueous solutions to better understand their behaviors in the soil remediated by relevant materials.

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.

Insight into the co-adsorption behaviors and interface interactions mechanism of chlortetracycline and lead onto struvite loaded diatomite

Finding effective methods for simultaneous removal of antibiotics and heavy metals has attracted increasing concerns. Herein, we investigated the co-adsorption behaviors of chlortetracycline (CTC) and Pb (II) onto struvite loaded diatomite (SD) in aqueous solution, and their interface interactions mechanism was investigated using crystal and microstructure analysis combined with density functional theory (DFT) calculations. The adsorption capacity at equilibrium of CTC increased from 44.28 to 87.58 mmol/kg with the presence of Pb (II), but the adsorption capacity at equilibrium of Pb (II) decreased from 4289.70 to 3559.9 mmol/kg with the presence of CTC. Besides, the effect of environmental factors (solution pH and humic acid) was also evaluated. Microstructure analysis for recovered products demonstrated that the interface interactions brought by the surface Pb(II) of Pb₅(PO₄)₃OH and carbonyl-O of CTC could facilitate the removal of CTC but inhibit the removal of Pb(II) by suppressing the crystal growth of Pb₅(PO₄)₃OH, especially the orientation growth of (0 0 1) crystal plane. DFT calculations gave theoretical support for the interface interactions between Pb₅(PO₄)₃OH and CTC.

Aqueous solubility of Pb at equilibrium with hydroxypyromorphite over a range of phosphate concentrations

Hydroxypyromorphite (HPM) is a low-solubility Pb phosphate mineral that has the potential to limit solubility and bioavailability of Pb in soils and water. Because of reported uncertainty regarding the solubility product of this important mineral, we re-evaluated the solubility of Pb and activity of the free Pb2+ ion in aqueous suspensions of microcrystalline HPM equilibrated up to 30 days over a wide range of added soluble phosphate. A small addition of phosphate (0.1 mM) reduced Pb solubility as measured by ICP-OES, but greater phosphate additions (up to 50 mM) had no further effect in lowering HPM solubility. However, free Pb2+ ion activity measured by ion-selective electrode progressively decreased from about 10-6.5 with no added phosphate to 10-9 as soluble phosphate was increased. The effect of soluble phosphate in lowering Pb2+ activity is attributed to inhibited dissolution of HPM as well as increased Pb2+-phosphate ion pair formation in solution at higher solution concentrations of phosphate. Measurement of the ion activity products (IAP) of the solutions at equilibrium with HPM gave highly variable IAP values that were sensitive to pH and were generally not consistent with the reported solubility product of this mineral. The high variability of the IAPs for solutions with variable pH and phosphate concentrations indicates that dissolution-precipitation reactions of HPM are not described by a constant solubility product at equilibrium, possibly because of the incongruent dissolution behavior of this mineral at near-neutral pH.

Anomalous Fluctuation of Halogens in Relation to the Pollution Status along Lake Mariout, Egypt

This paper aimed to study the anomalous fluctuation of halogens with respect to the pollution status in surface water (w), pore water (p), and sediments (s) of Lake Mariout. It provided a framework for understanding the distribution of dissolved and precipitated halogen salts related to the pollution status of the lake. The study cleared out that bromide was only the most abundant halogen in the three studied partitions. On contrast, sediment’s partition contained the lowest chloride content. Fluoride minerals, especially, fluorapatites and carbonate-fluorapatite (FAP and CFAP), had high Saturation Index (SI) values in surface water (42.77–51.95 and 16.04–60.89, respectively) and in pore water (51.26–54.60 and 17.52–78.33, respectively). Bromide and chloride were mainly found in the soluble forms in the surface water and pore waters. Iodide salts, (Ca(IO3)2 and Ca(IO3)2.6H2O), were moderately precipitated in surface and pore waters. Thus, SI content reflected that halogens, especially fluoride and iodide, played a vital role in reducing lake pollution. Fluorite (CaF2) and sellaïte (MgF2) could only be formed in pore water, while calcite and aragonite could be deposited from surface water. In addition, Cl was mainly found in the forms of NaCl, CaCl2, MgCl2, and KCl in surface and pore waters. The multivariate analysis revealed that fluoride precipitate may serve in decreasing the dissolved salt pollution. Multivariate analysis showed that in the long run, the fluoride precipitation in FAP and CFAP can significantly adsorb and absorb various pollutants and can protect the lake from pollution. The ecological risk assessment conducted by calculating the enrichment factor (EF) showed that the lake was still unpolluted. Regarding human health risks, at appropriate levels of human health and safety, the hazard quotient (HQ) and hazard index (HI) of halogens found to be lower than these reported levels. Hence, ingestion and dermal absorption routes of halogens by surface water and sediments did not pose any adverse effects to population reflecting uncontaminated status of Lake Mariout.

The factors influencing sludge incineration residue (SIR)-based magnesium potassium phosphate cement and the solidification/stabilization characteristics and mechanisms of heavy metals

Magnesium potassium phosphate cement (MKPC) is prepared from MgO and KH₂PO₄ through an acid-base reaction and has been widely used in the rapid repairs of building structures and the solidification/stabilization (S/S) of heavy metals (HMs). The use of sludge incineration residue (SIR) rich in phosphorus resources to prepare SIR-based MKPC can achieve the reclamation of SIR and efficient HM S/S. Herein, based on the exploration of the optimal MKPC magnesia/phosphate ratio (M/P), the effects of SIR and HMs on the performance of the matrix and its interaction mechanism were comprehensively investigated. The results indicated that the compressive strength of the SIR-based MKPC increased first and then decreased with the gradual increase of SIR incorporation; the optimal was reached at 40.31 MPa when the SIR incorporation was 5 wt%. The peak signal and crystal lattice of Pb₂(PO₄)₃ indicated that there is a mixed effect between HMs (in SIR) and KH₂PO₄. The Visual MINTEQ analysis results also indicated that HMs are precipitated as HM phosphates. The formation of HM phosphates not only increases the M/P (with 30 wt% SIR, M/P increased by 0.019), affecting the microstructure and changing the compressive strength of the matrix, but also promotes the transformation of HMs from the bioavailable to the more stable residual forms. The residual forms of the six HMs were all above 84% after S/S. Therefore, the SIR-based MKPC preparation significantly immobilized the HMs; particularly, the leaching toxicities of Cu (96.6%) and Zn (96.3%) were alleviated.

Divalent heavy metals and uranyl cations incorporated in calcite change its dissolution process

Due to the high capacity of impurities in its structure, calcite is regarded as one of the most attractive minerals to trap heavy metals (HMs) and radionuclides via substitution during coprecipitation/crystal growth. As a high-reactivity mineral, calcite may release HMs via dissolution. However, the influence of the incorporated HMs and radionuclides in calcite on its dissolution is unclear. Herein, we reported the dissolution behavior of the synthesized calcite incorporated with cadmium (Cd), cobalt (Co), nickel (Ni), zinc (Zn), and uranium (U). Our findings indicated that the HMs and U in calcite could significantly change the dissolution process of calcite. The results demonstrated that the incorporated HMs and U had both inhibiting and enhancing effects on the solubility of calcite, depending on the type of metals and their content. Furthermore, secondary minerals such as smithsonite (ZnCO3), Co-poor aragonite, and U-rich calcite precipitated during dissolution. Thus, the incorporation of metals into calcite can control the behavior of HMs/uranium, calcite, and even carbon dioxide.

Insights into dynamic adsorption of lead by nano-hydroxyapatite prepared with two-stage ultrasound

Nano-hydroxyapatite (nHAP) has an excellent effect on the remediation of Pb contaminated water and soil. In this study, an efficient modified nHAP was prepared assisted with two-stage ultrasonic irradiation. The effects of ultrasound modification on the nHAP were tested using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform InfraRed spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) and combined batch Pb uptake experiments. The nHAP with ultrasound has a fine structure with the width and length of around 9 nm and 40 nm respectively. The ultrasound parameter of 1s/36s in stage 1 and 16s/18s in stage 2 was verified as the optimum under which the nHAP prepared performed the best with the maximum adsorption capacity of 1300.93 mg/g. The results of XRD and SEM indicated that the sorbent after uptake of Pb²⁺ was mainly Pb₁₀(PO₄)₆OH₂ (HPY) with insignificant Ca₁₀Pb_10-x(PO₄)₆OH₂. Compared the results of Pb/Ca, pH and XRD with the metal fraction of Pb in adsorbents during the dynamic sorption process, this research proved that the effects of complexation, cation exchange and dissolution and precipitation coexisted in the initial stage, while the dissolution and precipitation gradually dominated the adsorption mechanism with contact time. The processes of Pb²⁺ uptake by nHAP sorbents prepared under different ultrasound parameter presented almost the same dynamic mechanism with a little difference in time node. The research of dynamic mechanism of Pb²⁺ uptake by a superior nHAP is essential for both contaminated water and soil remediation.

Micro-nano-engineered nitrogenous bone biochar developed with a ball-milling technique for high-efficiency removal of aquatic Cd(II), Cu(II) and Pb(II)

A cost-effective and eco-friendly engineering method to improve biochar's physicochemical and sorption performance is critical in various environmental applications. In this study, micro-nano-engineered nitrogenous biochars derived from cow bone meal pyrolyzed at different temperatures and were engineered with the assistance of a ball-milling technique. The ball-milled bone biochars were natural composites combined with plant biochars and hydroxyapatite components on the micro-nanoscale. Both the micropore area and the external specific surface area of the bone biochars were significantly improved after ball-milling. The sorption capacities for heavy metal ions were heavy metal ions were MBC-600 > MBC-450 > BC-600 > MBC-300 > BC-450 > BC-300, consistent with the variation tendency in the specific surface areas of the bone biochars. The adsorption capacities of MBC-600 for Cd(II), Cu(II) and Pb(II) were 165.77, 287.58 and 558.88 mg/g, respectively (T 298K, pH 5.0), representing increases of 93.91.%, 75.56% and 64.61% compared with the un-milled preparation. Surface complexation, cation exchange, chemical precipitation, electrostatic interaction and cation-π bonding were responsible for the removal of heavy metal ions by bone biochar materials. Taken together, the results show that micro-nano-engineered nitrogenous bone biochar prepared using ball-milling technology is a promising material for the remediation of heavy metals-bearing aquatic environments.

Factors Affecting the Environmentally Induced, Chronic Kidney Disease of Unknown Aetiology in Dry Zonal Regions in Tropical Countries—Novel Findings

A new form of chronic tubulointerstitial kidney disease (CKD) not related to diabetes or hypertension appeared during the past four decades in several peri-equatorial and predominantly agricultural countries. Commonalities include underground stagnation of drinking water with prolonged contact with rocks, harsh climatic conditions with protracted dry seasons, and rampant poverty and malnutrition. In general, the cause is unknown, and the disease is therefore named CKD of unknown aetiology (CKDu). Since it is likely caused by a combination of factors, a better term would be CKD of multifactorial origin (CKDmfo). Middle-aged malnourished men with more than 10 years of exposure to environmental hazards are the most vulnerable. Over 30 factors have been proposed as causative, including agrochemicals and heavy metals, but none has been properly tested nor proven as causative, and unlikely to be the cause of CKDmfo/CKDu. Conditions such as, having favourable climatic patterns, adequate hydration, and less poverty and malnutrition seem to prevent the disease. With the right in vivo conditions, chemical species such as calcium, phosphate, oxalate, and fluoride form intra-renal nanomineral particles initiating the CKDmfo. This article examines the key potential chemical components causing CKDmfo together with the risk factors and vulnerabilities predisposing individuals to this disease. Research findings suggest that in addition to drinking water from stagnant sources that contain high ionic components, more than 10 years of exposure to environmental nephrotoxins and micronutrient malnutrition are needed to contract this fatal disease.

Solidification/stabilization of Pb2+ and Zn2+ in the sludge incineration residue-based magnesium potassium phosphate cement: Physical and chemical mechanisms and competition between coexisting ions

In order to exhaustively investigate the physical and chemical mechanisms of heavy metal immobilization in sludge incineration residue (SIR)-based magnesium potassium phosphate cement (MKPC), this work investigated the influence of Pb²⁺ and Zn²⁺ on the compressive strength and microstructure of SIR-based MKPC, and the efficiency of Pb and Zn immobilization. Taking the difference of Ksp (solubility product) of different heavy metal compounds as the entry point, the physical and chemical mechanisms of Pb and Zn immobilization, and the competitive mechanism between coexisting ions, were comprehensively analyzed. It was discovered that Pb²⁺ is in the form Pb₃(PO₄)₂, and Zn²⁺ is immobilized in the form Zn₂(OH)PO₄ [Zn₃(PO₄)₂ is preferentially formed, when the pH > 7, Zn₃(PO₄)₂ is converted to Zn₂(OH)PO₄]. The low solubility of heavy metal phosphates is the main reason that Pb²⁺ and Zn²⁺ are well immobilized. The preferential formation of Pb₃(PO₄)₂ (K_sp = 8 × 10^-43) and Zn₃(PO₄)₂ (K_sp = 9.0 × 10^-33) reduced the amount of MgKPO₄·6H₂O (K_sp = 2.4 × 10^-11), resulting in a decrease in compressive strength. Besides, coexisting Pb²⁺ and Zn²⁺ has a competitive effect: Pb²⁺ will weaken the immobilization efficiency of Zn²⁺. The new exploration of these mechanisms provide a theoretical basis for rationally adjusting the Magnesia/Phosphate ratio to enhance the compressive strength and improve the efficiency of heavy metals immobilization.

Oxalate-enhanced solubility of lead (Pb) in the presence of phosphate: pH control on mineral precipitation

Here we study the precipitation of lead (Pb)-phosphate minerals over the pH range of 4.0 to 8.0 with and without oxalate, a ubiquitous and abundant low-molecular-weight organic acid derived from plants and microorganisms in environmental matrices. In the aqueous Pb-phosphate systems, phosphate precipitated Pb efficiently, reducing the dissolved Pb concentration below 1 μM at all the tested pH values, with the minimum solubility of about 0.1 μM measured at the intermediate pH of 6.0. The measured dissolved Pb and free Pb2+ ion activity were not in agreement with predictions from generally-accepted solubility products of the Pb phosphate minerals, particularly hydroxypyromorphite [Pb5(PO4)3OH]. Discrepancies between our measured Pb phosphate solubility products and older reported values are attributed to non-ideal behavior of these minerals (incongruent dissolution) as well as uncertainties in stability constants for soluble Pb-phosphate ion pairs. The presence of equimolar levels of oxalate and phosphate resulted in up to 250-fold increase in Pb solubility at acidic pH and about a 4-fold increase at pH 7.0, due to the strong suppression of Pb phosphate precipitation by oxalate and formation of soluble Pb-oxalate complexes. At pH 4.0 and 5.0, Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) identified a Pb-oxalate mineral phase as the only precipitate despite the presence of phosphate; in the absence of oxalate, Pb hydrogen phosphate, PbHPO4, stably formed under these acidic conditions. At pH 6.0 and greater, FTIR and XRD data revealed that Pb-phosphate [Pb3(PO4)2], and hydroxypyromorphite [Pb5(PO4)3OH] to a lesser extent, were the predominant precipitates both in the absence and presence of oxalate. Therefore, oxalate did not strongly interfere with Pb-phosphate mineral formation at aqueous pH greater than 6.0 but oxalate controlled Pb solubility at acidic pH values.

Assessing the Lead Solubility Potential of Untreated Groundwater of the United States

In the U.S., about 44 million people rely on self-supplied groundwater for drinking water. Because most self-supplied homeowners do not treat their water to control corrosion, drinking water can be susceptible to lead (Pb) contamination from metal plumbing. To assess the types and locations of susceptible groundwater, a geochemical reaction model that included pure Pb minerals and solid solutions of calcite (Ca _xPb_{1- x}CO₃) and apatite [Ca _xPb_5-x(PO₄)₃(OH; Cl; F)] was developed to estimate the lead solubility potential (LSP) for over 8300 untreated groundwater samples collected from domestic and public-supply sites between 2000 and 2016 in the U.S. The LSP is the calculated amount of Pb metal that could dissolve at 25 °C before a Pb-bearing mineral precipitates. About 33% of untreated groundwater samples had LSP greater than 15 μg/L-the USEPA action level for dissolved plus particulate forms of Pb. Five percent of samples had high LSP (above 300 μg/L) and tended to occur in the eastern and southeastern U.S. Measured Pb concentrations above 15 μg/L were rarely detected (<1%) but always coincided with high LSP values. Future work will provide a better understanding of the relation between water chemistry, Pb-mineral formation, and dissolved Pb concentrations in tap water.

Direct Observation of Simultaneous Immobilization of Cadmium and Arsenate at the Brushite-Fluid Interface

Cadmium (Cd²⁺) and Arsenate (As⁵⁺) are the main toxic elements in soil environments and are easily taken up by plants. Unraveling the kinetics of the adsorption and subsequent precipitation/immobilization on mineral surfaces is of considerable importance for predicting the fate of these dissolved species in soils. Here we used in situ atomic force microscopy (AFM) to image the dissolution on the (010) face of brushite (dicalcium phosphate dihydrate, CaHPO₄·2H₂O) in CdCl₂- or Na₂HAsO₄-bearing solutions over a broad pH and concentration range. During the initial dissolution processes, we observed that Cd or As adsorbed on step edges to modify the morphology of etch pits from the normal triangular shape to a four-sided trapezium. Following extended reaction times, the respective precipitates were formed on brushite through a coupled dissolution-precipitation mechanism. In the presence of both CdCl₂ and Na₂HAsO₄ in reaction solutions at pH 8.0, high-resolution transmission electron microscopy (HRTEM) showed a coexistence of both amorphous and crystalline phases, i.e., a mixed precipitate of amorphous and crystalline Cd_{(5- x)}Ca _x(AsO₄)_{(3- y)}(PO₄) _yOH phases was detected. These direct dynamic observations of the transformation of adsorbed species to surface precipitates may improve the mechanistic understanding of the calcium phosphate mineral interface-induced simultaneous immobilization of both Cd and As and subsequent sequestration in diverse soils.