Immobilization of cadmium by hydroxyapatite converted from microbial precipitated calcite

For aqueous/soil cadmium immobilization under acid attack, does the hydroxyapatite converted from Pseudochrobactrum sp. DL-1 induced vaterite necessarily show higher stability?

Microbial induced carbonate precipitation (MICP) technology was widely applied to immobilize heavy metals, but its long-term stability is tough to maintain, particularly under acid attack. This study successfully converted Pseudochrobactrum sp. DL-1 induced vaterite (a rare crystalline phase of CaCO3) to hydroxyapatite (HAP) at 30 ℃. The predominant conversion mechanism was the dissolution of CdCO3-containing vaterite and the simultaneous recrystallization of Ca4.03Cd0.97(PO4)3(OH)-containing HAP. For aqueous Cd immobilization, stability test at pH 2.0-10.0 showed that the Cd2+ desorption rate of Cd-adsorbed vaterite (3.96-4.35 ‱) were 7.13-20.84 times greater than that of Cd-adsorbed HAP (0.19-0.61 ‱). For soil Cd immobilization under 60 days of acid-rain erosion, the highest immobilization rate (51.00 %) of exchangeable-Cd and the lowest dissolution rate (-0.18 %) of carbonate-Cd were achieved with 2 % vaterite, while the corresponding rates were 16.78 % and 1.31 % with 2 % HAP, respectively. Furthermore, vaterite outperformed HAP in terms of soil ecological thorough evaluation. In conclusion, for Cd immobilization by MICP under acid attack, DL-1 induced vaterite displayed direct application value due to its exceptional stability in soil and water, while the mineral conversion strategy we presented is useful for further enhancing the stability in water.

Bio-based resources: systemic & circular solutions for (agro)environmental services

The global promotion of decarbonisation through the circular solutions and (re)use of bio-based resources (BBR), i.e. waste streams, notably from the agricultural, forest and municipal sectors has steadily increased in recent decades. Among the transformative solutions offered by BBR, biosolids (BS), biochars (BC), and bioashes (BA) specifically attract scientific attention due to their highly complex organo-mineral matrices, which present significant potential for recovery in the agro-/forest-ecosystems. These materials enhance various soil (i) chemical (pH, macro/micro nutrient concentrations, organic matter content), (ii) physical (porosity, water-air relations, compaction) or (iii) microbial (diversity, activity) properties. Furthermore, some of transformed BBR contribute to a multitude of environmental services such as the remediation of contaminated sites and wastewater treatment, employing cost-effective and eco-friendly approaches that align with circular economy/waste management principles, ultimately contributing to climate change mitigation. However, several challenges impede the widespread utilization/transformation of BBR, including technological limitations in processing and application, concerns about contamination (e.g., PAHs, PCBs, micro/nano plastics present in BS), toxicity issues (e.g., heavy metals in BA or nanoparticles in BC), and regulatory constraints (e.g., non-uniform regulations governing the reuse of BA and BS). Addressing these challenges demands an interdisciplinary and intersectoral approach to fully unlock the potential of BBR in sustainable decarbonisation efforts.

The Capture of Cadmium from Solution during the Replacement of Calcite by Apatite

The capture of cadmium (Cd) from phosphate-containing solutions during the replacement of CaCO3 by phosphate phases such as hydroxylapatite (HAP) and tricalcium phosphate (TCP) has been studied under high and low temperature and pressure conditions using atomic force microscopy, scanning electron microscopy equipped with an X-ray spectrometer and a backscattered electron detector, Raman spectroscopy, and microprobe analysis. Starting with cubes of Carrara Marble (polycrystalline calcite) and single crystals of calcite, a new solid phosphate phase was observed to incorporate Cd from solution, formed under different pressure and temperature conditions tested. Results showed that Cd precipitated in a new phase on the surface of all samples tested. In Carrara Marble, pseudomorphic replacement of CaCO3 is restricted possibly due to kinetic limitation caused by the adsorption of Cd complexes formed in solution at reactive surface sites and the variation of fluid composition inside the sample. However, on the sample surface, this kinetic limitation is less influential, so the new phase could incorporate higher amounts of Cd faster. Furthermore, this reaction at room temperature was found to have similar and/or better Cd-uptake efficiency as HAP and CaCO3 in pure Cd solution through the precipitation of Cd-containing phosphate crystals on the sample surface. Both reactions were able to capture Cd in the precipitating phase structure and could provide a mechanism for simultaneous Cd and phosphate removal from solutions contaminated with both, in industrial or natural settings.

Ammonium nitrogen and phosphorus removal by bacterial-algal symbiotic dynamic sponge bioremediation system in micropolluted water: Operational mechanism and transformation pathways

Construct a bacteria-algae symbiotic dynamic sponge bioremediation system to simultaneously remove multiple pollutants under micro-pollution conditions. The average removal efficiencies of NH4+-N, PO43--P, total nitrogen (TN), and Ca2+ were 98.35, 78.74, 95.64, and 84.92 %, respectively. Comparative studies with Auxenochlorella sp. sponge and bacterial sponge bioremediation system confirmed that NH4+-N and TN were mainly removed by bacterial heterotrophic nitrification - aerobic denitrification (HN-AD). PO43--P was removed by algal assimilation and the generation of Ca3(PO4)2 and Ca5(PO4)3OH, and Ca2+ was removed by algal electron transfer formation of precipitates and microbially induced calcium precipitation (MICP) by bacteria. Algae provided an aerobic environment for the bacterial HN-AD process through photosynthesis, while respiration produced CO2 and adsorbed Ca2+ to promote the formation of calcium precipitates. Immobilization of Ca2+ with microalgae via bacterial MICP helped to lift microalgal photoinhibition. The bioremediation system provides theoretical support for research on micropolluted water treatment while increasing phosphorus recovery pathways.

Penicillium oxalicum induced phosphate precipitation enhanced cadmium (Cd) immobilization by simultaneously accelerating Cd biosorption and biomineralization

Soil cadmium (Cd) is immobilized by the progressing biomineralization process as microbial induced phosphate precipitation (MIPP), which is regulated by phosphate (P) solubilizing microorganisms and P sources. However, little attention has been paid to the implications of Cd biosorption during MIPP. In this study, the newly isolated Penicillium oxalicum could immobilize 5.4-12.6 % of Cd2+, while the presence of hydroxyapatite (HAP) considerably enhanced Cd2+ immobilization in P. oxalicum and reached over 99 % Cd2+ immobilization efficiency within 7 days. Compared to P. oxalicum mono inoculation, MIPP dramatically boosted Cd biosorption and biomineralization efficiency by 71 % and 16 % after 96 h cultivation, respectively. P. oxalicum preferred to absorbing Cd2+ and reaching maximum Cd2+ biosorption efficiency of 87.8 % in the presence of HAP. More surface groups in P. oxalicum and HAP mineral involved adsorption which resulted in the formation of Cd-apatite [Ca8Cd2(PO4)6(OH)2] via ion exchange. Intracellular S2-, secreted organic acids and soluble P via HAP solubilization complexed with Cd2+, progressively mineralized into Cd5(PO4)3OH, Cd(H2PO4)2, C4H6CdO4 and CdS. These results suggested that Cd2+ immobilization was enhanced simultaneously by the accelerated biosorption and biomineralization during P. oxalicum induced P precipitation. Our findings revealed new mechanisms of Cd immobilization in MIPP process and offered clues for remediation practices at metal contaminated sites.

Statistical modelling, optimization, and mechanistic exploration of novel ureolytic Enterobacter hormaechei IITISM-SA3 in cadmium immobilization under microbial inclusive and cell-free conditions through microbially induced calcite precipitation

The study aimed to evaluate the potential of a novel isolated ureolytic Enterobacter hormaechei IITISM-SA3 in cadmium bioremoval through MICP. The optimization and modelling of the biotic and abiotic factors affecting the process of mineralization were also performed. In addition, the underlying mechanism of MICP-driven Cd mineralization under microbial-inclusive and cell-free conditions was revealed and supported through the characterization of the bio-precipitates obtained using various characterization techniques. The results indicated that the isolate could remove 97.18% Cd2+ of 11.4 ppm under optimized conditions of 36.86 h, pH 7.63, and biomass dose of 1.75 ml. Besides, the presence and absence of bacterial cells were found to influence both the morphologies and crystalline structures of precipitates. The precipitates obtained under microbial-inclusive conditions showed typical rhombohedral crystalline structures of the composition comprising CaCO3, CdCO3, and 0.67Ca0.33CdCO3. However, the crystalline nature of the precipitate reduced to a nano-sized granular structure in cell-free media. Unlike the cadmium mineralization process under microbial-inclusive media, where bacterial cells serve as nucleation sites for crystallization, the carbonate precipitation effectively captures Cd2+ through co-precipitation, chemisorption, or alternative mechanisms involving interactions between metal ions and CaCO3 under cell-free conditions. The findings presented suggest that using cell-free culture supernatant enriched with carbonate ions provides an avenue that could be harnessed for sustainable metal remediation.

Biogenic calcium improved Cd2+ and Pb2+ immobilization in soil using the ureolytic bacteria Bacillus pasteurii

Bioremediation based on microbial-induced carbonate precipitation (MICP) was conducted in cadmium and lead contaminated soil to investigate the effects of MICP on Cd and Pb in soil. In this study, soil indigenous nitrogen was shown to induce MICP to stabilize heavy metals without inputting exogenous urea. The results showed that applying Bacillus pasteurii coupled with CaCl2 reduced Cd and Pb bioavailability, which could be clarified through the proportion of exchangeable Cd and Pb in soil decreasing by 23.65 % and 12.76 %, respectively. Moreover, B. pasteurii was combined separately with hydroxyapatite (HAP), eggshells (ES), and oyster shells (OS) to investigate their effects on soil heavy metals' chemical fractions, toxicity characteristic leaching procedure (TCLP)-extractable Cd and Pb as well as enzymatic activity. Results showed that applying B. pasteurii in soil significantly decreased the heavy metals in the exchangeable fraction and increased them in the carbonate phase fraction. When B. pasteurii was combined with ES and OS, the content of carbonate-bound Cd increased by 114.72 % and 118.81 %, respectively, significantly higher than when B. pasteurii was combined with HAP, wherein the fraction of carbonate-bound Cd increased by 86 %. The combination of B. pasteurii and biogenic calcium effectively reduced the leached contents of Cd and Pb in soil, and the TCLP-extractable Cd and Pb fractions decreased by 43.88 % and 30.66 %, respectively, in the BP + ES group and by 52.60 % and 41.77 %, respectively, in the BP + OS group. This proved that MICP reduced heavy metal bioavailability in the soil. Meanwhile, applying B. pasteurii and calcium materials significantly increased the soil urease enzyme activity. The microstructure and chemical composition of the soil samples were studied, and the results from scanning electron microscope, Fourier transform infra-red spectroscopy, and X-ray diffraction demonstrated the MICP process and identified the formation of CaCO3, Ca0.67Cd0.33CO3, and PbCO3 in heavy metal-contaminated soil.

Microbial induced carbonate precipitation for remediation of heavy metals, ions and radioactive elements: A comprehensive exploration of prospective applications in water and soil treatment

Improper disposal practices have caused environmental disruptions, possessing by heavy metal ions and radioactive elements in water and soil, where the innovative and sustainable remediation strategies are significantly imperative in last few decades. Microbially induced carbonate precipitation (MICP) has emerged as a pioneering technology for remediating contaminated soil and water. Generally, MICP employs urease-producing microorganisms to decompose urea (NH2CONH2) into ammonium (NH4+and carbon dioxide (CO2), thereby increasing pH levels and inducing carbonate precipitation (CO32-), and effectively removing remove contaminants. Nonetheless, the intricate mechanism underlying heavy metal mineralization poses a significant challenge, constraining its application in contaminants engineering, particularly in the context of prolonged heavy metal leaching over time and its efficacy in adverse environmental conditions. This review provides a comprehensive idea of recent development of MICP and its application in environmental engineering, examining metabolic pathways, mineral precipitation mechanisms, and environmental factors as well as providing future perspectives for commercial utilization. The use of ureolytic bacteria in MICP demonstrates cost-efficiency, environmental compatibility, and successful pollutant abatement over tradition bioremediation techniques, and bio-synthesis of nanoparticles. limitations such as large-scale application, elevated Ca2+levels in groundwater, and gradual contaminant release need to be overcome. The possible future research directions for MICP technology, emphasizing its potential in conventional remediation, CO2 sequestration, bio-material synthesis, and its role in reducing environmental impact for long-term economic benefits.

Mechanisms of ammonia, calcium and heavy metal removal from nutrient-poor water by Acinetobacter calcoaceticus strain HM12

An Acinetobacter calcoaceticus strain HM12 capable of heterotrophic nitrification-aerobic denitrification (HN-AD) under nutrient-poor conditions was isolated, with an ammonia nitrogen (NH4+-N) removal efficiency of 98.53%. It can also remove heavy metals by microbial induced calcium precipitation (MICP) with a Ca2+ removal efficiency of 75.91%. Optimal conditions for HN-AD and mineralization of the strain were determined by kinetic analysis (pH = 7, C/N = 2.0, Ca2+ = 70.0 mg L-1, NH4+-N = 5.0 mg L-1). Growth curves and nitrogen balance elucidated nitrogen degradation pathways capable of converting NH4+-N to gaseous nitrogen. The analysis of the bioprecipitation showed that Zn2+ and Cd2+ were removed by the MICP process through co-precipitation and adsorption (maximum removal efficiencies of 93.39% and 80.70%, respectively), mainly ZnCO3, CdCO3, ZnHPO4, Zn3(PO4)2 and Cd3(PO4)2. Strain HM12 produces humic and fulvic acids to counteract the toxicity of pollutants, as well as aromatic proteins to increase extracellular polymers (EPS) and promote the biomineralization process. This study provides a experimental evidence for the simultaneous removal of multiple pollutants from nutrient-poor waters.

Immobilization of Cd2+ in an aqueous environment using a two-step microbial-induced carbonate precipitation method

Previous researches indicate that the potent toxicity of cadmium hinders the efficacy of the microbial-induced carbonate precipitation (MICP) process for bioremediation of Cd2+ in aqueous environment. Increasing urea and calcium resource doses, introducing synergists, and utilizing urease-producing consortia can improve bio-immobilization performance of MICP. However, such measures may incur cost increases and/or secondary contamination. This study first verifies the substantial biotoxicity of Cd2+ for urease activity and then analyzes the practical limitation of traditional MICP using Bacillus pasteurii for bioremediation of Cd2+ in an aqueous environment containing 1-40 mM Cd2+ by a series tube tests and numerical simulation. Subsequently, a two-step MICP method, which separates urea hydrolysis and heavy metal precipitation, is introduced in this study to eliminate the inhibitory effect of heavy metal on urease activity. The concentrations of ammonium, Cd2+, and pH were monitored over time. The results indicate that the urease expression in B. pasteurii can be significantly inhibited by Cd2+ particularly at the concentration ranging from 10 to 40 mM, leading to pretty low efficacy of traditional MICP for bioremediation of Cd2+ (Cd2+ removal rate as low as 21.55-38.47% when the initial Cd2+ concentration = 40 mM). In contrast, when the two-step MICP method is applied, the Cd2+ can be almost completely immobilized, even though the concentration ratio of urea to Cd2+ is as low as 1.5:1.0, which is close to the theory minimum concentration ratio for the complete precipitation of carbonate to cadmium ions(1.0:1.0). Therefore, the cost-effective, environmentally sustainable, and straightforward two-step MICP method holds great potential for application in the bioremediation of Cd2+-contaminated solutions in high concentration.

Assessment and machine learning prediction of heavy metals fate in mining farmland assisted by Positive Matrix Factorization

The accurate pollutant prediction by Machine Learning (ML) is significant to efficient environmental monitoring and risk assessment. However, application of ML in soil is under studied. In this study, a Positive Matrix Factorization (PMF) assisted prediction method was developed with Support Vector Machine (SVM) and Random Forest (RF) for heavy metals (HMs) prediction in mining farmland. Principal Component Analysis (PCA) and Redundancy Analysis (RDA) were selected to pretreat data. Experiment results illustrated Cd was the main pollutant with heavy risks in the study area and Pb was easy to migrate. The method effects of HMs total concentration predicting were PMF > Simple > PCA > PCA - PMF, and RF predicted better than SVM. Data pretreatment by RDA prior inspection improved the model results. Characteristic HMs Tessier fractions prediction received good effects with average R value as 0.86. Risk classification prediction performed good in Cd, Cu, Ni and Zn, however, Pb showed weak effect by simple model. The best classifier method for Pb was PMF - RF method with relatively good effect (Area under ROC Curve = 0.896). Overall, our study suggested the combination between PMF and ML can assist the prediction of HMs in soil. Spatial weighted attribute of HMs can be provided by PMF.

Reduction of bioavailability and phytotoxicity effect of cadmium in soil by microbial-induced carbonate precipitation using metabolites of ureolytic bacterium Ochrobactrum sp. POC9

The application of ureolytic bacteria for bioremediation of soil contaminated with heavy metals, including cadmium (Cd), allows for the efficient immobilization of heavy metals by precipitation or coprecipitation with carbonates. Microbially-induced carbonate precipitation process may be useful also in the case of the cultivation of crop plants in various agricultural soils with trace but legally permissible Cd concentrations, which may be still uptaken by plants. This study aimed to investigate the influence of soil supplementation with metabolites containing carbonates (MCC) produced by the ureolytic bacterium Ochrobactrum sp. POC9 on the Cd mobility in the soil as well as on the Cd uptake efficiency and general condition of crop plants (Petroselinum crispum). In the frame of the conducted studies (i) carbonate productivity of the POC9 strain, (ii) the efficiency of Cd immobilization in soil supplemented with MCC, (iii) crystallization of cadmium carbonate in the soil enriched with MCC, (iv) the effect of MCC on the physico-chemical and microbiological properties of soil, and (v) the effect of changes in soil properties on the morphology, growth rate, and Cd-uptake efficiency of crop plants were investigated. The experiments were conducted in soil contaminated with a low concentration of Cd to simulate the natural environmental conditions. Soil supplementation with MCC significantly reduced the bioavailability of Cd in soil with regard to control variants by about 27-65% (depending on the volume of MCC) and reduced the Cd uptake by plants by about 86% and 74% in shoots and roots, respectively. Furthermore, due to the decrease in soil toxicity and improvement of soil nutrition with other metabolites produced during the urea degradation (MCC), some microbiological properties of soil (quantity and activity of soil microorganisms), as well as the general condition of plants, were also significantly improved. Soil supplementation with MCC enabled efficient Cd stabilization and significantly reduced its toxicity for soil microbiota and plants. Thus, MCC produced by POC9 strain may be used not only as an effective Cd immobilizer in soil but also as a microbe and plant stimulators.

A promising approach for simultaneous removal of ammonia and multiple heavy metals from landfill leachate by carbonate precipitating bacterium

The effective and cheap remediation of ammonia (NH⁺₄) and multiple heavy metals from landfill leachate is currently a grand challenge. In this study, Paracoccus denitrificans AC-3, a bacterial strain capable of heterotrophic nitrification aerobic denitrification (HNAD) and carbonate precipitation, exhibited good tolerance to a variety of heavy metals and could remove 99.70% of NH⁺₄, 99.89% of zinc (Zn²⁺), 97.42% of cadmium (Cd²⁺) and 46.19% of nickel (Ni²⁺) simultaneously after 24 h of incubation. The conversion pathway of NH⁺₄ by strain AC-3 was dominated by assimilation (84.68%), followed by HNAD (14.93%), and the increase in environmental pH was mainly dependent on assimilation rather than HNAD. Calcium (Ca²⁺) primarily played four roles in heavy metal mineralization: (ⅰ) improving bacterial tolerance to heavy metals; (ⅱ) ensuring the HNAD capacity of strain AC-3; (ⅲ) co-precipitating with heavy metals; and (ⅳ) precipitating into calcite to adsorb heavy metals. The heavy metals removal mechanisms were mainly calcite adsorption and formation of carbonate and hydroxide precipitation for Zn²⁺, co-precipitation for Cd²⁺, and adsorption for Ni²⁺. The Zn²⁺, Cd²⁺, and Ni²⁺ precipitates displayed unique morphologies. This research provided a promising biological resource for the simultaneous remediation of NH⁺₄ and heavy metals from landfill leachate.

Immobilization of Cd in the soil of mining areas by FeMn oxidizing bacteria

Microorganisms are widely used in large-scale pollution remediation due to their rapid reproduction and low cost. In this study, bioremediation batch experiments and characterization methods were adopted to investigate the mechanism of FeMn oxidizing bacteria on the immobilization of Cd in mining soil. The results showed that the FeMn oxidizing bacteria successfully reduced 36.84 % of the extractable Cd in the soil. The exchangeable forms, carbonate-bound forms, and organic-bound forms of Cd in the soil decreased by 11.4 %, 8 %, and 7.4 %, respectively, due to the addition of FeMn oxidizing bacteria, while FeMn oxides-bound and residual forms of Cd increased by 19.3 % and 7.5 %, as compared to the control treatments. The bacteria promotes the formation of amorphous FeMn precipitates such as lepidocrocite and goethite, which have high adsorption capacity on soil Cd. The oxidation rates of Fe and Mn in the soil treated with the oxidizing bacteria reached 70.32 % and 63.15 %, respectively. Meanwhile, the FeMn oxidizing bacteria increased soil pH and decreased soil organic matter content, further decreasing the extractable Cd in the soil. The FeMn oxidizing bacteria have the potential to be used in large mining areas to assist in the immobilization of heavy metals.

Periplasmic space is the key location for Pb(II) biomineralization by Burkholderia cepacia

Phosphate solubilizing bacteria (PSB) induced phosphate precipitation is considered as an effective method for Pb(II) removal through the formation of stable Pb(II)-phosphate compound, but the location of end-products is still unclear. Herein, the PSB strain of Burkholderia cepacia (B. cepacia) coupled with the hydroxyapatite (HAP) was used in this study to investigate the Pb(II) removal mechanism and the biomineralization location. The dissolving phosphate of three particle sizes of HAP and Pb(II) resistant capabilities, and the effect factors such as HAP dosage, initial concentrations of Pb(II), pH, temperature, and different treatments were determined. The results indicated that the highest soluble phosphate could reach 224.85 mg/L in a 200 nm HAP medium and the highest removal efficiency of Pb(II) was about 96.32 %. Additionally, it was interesting that Pb(II) was mainly located in the periplasmic space through the cellular distribution experiment, which was further demonstrated by scanning electron microscope (SEM) and transmission electron microscopy (TEM). Besides, the characterization results showed that the functional groups such as amide, hydroxy, carboxy and phosphate played an important role in Pb(II) biomineralization, and the free Pb(II) in aqueous solution could be transformed into pyromorphite through phosphate dissolution, extracellular adsorption/complexation, and intracellular precipitation.

Application of microbial-induced carbonate precipitation (MICP) techniques to remove heavy metal in the natural environment: A critical review

The occurrence of imbalanced heavy metals concentration due to anthropogenic hindrances in the aquatic and terrestrial environment has become a potential risk to life after circulating through different food chains. The microbial-induced carbonate precipitation (MICP) method has gradually received great attention from global researchers but the underlying mechanism of heavy metal mineralization is not well-understood and challenging, limiting the applications in wastewater engineering. This paper reviews the metabolic pathways, mechanisms, operational factors, and mathematical/modeling approaches in the MICP process. Subsequently, the recent advancement in MICP for the remediation of heavy metal pollution is being discussed. In the follow-up, the key challenges and prospective associated with technical bottlenecks of MICP method are elaborated. The prospective study reveals that MICP technology could be efficiently used to remediate heavy metal contaminants from the natural environment in a cost-effective way and has the potential to improve soil properties while remediating heavy metal contaminated soil.

Soil heavy metal pollution from Pb/Zn smelting regions in China and the remediation potential of biomineralization

Smelting activities pose serious environmental problems due to the local and regional heavy metal pollution in soils they cause. It is therefore important to understand the pollution situation and its source in the contaminated soils. In this paper, data on heavy metal pollution in soils resulting from Pb/Zn smelting (published in the last 10 years) in China was summarized. The heavy metal pollution was analyzed from a macroscopic point of view. The results indicated that Pb, Zn, As and Cd were common contaminants that were present in soils with extremely high concentrations. Because of the extreme carcinogenicity, genotoxicity and neurotoxicity that heavy metals pose, remediation of the soils contaminated by smelting is urgently required. The primary anthropogenic activities contributing to soil pollution in smelting areas and the progressive development of accurate source identification were performed. Due to the advantages of biominerals, the potential of biomineralization for heavy metal contaminated soils was introduced. Furthermore, the prospects of geochemical fraction analysis, combined source identification methods as well as several optimization methods for biomineralization are presented, to provide a reference for pollution investigation and remediation in smelting contaminated soils in the future.

Effect of phosphate on cadmium immobilized by microbial-induced carbonate precipitation: Mobilization or immobilization?

Microbial-induced carbonate precipitation (MICP) is a promising technology to immobilize/remediate heavy metals (HMs) like cadmium (Cd). However, the long-term stability of MICP-immobilized HMs is unclear, especially in farmland where chemical fertilization is necessary. Therefore, we performed MICP treatment on soils contaminated with various Cd compounds (CdCO₃, CdS, and CdCl₂) and added diammonium phosphate (DAP) to explore the impact of phosphate on the MICP-immobilized Cd. The results showed that MICP treatment was practical to immobilize the exchangeable Cd but to mobilize the carbonate and Fe/Mn oxide-bound Cd. After applying DAP, soil pH declined due to ammonium nitrification. At high P/Ca molar ratios (1/2 and 1), partial previously immobilized Cd was released due to the carbonate dissolution. Contrarily, exchangeable Cd transformed to less mobilizable Fe/Mn oxide-bound at low P/Ca molar ratios (1/4 and 1/8). Meanwhile, other treatments were also helpful in avoiding the release of immobilized Cd, such as applying non-ammonium phosphate and adding lime material after soil acidification. Our investigation suggested that the long-term stability of HMs in remediated sites should be carefully evaluated, especially in agricultural areas with phosphate and nitrogen fertilizer input.

The fate of Cd during the replacement of Cd-bearing calcite by calcium phosphate minerals

Carbonate-bound speciation is a critical sink of potentially toxic elements (PTEs) like cadmium (Cd) in soil and sediment. In a phosphate-rich environment, carbonate minerals could be replaced by phosphate minerals such as dicalcium phosphate dihydrate (DCPD, also known as brushite), octacalcium phosphate (OCP), and hydroxylapatite (HAP). Currently, it is unclear the migration and fate of PTEs during the replacement of PTEs-bearing carbonates by HAP and related intermediate minerals. Therefore, we synthesized Cd-bearing calcite by the coprecipitation method and converted it to DCPD, OCP, and HAP to investigate the redistribution and fate of Cd. The results showed that Cd incorporation in calcite significantly inhibited their replacement by DCPD and OCP, respectively. 1.26% of Cd in calcite was released into the solution when DCPD replaced calcite, and subsequently, most of the released Cd was recaptured by OCP. Significantly, the released Cd was below 0.05‰ when all the solid converted to HAP. These results suggested that with the application of phosphate fertilizer in alkaline soil, the secondary calcium phosphate minerals could control the environmental behavior of Cd.

Mechanism of microbiologically induced calcite precipitation for cadmium mineralization

Microbiologically induced calcite precipitation (MICP) technology shows potential for remediating heavy metal pollution; however, the underlying mechanism of heavy metal mineralization is not well-understood, limiting the application of this technology. In this study, we targeted Cd contamination (using 15:1, 25:1, and 50:1 Ca²⁺/Cd²⁺ molar ratios) and showed that the ureolytic bacteria Sporosarcina ureilytica ML-2 removed >99.7 % Cd²⁺ with a maximum fixation capacity of 75.61 mg-Cd/g-CaCO₃ and maximum precipitation production capacity of 135.99 mg-CaCO₃/mg-cells. Quantitative PCR analysis showed that Cd²⁺ inhibited the expression of urease genes (ureC, ureE, ureF, and ureG) by 70 % in the ML-2 strain. Additionally, the pseudo-first-order kinetics model (R² = 0.9886), intraparticle diffusion model (R² = 0.9972), and Temkin isotherm model (R² = 0.9828) described the immobilization process of Cd²⁺ by bio calcite in MICP-Cd system. The three Cd²⁺ mineralization products generated by MICP were attributed to surface precipitation (Cd²⁺ → Cd(OH)₂), direct binding with the CO₃^2-/substitution calcium site of calcite (Cd²⁺ → CdCO₃, otavite), and calcite lattice vacancy anchors (Cd²⁺ → (Ca_xCd_1-x)CO₃). Our findings improve the understanding of the mechanisms by which MICP can achieve in situ stabilization of heavy metals.

Remediation of chromium, zinc, arsenic, lead and antimony contaminated acidic mine soil based on Phanerochaete chrysosporium induced phosphate precipitation

Microbial induced phosphate precipitation (MIPP) is an advanced bioremediation technology to reduce the mobility and bioavailability of heavy metals (HMs), but the high level of HMs would inhibit the growth of phosphate solubilizing microbes. This study proposed a new combination system for the remediation of multiple HMs contaminated acidic mine soil, which included hydroxyapatite (HAP) and Phanerochaete chrysosporium (P. chrysosporium, PC) that had high phosphate solubilizing ability and HMs tolerance. Experimental data suggested that in HAP/PC treatment after 35 d of remediation, labile Cr, Zn and As could be transformed into the stable fraction with the maximum immobilization efficiencies increased by 53.01 %, 22.43 %, and 35.65 %, respectively. The secretion of organic acids by P. chrysosporium was proved to promote the dissolution of HAP. Besides, the pH value, available phosphorus (AP) and organic matter (OM) increased in treated soil than in original soil, which also indicated the related dissolution-precipitation mechanism of HMs immobilization. Additionally, characterization results revealed that adsorption and ion exchange also played an important role in the remediation process. The overall results suggested that applying P. chrysosporium coupled with HAP could be considered as an efficient strategy for the remediation of multiple HMs contaminated mine soil and laid a foundation for the future exploration of soil microenvironment response during the remediation process.

One stone two birds: Bone char as a cost-effective material for stabilizing multiple heavy metals in soil and promoting crop growth

Remediation of farmland soils contaminated with high levels of multiple heavy metals near PbZn smeltery is still a great challenge. It is of great significance to find cost-effective green remediation technologies for stabilization of multiple heavy metals in soil and reduce metal accumulation in crops with ensured yield. In this study, we demonstrated that bone char (BC) is an effective heavy metal stabilizer which can substantially increase residual fractions of heavy metals and reduce metal accumulation in pea (Pisum sativum) with its enhanced growth. We chose the soils contaminated with high levels of Pb, Zn, Cu and Cd near the Baiyin PbZn smeltery as the tested soil. After 2 months of BC application, the relative mobile fractions (non-residual fractions) of Cu, Zn, Pb and Cd in the contaminated soil decreased while the residual fraction increased significantly. The leachability of Cu, Zn, Pb and Cd decreased by 91.2%, 38.6%, 67.6% and 54.3%, respectively compared with the control. BC application remarkably promoted pea growth and reduced accumulation of heavy metals in shoots. The mechanisms for stabilization of multiple heavy metals BC include ion exchange, surface complexation and subsequent mineralization, accompanied with release of Ca and phosphate. The immobilization of heavy metals led to their reduced toxicity to plant, and thus increased pea growth. The results show that BC is a cost-effective and sustainable heavy metal stabilizer with phosphate fertilization function. It can simultaneously immobilize multiple heavy metals in soil and facilitate crop production.

Effect of application mode (capping and amendment) on the control of cadmium release from sediment by apatite/calcite mixture and its phosphorus release risk

In this research, the influence of application mode (capping and amendment) on the control of cadmium (Cd) liberation from sediment by apatite/calcite mixture and its phosphorus release risk were investigated. The results showed that calcite addition had a limited effect on the speciation of Cd in sediment, but apatite addition had a significant impact on the fractionation of Cd in sediment. Apatite amendment could effectively immobilize the most readily mobilized Cd by transferring the acid-soluble fraction to the reducible and residual fractions. Apatite addition also could effectively reduce the concentration of toxicity characteristic leaching procedure (TCLP)-leachable Cd in sediment, and apatite had a much higher reduction efficiency of TCLP-leachable Cd than calcite. Apatite/calcite mixture capping could reduce the risk of Cd liberation from sediment into the overlying water, and the controlling efficiency of apatite/calcite mixture capping was higher than that of apatite/calcite mixture amendment. The effect of apatite/calcite mixture addition on the concentration of reactive soluble phosphorus (SRP) in the overlying water was limited. The introduction of calcite into the apatite capping layer could lower the risk of phosphorus release from apatite to the overlying water as compared to single apatite capping. However, the apatite/calcite mixture capping layer still had a relatively high risk of phosphorus liberation into the overlying water. Results of this work suggest that apatite/calcite mixture has a high potential to be used as a capping material to control Cd release from sediment from the perspective of controlling efficiency and application convenience.

Process optimization for the rapid conversion of calcite into hydroxyapatite microspheres for chromatographic applications

Microsphere hydroxyapatite (HAp) is widely used in various biomedical and chromatographic applications. The work described in this manuscript focuses on a dissolution precipitation method for production of HAp microspheres. This method overcomes certain drawbacks of conventional preparation methods used for HAp preparation, which produce polydisperse particles and are time-consuming and expensive. In the present work, the calcium carbonate (calcite) particles were directly and rapidly converted into HAp microspheres using an inexpensive dissolution precipitation method. The effects of the reaction temperature, time, and mechanical stirring rates were studied, and the reaction parameters were optimized. As confirmed by the XRD studies, the higher reaction temperature and time promote complete HAp conversion, while calcite residues were observed for lower temperatures and times. SEM images show the influence of reaction parameters on the surface microstructure of the microspheres produced. It was observed that the HAp microspheres undergo disintegration at a higher stirring rate. The reaction parameters optimized in this work were ideal for preparing HAp microspheres. The resultant HAp particles were utilized as matrices for chromatographic separation of protein mixtures.

Enhanced immobilization of uranium(VI) during the conversion of microbially induced calcite to hydroxylapatite

Carbonate-bound uranium (U) is critical in controlling the migration of U in circumneutral to alkaline conditions. The potential release risk of carbonate-bound U should be concerned due to the contribution of mineral replacement. Herein, we explored the fate of U during the conversion process from microbial-induced calcite to hydroxylapatite (HAP) and investigated the phase and morphology evolution of minerals and the immobilization efficiency, distribution, and stability of U. The results showed that most calcite could convert to HAP during the conversion process. The aqueous residual U was below 1.0 mg/L after U-HAP formation, and the U removal efficiencies were enhanced by 20.0-74.4% compared to the calcite precipitation process. XRD and TEM results showed that the products were a mixture of HAP and uramphite. The elemental mapping results showed that most U concentrated on uramphite while a handful of U distributed homogeneously in calcite and HAP matrixes. The stability test verified that U-bearing HAP decreased the U solubility by 98-100% relative to calcite due to the uramphite formation and U incorporation into HAP. Our findings demonstrated that the combinations of microbial-induced calcite precipitation and calcite-HAP conversion could facilitate the U immobilization in treating radioactive wastewater and soil.

Fungal-sponge composite carriers coupled with denitrification and biomineralization bacteria to remove nitrate, calcium, and cadmium in a bioreactor

The coexistence of nitrate (NO₃^--N) and heavy metals in the aquatic environment causes harm to both the aquatic ecosystem and human health. Here, fungal-sponge composite carriers (FSC) were assembled and immobilized with strain WZ39 in a bioreactor to remove NO₃^--N, Ca²⁺, and Cd²⁺. Stable bioreactor performance under heavy metal pressure was achieved. The highest removal efficiencies of NO₃^--N, Ca²⁺, and Cd²⁺ reached 100, 71.81, and 92.50%, respectively. Bacteria and precipitates were found in fungal mycelium and sponge. The precipitates composed of Ca_3.9(Ca_4.7Cd_0.7)(PO₄)₆(OH)_1.8, CaCO₃, and CdCO₃. Fluorescence excitation-emission matrix (EEM) and flow cytometric (FCM) analysis indicated bacteria in FSC exhibited a strong metabolic activity and high percentage of intact cells under heavy metal stress. High-throughput sequencing results showed Pseudomonas sp. WZ39 played a major role in the bioreactor. The potential functions associated with metabolism, heavy metal transfer, and biofilm formation had high relative abundance in the bioreactor.

Mechanically activated calcium carbonate and zero-valent iron composites for one-step treatment of multiple pollutants

The growing presences of conventional and emerging contaminants make the wastewater treatment increasingly difficult and expensive on a global scale. ZVI tends to be an expectable material for the detoxification of some difficult contaminants such as chlorinated solvents and nitroaromatics. In this work, together use with calcium carbonate (CaCO₃), which serves as a green supporter to ZVI and also direct participant toward the purification process, has been carried out by cogrinding to give a synergistic effect, particularly for treating multiple pollutants including both inorganic and organic compositions. Based on a set of analytical methods of XRD, FTIR, SEM, XPS, and other test methods, the activation mechanism of the ball milling process and the removal performances of the prepared composites were examined. The results prove that the mechanically activated calcium carbonate and ZVI composite samples exhibited extremely high removal capacity on a variety of pollutants contaminated water. The decolorization of azo dyes is mainly attributed to the breaking of chromogenic functional group nitrogen and nitrogen double bonds, and the removal mechanism of aromatic series occurs through a hydrogenation substitution reaction. As to the inorganic pollutant removals, besides the efficient heavy metal ion precipitations, phosphate and fluoride ions are co-precipitated through the formation of fluorapatite to achieve a simultaneous and synergistic removal effect. Under the optimal reaction conditions, the concentration of PO₄^3- is reduced from 250 to 0 mg/L, and that of F^- is reduced from 51.07 to 1.20 mg/L. The prepared composite sample of ZVI rand calcium carbonate allowed simultaneous removals of both inorganic and organic pollutants, simplifying the remediation process of complicated multiple contaminations.

Remediation of soil cadmium pollution by biomineralization using microbial-induced precipitation: a review

In recent years, with industrial pollution and the application of agricultural fertilizers with high cadmium (Cd) content, soil Cd pollution has become increasingly serious. A large amount of Cd is discharged into the environment, greatly endangering the stability of the ecological environment and human health. The use of microorganisms to induce Cd precipitation and mineralization is an important bioremediation method. Itis highly efficient, has a low cost, enables environmental protection, and convenient to operate. This article summarizes the pollution status, pollution source, biological toxicity and existing forms of Cd, as well as the biomineralization mechanism of microbial induced Cd(II) precipitation, mainly including microbial-induced carbonate precipitation, microbial-induced phosphate precipitation and microbial-induced sulfide precipitation. Factors affecting the bioremediation of Cd, such as pH, coexisting ions, and temperature, are introduced. Finally, the key points and difficulties of future microbe-induced Cd(II) biomineralization research are highlighted, providing a scientific basis and theoretical guidance for the application of microbe-induced Cd(II) immobilization in soil.

Microbiologically induced calcite precipitation technology for mineralizing lead and cadmium in landfill leachate

As a new bioremediation technology for toxic metals, microbiologically induced calcite precipitation (MICP) is gradually becoming a research focus. This study investigated the application of MICP to mineralize toxic metals (lead and cadmium) in landfill leachate for the first time. In the experiment of remediating synthetic landfill leachate (SLL) contaminated by Pb²⁺, 100% of the 20 mg/L Pb²⁺ was removed when the maximum urease activity was only 20.96 U/ml. Scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) and laser particle size characterizations of the precipitates indicate the formation of agglomerated square particles, 76.9% of which had sizes that ranged from 33.93 to 57.06 μm. Fourier transform infrared spectroscopic and X-ray diffraction analyses confirmed that the precipitates consisted predominantly of calcite crystals, and the unit cell lattice constants of the precipitates (a = b = 4.984 Å, c = 17.171 Å) matched those of calcite, while lead was fixed as hydrocerussite. In addition, the Pb-MICP precipitates were stable under continuous acid degradation (pH = 5.5), and only 1.76% of the lead was released after 15 days. In the verification test of toxic metals remediation in a real landfill leachate (RLL), all of the Pb²⁺ and Cd²⁺ (initial concentrations: Pb²⁺ = 25 mg/L; Cd²⁺ = 5.6205 mg/L) was mineralized simultaneously, which further confirmed the feasibility of MICP for toxic metal remediation in landfill leachate. However, optimizing the urea dosage and combining the ammonium recovery are necessary strategies required for improving the economic and environmental benefits of the MICP process.

Kinetic model of a newly-isolated Lysinibacillus sp. strain YL and elastic properties of its biogenic CaCO3 towards biocement application

BACKGROUND

Biocement, calcifying bacteria-incorporated cement, offers an environmentally-friendly way to increase the cement lifespan. This work aimed to investigate the potential use of Lysinibacillus sp. strain YL towards biocement application in both theoretical and experimental ways.

METHODS AND RESULTS

Strain YL was grown using calcium acetate (Ca(C₂ H₃ O₂ )₂ ), calcium chloride (CaCl₂ ) and calcium nitrate (Ca(NO₃ )₂ ). Maximum bacterial growth of ~0.09 hr-1 and the highest amount of CaCO₃ precipitation of ~8.0 g/L were obtained when using Ca(C₂ H₃ O₂ )₂ . The SEM and XRD results confirmed that biogenic CaCO₃ were calcites. The bulk, Young's and shear moduli of biogenic CaCO₃ calculated via the VRH approximation were ~1.5-2.3 times larger than those of ordinary Portland cement. The Poisson's ratio was 0.382 and negative in some directions, suggesting its ductility and auxetic behaviors. The new model was developed to explain the growth kinetic of strain YL in the presence of Ca(C₂ H₃ O₂ )₂ , whose concentration was optimized for biocement experiments. Strain YL could increase the compressive strength of cement up to ~50% higher than that of the uninoculated cement.

CONCLUSION

Strain YL is a promising candidate for biocement applications. This work represents the trials of experiments and models allowing quantitatively comparison with large-scale production in the future.

Polymer-coated manganese fertilizer and its combination with lime reduces cadmium accumulation in brown rice (Oryza sativa L.)

Manganese (Mn) has the potential to reduce cadmium (Cd) uptake by rice; however, the efficiency depends on its soil availability. Therefore, this study designed a slow-release Mn fertilizer by employing a polyacrylate coating. Pot trials were conducted to study the effects of coated-Mn and uncoated-Mn alone or in combination with lime on the dynamics of soil dissolved-Mn and available Cd, and the transportation of Mn and Cd within rice. The results showed that coated-Mn declined the release of Mn until the 7th day of application; however, it consistently supplied more dissolved-Mn than uncoated-Mn. As a result, coated-Mn induced a greater Cd reduction (45.8%) in brown rice than uncoated-Mn (9.7%). The total Cd of rice and its proportion in brown rice were greatly reduced by coated-Mn, indicating the inhibition of root uptake and interior transport of Cd. Additionally, lime addition prominently increased the soil pH and decreased the CaCl₂-extractable Cd (90.1-93.9%). However, since lime reduced the soil dissolved-Mn, downregulated the OsHMA3 expression and upregulated the OsNramp5 expression, brown rice Cd was reduced by only 43.0%. The combined addition of lime and coated-Mn alleviated the liming effect on soil Mn and gene expression in roots, thereby reducing brown rice Cd by 71.5%.

Ferric iron incorporation promotes brushite hydrolysis and enhances cadmium immobilization

Dissolution-precipitation processes on the surface of brushite (dicalcium phosphate dihydrate, DCPD) control the migration and transformation of potentially harmful elements (PHEs). The incorporation of impurities could affect the properties of DCPD and its interactions with PHEs. In this study, we synthesized Fe³⁺-bearing DCPD via coprecipitation and investigated the influence of Fe³⁺ incorporation on the crystal structure, hydrolysis process, and Cd removal performance. Fe-bearing DCPD had lattice expansion due to the coupled substitution of Fe³⁺ and NH₄⁺ for Ca²⁺. Therefore, the Cd removal performance of Fe-DCPD was enhanced, with a maximum Cd uptake capacity of 431.6 mg/g, which is 1.77 times that of Fe-free DCPD (244.4 mg/g). Furthermore, Fe-DCPD also exhibited a faster hydrolysis rate, which was up to 2.67 times that of Fe-free DCPD and accelerated Cd's transfer to the stable host mineral, hydroxylapatite. Cd was first caught by the DCPD surface in a weakly crystalline form and then incorporated into the hydroxylapatite structure during crystallization. Based on the X-ray photoelectron spectroscopy and thermogravimetric analysis results, we concluded that the decrease in interstitial water due to Fe incorporation was responsible for accelerating hydrolysis and enhancing Cd immobilization. In all, the incorporation of Fe³⁺ into DCPD could promote its transformation and improve its Cd uptake capacity. Our results suggest that Fe-DCPD could be a promising candidate for environmental remediation.

Hydroxyapatite biomaterial production from chicken (femur and beak) and fishbone waste through a chemical less method for Cd2+ removal from shipbuilding wastewater

Hydroxyapatite (HAp) powder was produced from chicken (femur and beak) and fishbone wastes and used as a green adsorbent to decrease Cd²⁺ from aqueous media. The HAp powder was generated at 900 °C and characterized using physicochemical techniques. Chicken femur' HAp (16.72 m²/g) had a higher surface compared to chicken beak and fishbone ones. The solution pH was the most important parameter in removing Cd²⁺. The highest Cd²⁺ removal was achieved at pH 6, temperature of 25 °C, contact time of 80 min, and adsorbent mass of 2 g/L. The Cd²⁺ adsorption data fitted well with the quasi-second-order model in kinetics and the Freundlich model in isotherm. The highest adsorption capacity of Cd²⁺ using HAp-chicken femur, HAp-fish bone, and HAp-chicken beak was determined 22.94 mg/g, 21.54 mg/g, and 21.45 mg/g, respectively. The Cd²⁺ adsorption using HAp powder was a spontaneous and exothermic process and accidental collisions at the liquid-solid interface were reduced. The decrease of Cd²⁺ adsorption efficiency was not significant after multiple recovery steps of the desired powders. In addition to Cd²⁺, other parameters of real wastewater (shipbuilding industry) were reduced by the proposed adsorbents. The utilization of hydroxyapatite powder is expected to be a cheap and eco-friendly method for eliminating metals such as Cd²⁺.

Multiple heavy metals immobilization based on microbially induced carbonate precipitation by ureolytic bacteria and the precipitation patterns exploration

Biomineralization to immobilize the toxic metal has great potential for the bioremediation of multiple heavy metal contamination. In this study, the efficiency of Microbially Carbonate Induced Precipitation (MICP) for several common heavy metals (Cu, Zn, Ni, Cd) in mining areas as well as their precipitation patterns were researched. After urease activity and precipitation ability comparison, Sporosarcina kp-4 and kp-22 were selected for subsequent studies. The removal of Cd was mainly based on the formation of cadmium carbonate induced by bacteria activity, while the removal of Cu was depended on the pH increase generated by the same process. Precipitation contributed to Zn and Ni removal was more complex, which was also based on the MICP process. Removal rates of Cu, Zn, Ni, and Cd (the concentration of all metals was 160 mg/L) reached 75.10%, 98.03%, 59.46% and 96.18%, respectively, within 2 h. For the immobilization of Cu, Zn, Ni and Cd at 160 mg/L, the optimal dosages of bacterial cultured solution were about 0.25 mL, 0.8 mL, 0.5 mL and 0.8 mL, respectively. Minimum inhibitory concentrations (MIC) revealed the toxicity of these heavy metals for MICP bacteria was arrange as: Cd > Zn > Ni > Cu. Our study confirmed that urease-producing bacteria could coprecipitate multiple heavy metals even without the ability tolerate them, and the MICP process was an effective biological approach that was worth investigating further to immobilize multiple heavy metals in ecological restoration.

Immobilization and migration of arsenic during the conversion of microbially induced calcium carbonate to hydroxylapatite

Coprecipitation with calcium carbonate (CaCO₃) could decrease the bioavailability of arsenic (As). However, in a phosphate-rich environment, some CaCO₃ will be converted to hydroxylapatite (HAP). Currently, the behavior of carbonate-bound As during conversion is unclear. Therefore, we prepared bio-induced CaCO₃ in an As solution and converted it to HAP. The results showed that a high concentration of arsenate promoted vaterite precipitation and the conversion of CaCO₃ to HAP. The dissolution data verified the low solubility of As in HAP, though its As-bearing CaCO₃ precursor released up to 88.19% As during the conversion. Furthermore, HPLC-ICP-MS data showed partial oxidation of arsenite to arsenate, suggesting that CaCO₃ and HAP's structure favored the incorporation of arsenate. Our results demonstrated that the stability of heavy metal-bearing CaCO₃ should be considered, and the role of HAP in the immobilization of heavy metals such as As should not be overestimated.

Immobilization of cadmium by Burkholderia sp. QY14 through modified microbially induced phosphate precipitation

Microbially induced phosphate precipitation (MIPP) is an advanced bioremediation technology to immobilize heavy metals. An indigenous bacterium QY14 with the function of mineralization isolated from Cd contaminated farmland soil was identified as Burkholderia ambifaria. The minimum inhibitory concentration value for QY14 was 550 mg/L for soluble Cd concentration. This study found that the addition of 10 mM Ca²⁺ during MIPP process could significantly increase the removal ratio of Cd, and the maximum removal ratio of Cd with 10 mM Ca²⁺ and without Ca²⁺ in solution was 99.97% and 76.14%, respectively. The increase of acid phosphatase activity and the formation of precipitate containing calcium caused by 10 mM Ca²⁺ addition contributed the increase of Cd removal efficiency. The results of SEM-EDS, FTIR and XRD showed that Cd was removed by forming Cd containing hydroxyapatite (Cd-HAP). In addition, the dissolution experiment showed the Cd release ratio of Cd-HAP (0.01‰ at initial pH 3.0 of solution) was lower than Cd-absorbed HAP, indicating that Cd was more likely removed by the formation of Ca_10-xCd_x(PO₄)₆(OH)₂ solid solution. Our findings revealed MIPP-based bioremediation supplied with 10 mM Ca²⁺ could increase the Cd removal and could potentially be applied for Cd remediation.

Environmental investigation of bio-modification of steel slag through microbially induced carbonate precipitation

Steel slag (SS) is one of byproduct of steel manufacture industry. The environmental concerns of SS may limit their re-use in different applications. The goal of this study was to investigate the leaching behavior of metals from SS before and after treated by microbially induced carbonate precipitation (MICP). Toxicity characteristic leaching procedure, synthetic precipitation leaching procedure and water leaching tests were performed to evaluate the leaching behavior of major elements (Fe, Mg and Ca) and trace elements (Ba, Cu and Mn) in three scenarios. The concentrations of leaching metals increased with the content of SS. After it reached the peak concentration, the leaching concentration decreased with the content of SS. The leachability of all elements concerned in this study was below 0.5%. The carbonate generated from the MICP process contributed to the low leachability of metals. After bio-modified by MICP process, the leaching concentrations of Ba from TCLP, SPLP and WLT tests were below 2.0 mg/L, which was the limit in drinking water regulated by U.S. EPA. The concentrations of Cu leached out from MICP-treated SS-sand samples were below 1.3 mg/L which is the limit regulated by national secondary drinking water. Compared with the regulations of U.S.EPA and Mississippi Department of Environment Quality (MDEQ), MICP-treated samples were classified as non-hazardous materials with respects to the leaching of metals. Meanwhile, maximum contaminant limits regulated by U.S.EPA states that MICP-treated SS are eco-friendly materials that can be reused as construction materials.

Biomass bottom ash & dolomite similarly ameliorate an acidic low-nutrient soil, improve phytonutrition and growth, but increase Cd accumulation in radish

One of negative side-effects of usage of bio-renewables might be generation of mineral (ash) material, potential source of environmental pollution. A hypothesis was that bottom ash (BA; from biomass cogeneration facility) could be efficiently (re) used in soil chemical conditioning similarly to widely-used dolomite-based soil conditioner (DO; from Croatian Dinaric-coastal region) which we tested by: i) physicochemical characterisation of BA and DO, and ii) bioassay with Raphanus sativus cultivated in acidic soil amended with BA or DO. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) confirmed complex chemical/physical structures and morphology between amendments, X-ray diffraction (XRD) showed their distinctive mineralogy with predominantly dolomite (in DO) vs. quartz and calcite (in BA), while secondary ion mass spectrometry (SIMS) revealed their diverse elemental/isotopic composition. The BA or DO amendments ameliorated soil acidity, increased available P, K and most other nutrients, but not Cd. The BA or DO amendments improved vegetative growth and edible hypocotyl yield. However, both amendments also increased Cd accumulation in all radish tissues, which was unexpected given the alkaline matrix of bio-ash and dolomite that would be likely to facilitate retention and immobilisation of toxic Cd. Thus, thorough characterisation and evaluation of BA- and/or DO-based materials and relevant soils (with an emphasis on metal sorption/immobilisation) prior to application in (agro) ecosystems is crucial for producing food clean of toxic metals.

Application of a heavy metal-resistant Achromobacter sp. for the simultaneous immobilization of cadmium and degradation of sulfamethoxazole from wastewater

Little information is available regarding the kinetics, products, and pathways of simultaneous SMX degradation and Cd(II) immobilization from wastewater. In this study, a novel bacterium (Achromobacter sp. L3) with SMX degradation and Cd(II) immobilization capabilities was isolated. The boundary conditions of SMX degradation were as follows: initial pH 6-8, temperature 25-30 °C, and SMX concentration 10-40 mg/L^-1. The boundary conditions of Cd(II) immobilization were as follows: initial pH 7-9, temperature 25-35 °C, and SMX concentration 10-30 mg/L^-1. The maximum SMX degradation and Cd(II) removal were 91.98% and 100%, respectively. The SMX degradation and Cd(II) immobilization data fitted well with the pseudo-first-order kinetic model, indicating that the two pollutants conform to the same degradation rule. Moreover, the microbial degradation, sediment adsorption, and intermediates identified in the experiments were used to explore the mechanisms of SMX and Cd(II) removal. These results indicate that microbial removal and sediment adsorption play equally important roles in Cd(II) immobilization; however, microbial degradation plays a decisive role in SMX degradation. Furthermore, the relationship between aerobic denitrification, SMX degradation, and Cd(II) immobilization was proposed. These results may provide valuable insights for treatment of wastewater polluted by antibiotics and heavy metals.

Evaluation of hydroxyapatite derived from flue gas desulphurization gypsum on simultaneous immobilization of lead and cadmium in contaminated soil

Flue gas desulphurization gypsum (FGD) is a major solid waste in coal-fired energy plants, and the appropriate reuse of this resources is still a major challenge. In this study, the feasibility of FGD as a calcium source to produce hydroxyapatite (FGD-HAP) for the immobilization of lead (Pb) and cadmium (Cd) in spiked soil was investigated. The effects of FGD and FGD-HAP on soil properties and redistribution, bioaccessibility and plant uptake of Pb and Cd were examined. Results showed that application of FGD and FGD-HAP could significantly improve the enzymes activities of contaminated soils, but the effectiveness was more pronounced with FGD-HAP. Addition of only 1% FGD-HAP could effectively reduce bioavailable Pb and Cd concentration in soil as measured by CaCl₂ extraction by 60.6% and 65.4%, respectively. On the other hand, plant available Pb and Cd could significantly decrease by 93.8% and 73.2% after amendment of 5% FGD-HAP. Significant changes in the micro-scale distribution of heavy metals before and after FGD-HAP treatment demonstrated that while heavy metals were predominantly associated with iron/manganese oxides in untreated soil, high correlation between heavy metals and phosphorus/sulfur was observed in FGD-HAP treated soil. In addition, results of the leaching tests showed that incorporation of FGD-HAP enhanced the retention capacity of heavy metals in soil, indicating that application of FGD-HAP could diminish the environmental risk of leachable heavy metals to groundwater. Overall, this study highlighted the potential value of FGD-HAP as a low-cost and high-efficient amendment for remediation of Pb and Cd contaminated soils.

Cadmium Immobilization in the Rhizosphere and Plant Cellular Detoxification: Role of Plant-Growth-Promoting Rhizobacteria as a Sustainable Solution

Food is the major cadmium (Cd)-exposure pathway from agricultural soils to humans and other living entities and must be reduced in an effective way. A plant can select beneficial microbes, like plant-growth-promoting rhizobacteria (PGPR), depending upon the nature of root exudates in the rhizosphere, for its own benefits, such as plant growth promotion as well as protection from metal toxicity. This review intends to seek out information on the rhizo-immobilization of Cd in polluted soils using the PGPR along with plant nutrient fertilizers. This review suggests that the rhizo-immobilization of Cd by a combination of PGPR and nanohybrid-based plant nutrient fertilizers would be a potential and sustainable technology for phytoavailable Cd immobilization in the rhizosphere and plant cellular detoxification, by keeping the plant nutrition flow and green dynamics of plant nutrition and boosting the plant growth and development under Cd stress.

Paddy periphyton reduced cadmium accumulation in rice (Oryza sativa) by removing and immobilizing cadmium from the water-soil interface

Periphyton plays a significant role in heavy metal transfer in wetlands, but its contribution to cadmium (Cd) bioavailability in paddy fields remains largely unexplored. The main aim of this study was to investigate the effect of periphyton on Cd behavior in paddy fields. Periphyton significantly decreased Cd concentrations in paddy waters. Non-invasive micro-test technology analyses indicated that periphyton can absorb Cd from water with a maximum Cd²⁺ influx rate of 394 pmol cm^-2 s^-1 and periphyton intrusion significantly increased soil Cd concentrations. However, soil Cd bioavailability declined significantly due to soil pH increase and soil redox potential (Eh) decrease induced by periphyton. With periphyton, more Cd was adsorbed and immobilized on organic matter, carbonates, and iron and manganese oxides in soil. Consequently, Cd content in rice decreased significantly. These findings give insights into Cd biogeochemistry in paddy fields with periphyton, and may provide a novel strategy for reducing Cd accumulation in rice.

Natural fluorapatite dissolution kinetics and Mn2+ and Cr3+ metal removal from sulfate fluids at 35 °C

In light of the consequences of global warming and population growth, access to safe drinking water becomes an ever greater challenge, in particular in low to middle income countries in arid regions. Moreover, mining which may cause acid mine drainage and heavy metal contamination puts further pressure on management of limited water resources. Hence, the development of cost effective water treatment methods is critical. Here, using batch reactor experiments we investigate the kinetics and mechanisms behind divalent Mn and trivalent Cr removal from sulfate fluids using natural fluorapatite at 35 °C. The results show that the fluorapatite dissolution rate depends on fluid pH, and that dissolution is the dominant mechanism in fluids with pH below 4. Apatite can thus serve as remediation to neutralize acidic fluids. Fluid pH of 4-6 triggers a dissolution-precipitation mechanism, in some cases following upon a dissolution-only period, with the formation of a metal phosphate. In these experiments, Cr removal is two to ten times faster than Mn removal given similar solution pH. The results demonstrate that natural apatite represents a promising, cost effective material for use in passive remediation of mining-induced contamination of soils and groundwater in arid regions.

Phosphate microbial mineralization consolidation of waste incineration fly ash and removal of lead ions

This paper proposes a green environment-friendly Bacillus subtilis to mineralize and consolidate waste incineration fly ash and heavy metal cations, and there is no harmful by-product in the mineralization process. Different phosphate products can be prepared, and are more stable than the microbially-induced carbonate precipitation (MICP) in nature. Typical heavy metal oxides were mainly PbO, ZnO, CdO, NiO, CuO and Cr₂O₃ in the chemical composition of waste incineration fly ash. Microstructure and chemical composition of waste incineration fly ash before and after treatment were characterized by powder X-ray diffraction (XRD) analysis and scanning electron microscopy. Scanning electron microscopy (SEM) images showed that the morphology of the Bacillus subtilis was mainly a rod-like structure. The optimal hydrolysis dosage of the organic phosphate monoester sodium salt was 0.2mol in the bacterial solution (1L, 20 g/L). The optimum required mass of the bacterial powder was 15 g/kg in treatment process of the waste incineration fly ash. The initial concentration of lead ions was 40.28 mg/L in waste incineration fly ash solution. After the optimum dosage treatment, the removal efficiency of lead ions was 78.15%, 79.64%, 77.70% and 80.14% when curing time was 1, 2, 4 and 6d, respectively. The waste incineration fly ash had a Shore hardness of 22 after the optimum amount of bacterial liquid treatment. Results of wind erosion test showed that the wind erosion rate of waste incineration fly ash was 2.6, 0, 0, 0, 0 and 0 g/h when blank group, deionized water, 100, 200, 300 and 400 mL of bacterial solutions treated, respectively. The bio-mineralization method provides an approach for the safe disposal of heavy metals in the contaminated areas of tailings, electroplating sewage, waste incineration plants, and so on.

Facile hydrothermal synthesis of novel Fe-Cu layered double hydroxide/biochar nanocomposite with enhanced sonocatalytic activity for degradation of cefazolin sodium

This study reports the successful synthesis of Fe-Cu layered double hydroxide (Fe-Cu-LDH) /biochar (BC) nanocomposite by a hydrothermal method. The sonocatalytic performance of Fe-Cu-LDH/BC nanocomposite was investigated for the degradation of cefazolin sodium (CFZ), as a model emerging contaminant, from the solution. The physico-chemical properties of the synthesized samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDX), Brunauer-Emmett-Teller (BET), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR), and UV-Vis diffuse reflectance spectroscopy (DRS) analyses. The best sonocatalytic efficiency of 97.6% was achieved by using 1.0 g/L sonocatalyst, 0.1 mM CFZ, and an ultrasonic power of 300 W at pH = 6.5 (natural) within 80 min. Additionally, the effects of the addition of various oxidants, dissolved gases, and organic and inorganic scavengers on the degradation of CFZ were studied. Moreover, the possible sonocatalytic mechanism of the sonochemical degradation of CFZ in the presence of Fe-Cu-LDH/BC sonocatalyst was proposed based on the results of GC-MS analysis. The mineralization of CFZ solution was evaluated using COD and IC analyses. Finally, the reusability test of Fe-Cu-LDH/BC nanocomposite in the CFZ degradation revealed that almost 9% drop occurred after five successive cycles.

Heavy metal removal from aqueous systems using hydroxyapatite nanocrystals derived from clam shells

Hydroxyapatite (HA) was synthesized by wet chemical precipitation, using clam shell (CS) waste as feedstock. SEM and TEM observation of the produced hydroxyapatite revealed the presence of rod-shaped nanocrystals, while XRD and EDS analyses confirmed the characteristic patterns of hydroxyapatite molecules. This material was subsequently employed as a sorbent for heavy metal removal from aqueous solutions, both in batch and column equilibrium procedures. In batch studies, higher sorption efficiencies were obtained at pH 5, with the highest adsorption capacities of 265, 64, and 55 mg g⁻¹ for Pb(ii), Cd(ii), and Cu(ii), respectively. In addition, an adsorption capacity of 42.5 mg g⁻¹ was determined using a CS-HA packed bed column fed with a solution of Pb(ii). Finally, the breakthrough curve was fitted with Thomas model in order to predict column behavior and scaling up.

Removal of Pb(ii), Cu(ii) and Cd(ii) was attained using hydroxyapatite nanocrystals derived from clam shells, in batch and column experiments.