Characterization, dissolution and solubility of synthetic cadmium hydroxylapatite [Cd5(PO4)3OH] at 25-45°C

Efficient removal of Cd(II) and Pb(II) from aqueous solution using biochars derived from food waste

Massive amount of food waste has been generated annually, posing a threat to ecological sustainability and the social economy due to current disposal methods. Urgent action is needed worldwide to convert the traditional pathway for treating food waste into a sustainable bioeconomy, as this will significantly benefit food chain management. This study explores the use of pyrolysis to produce different types of food waste biochars and investigates their adsorption capabilities for removing Cd2+ and Pb2+ in aqueous solution. The results indicated that co-pyrolysis biochar from fresh food waste and rice husk (FWRB) exhibited superior adsorption performance for Cd2+ (61.84 mg·g-1) and Pb2+ (245.52 mg·g-1), respectively. Pseudo-second-order kinetics (0.74 ≤ R2 ≤ 0.98) and Langmuir isotherms (0.87 ≤ R2 ≤ 0.98) indicated that the immobilized Cd2+ and Pb2+ on biochars were mainly attributed to the chemisorption, including precipitation with minerals (e.g., carbonates, silicates, and phosphate), complexation with functional groups (-OH), cation exchange (-COO-), and coordination with π-electrons. Furthermore, FWRB demonstrated reduced EC and Na content in comparison to food waste digestate biochar (FWDB) and food waste digestate co-pyrolysis with sawdust biochar (FWSB), with levels of Cd and Pb falling below China's current guideline thresholds. These findings suggested that co-pyrolysis of fresh food waste with rice husk could be applicable to the recycling of food waste into biochar products for heavy metal stabilization in contaminated water and soils.

Recycled biochar adsorption combined with CaCl2 washing to increase rice yields and decrease Cd levels in grains and paddy soils: A field study

Field-scale trials were conducted to remove cadmium (Cd) from paddy soil by using recycled hydroxyapatite modified biochar (HBC) plus low-level CaCl₂ washing. Synergistic reduction efficiencies of total and available Cd in soil (45.6 % and 36.8 %) were achieved by the combined amendments compared with only HBC or CaCl₂. The release of Cd from soil particulates was facilitated by CaCl₂ washing and the increased soluble Cd in soil water (hardly removed by drainage) could be removed efficiently by recycled HBC adsorption. Significantly decreases in Cd translocation and accumulation in rice plants benefited from the decrease of Cd level and availability in soil and the increase of available silicon (Si). As a result, Cd contents in early/late rice grains decreased by ~85 % and met the Chinese national food standard. SOM, CEC, and soil nutrients after remediation were increased due to 10 %-15 % of HBC residual. Grain yields of the early and late rice increased by 34.1 % and 9.91 %, respectively. The collected HBC (>85 % of the total used HBC) was in-situ regenerated and could be used in the next field trials. The generated wastewater together with drainage from field treatment could be reused as irrigation water after the treatment with a small-scale reclamation ecosystem. The work provides a novelty remediation strategy for Cd-contaminated paddy soil. The noticeable remediation efficiency for Cd reduction in soil and grains, and improved productivity-relevant soil properties have important implications for paddy soil with poor fertility, severe desilicification, and Cd contamination in South China whereas the application of biochar or chemical washing alone did not.

Anaerobic syntrophic system composed of phosphate solubilizing bacteria and dissimilatory iron reducing bacteria induces cadmium immobilization via secondary mineralization

Secondary mineralization is a promising method for remediating cadmium (Cd) pollution in sediments, but the poor stability of Cd-containing secondary minerals is a bottleneck that limits the development of this approach. The existence of phosphate can enhance the formation of stable secondary minerals and points a new direction for Cd immobilization. In this research, a novel syntrophic system composed of phosphate solubilizing bacteria (PSB) and dissimilatory iron reducing bacteria (DIRB) was established and the effect and mechanism of Cd immobilization in the system were also explored. The results showed that under the conditions of DIRB:PSB (V:V)= 3:1, syntrophic bacteria dosage of 5% and glucose dosage of 5 g/L, Cd incorporated in the secondary minerals could account for about 60% of the total Cd. In the pH range of 5-9, alkaline environment was conducive to the immobilization of Cd and the percentage of combined Cd was up to 58%, while the combined Cd in secondary minerals decreased from 62% to 56% with the increase of initial Cd concentration from 0.1 to 0.3 mmol/L. In addition, XRD, XPS, Mössbauer and other characterization results showed that secondary minerals, such as Cd exchange hydroxyapatite (Cd-HAP) and kryzhanovskite (Fe₃(PO₄)₂(OH)₃) were formed in this new system. The established syntrophic system of PSB and DIRB is thus a prospective bioremediation technology for Cd immobilization in sediments and can avoid the potential risk might be caused by the addition of phosphorus-containing materials.

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.

General In Situ Photoactivation Route with IPCE over 80% toward CdS Photoanodes for Photoelectrochemical Applications

Cost-effective photoanodes with remarkable electronic properties are highly demanded for practical photoelectrochemical (PEC) water splitting. The ability to manipulate the surface carrier separation and recombination is pivotal for achieving high PEC performance for water splitting. Here, a facile and economical approach is reported for substantially improving the surface charge separation property of CdS photoanodes through in situ photoactivation, which significantly reduces surface charge recombination through the formation of thiosulfate ion which is favorable to the transfer of photogenerated holes and a uniform nanoporous morphology via the dissolving Cd²⁺ with phosphate ions on the surface of CdS. The resulting CdS electrodes through scalable particle transfer method exhibit nearly tripled photocurrents, with an incident-photon-to-current conversion efficiency (IPCE) at 480 nm exceeding 80% at 0.6 V versus reversible hydrogen electrode (RHE). And the CdS thin films prepared from chemical bath deposition display quadrupled photocurrents after the stir and PEC activation, with an IPCE of 91.7% at 455 nm and 0.6 V versus RHE. With the suppression of photocorrosion in alkaline borate buffer, the activated photoanodes show great stability for solar hydrogen production at the sacrifice of sulfite. This work brings insights into the design of nanoporous metal sulfide semiconductors for solar water splitting.

Effects of metal stabilizers on soil hydraulic characteristics and mobility of cadmium

This study investigated the effect of typical stabilizers on hydraulic properties, immobilization, and leachate characteristics based on the diffusive gradient thin-films technique (DGT) and a leaching experiment. Three types of stabilizers were classified based on various characteristics of soil field capacity (θf), and their immobilization effects were as follows: (i) θf increased and the immobilization of Cd was achieved with nanohydroxyapatite, increasing θf by 19.36% and decreasing the bioavailable Cd by 78.84%; (ii) the increasing θf conversely inhibited cadmium stabilization. Straw biochar averagely promoted θf by 17.39%, while the stabilization was suppressed; (iii) other stabilizers (zeolite, montmorillonite, and sepiolite) had no significant effect on θf and immobilization. It is suggested that stabilization depends on chemical mechanisms and is probably also affected by hydraulic mechanisms. The first types of stabilizers formed precipitates with poor solubility, and the strong affinity of heavy metals to soil particles can account for that the increasing θf had a negligible influence on the dissolution equilibrium of the heavy metals. Attapulgite also belongs to this type. The second and third types of stabilizers primarily adsorbed cadmium through ion exchange, resulting in the relatively easy heavy metal release. Increasing θf facilitated the desorption of heavy metals in the case of the second stabilizer type. However, the inconspicuous change in θf caused by the third stabilizer type had no impact on stabilization. Moreover, Cd leaching was positively correlated with bioavailable Cd and soil permeability. Heavy metal migration induced by colloids less than 90 nm in coarse biochar treatments deserves further research.

Transformation of Pb, Cd, and Zn Minerals Using Phosphates

Heavy metal contamination in soils has become one of the most critical environmental issues. The most efficient in-situ remediation technique is chemical immobilization that uses cost-effective soil amendments such as phosphate compounds to decrease Pb, Cd and Zn accessibility in the contaminated soils. The present study examined the effectiveness of KH2PO4 in immobilizing Pb, Cd and Zn in three samples of contaminated soils collected from ZGH “Bolesław” (Mining and Smelting Plant “Bolesław”). Effectiveness was evaluated using the following methods: a toxicity characteristic leaching procedure (TCLP)-based experiment, sequential extraction, X-ray diffraction analyses (XRD), and scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS). The most efficient percentage reduction of total leachable metal concentration assessed by TCLP was observed for lead (50%–80%), and the least reduction was observed for zinc (1%–17%). The most effective immobilization of stable compounds assessed by sequential extraction was noted for lead, while the weakest immobilization was noted for cadmium. New insoluble mineral phases were identified by SEM-EDS analysis. Cd, Zn, and Pb formed new stable mineral substances with phosphates. The predominant crystal forms were dripstones and groups of needles, which were easily formed by dissolved carbon rock surfaces containing zinc ions. The alkaline nature of the soil and a large number of carbonates mainly influenced the formation of new structures.

Dissolution and Solubility Product of Cd-Fluorapatite [Cd5(PO4)3F] at pH of 2–9 and 25–45°C

Dissolution of the synthetic cadmium fluorapatite [Cd5(PO4)3F] at 25°C, 35°C, and 45°C was experimentally examined in HNO3 solution, pure water, and NaOH solution. The characterization results confirmed that the cadmium fluorapatite nanorods used in the experiments showed no obvious variation after dissolution. During the dissolution of Cd5(PO4)3F in HNO3 solution (pH = 2) at 25°C, the fluoride, phosphate, and cadmium ions were rapidly released from solid to solution, and their aqueous concentrations had reached the highest values after dissolution for <1 h, 1440 h, and 2880 h, respectively. After that, the total dissolution rates declined slowly though the solution Cd/P molar ratios increased incessantly from 1.55∼1.67 to 3.18∼3.22. The solubility product for Cd5(PO4)3F (Ksp) was determined to be 10−60.03 (10−59.74∼10−60.46) at 25°C, 10−60.38 (10−60.32∼10−60.48) at 35°C, and 10−60.45 (10−60.33∼10−60.63) at 45°C. Based on the log Ksp values obtained at an initial pH of 2 and 25°C, the Gibbs free energy of formation for Cd5(PO4)3F (ΔGf0) was calculated to be −4065.76 kJ/mol (−4064.11∼−4068.23 kJ/mol). The thermodynamic parameters for the dissolution process were computed to be 342515.78 J/K·mol, −85088.80 J/mol, −1434.91 J/K·mol, and 2339.50 J/K·mol for ΔG0, ΔH0, ΔS0, and ΔCp0, correspondingly.