Green and effective remediation of heavy metals contaminated water using CaCO3 vaterite synthesized through biomineralization

Removal of copper, lead and cadmium from water through enzyme-induced carbonate precipitation by soybean urease

Enzyme-induced carbonate precipitation (EICP) is widely recognized as a green and sustainable technology for heavy metal remediation. In this study, a novel spherical porous vaterite (0.05-5 μm) was synthesized via EICP, demonstrating exhibited excellent performance in heavy metals removal from contaminated water. The Langmuir maximum adsorption capacity of vaterite for multiple heavy metals are in the order of Cu2+ (1207.20 mg/g) > Cd2+ (785.73 mg/g) > Pb2+ (654.95 mg/g), with adsorption primarily occurring on the vaterite surface. Notably, the vaterite exhibited a significantly higher removal capacity for Cd2+, which was 49.80 times that of Sinopharm-CaCO3 and 2.07 times that of Chemical-CaCO3, achieving over 90 % removal within the first 6 d in cyclic tests. On the 3th day of aqueous solution, calcite formation was first detected by X-ray Diffraction (XRD). Although 55 % of vaterite was transformed into calcite after 5 weeks, Cd2+ removal efficiency remained above 80 %, with XRD analysis confirmed that the formation of precipitate is CdCO3. Comprehensive characterization (SEM-EDS and XRD) showed that distinct immobilization products for Cd2+ and Pb2+ were identified as CdCO3, and PbCO3 or Pb3(CO3)2(OH)2, respectively. For Cu2+, the presence of Cl- promoted Cu2Cl(OH)3 formation rather than CuCO3 during biomineralization. These results demonstrate that EICP-derived vaterite maintains excellent long-term remediation performance while forming stable precipitates that effectively prevent secondary pollution.

Classification of additives and their influence mechanisms in improving the performance of biologically induced carbonate precipitation

Microbial/enzyme induced carbonate precipitation (MICP/EICP) is one of the hot topics in the field of civil engineering, environmental engineering in recent years, primarily attributed to its environmental friendliness and low energy consumption. However, how to enhance its economic and technical feasibility to ensure its stable and high-performance is still a significant challenge. This paper systematically explores the strategic incorporation of additives as a promising approach to enhance the efficiency and controllability of MICP/EICP process. An overview of MICP and EICP, including a comparison between them, is first compiled. According to the characteristics of various additives and the regulatory requirements, they are classified into the following categories: organic macromolecular additives, inorganic additives, biological additives and others. It then highlights the potential of additives to impact the mineralization dynamic process and the underlying mechanisms of their involvement in the reaction, such as providing nucleation sites, enhancing bioactivity, altering the properties of the calcium carbonate product, and reducing by-products. Whereas these additives either possess outstanding biocompatibility, specific functional groups, or particular viscosity, can work synergistically with MICP/EICP, they still have some intrinsic limits that need to be addressed. Therefore, future perspectives in additive-modified MICP/EICP systems are discussed in-depth. These insights establish a theoretical framework for additive selection tailored to specific MICP/EICP applications, making the incorporation of additives a powerful tool in the future to improve mineralization outcomes in different application scenarios.

Terahertz spectroscopy for analysis of vaterite-to-calcite crystal phase transition induced by distilled water

Vaterite is a crystalline polymorph of anhydrous calcium carbonate (CaCO3), which exhibits relatively low stability compared to other two polymorphs, calcite and aragonite. Vaterite particles have properties such as large specific surface area, high porosity, and high solubility; hence, research has been made in wide range of applications from material additive to drug delivery. X-ray diffractometry (XRD) is capable of identifying polymorphs of calcium carbonate. However, depending on the base material and the installed state, it may not always be suitable for non-destructive observation. This study reveals that vaterite-to-calcite crystal phase transition degree can be quantified by the absorption spectrum in terahertz range. The vaterite concentration and the terahertz absorption peak intensity near 3.3 THz showed linearity with correlation coefficient of R2 = 0.934 in our experiment. The findings allow us to quantitatively evaluate vaterite-to-calcite crystal phase transition in non-destructive and non-invasive way.

Calcium carbonate hollow microspheres encapsulated cellulose nanofiber/sodium alginate hydrogels as a sequential delivery system

By using folic acid (FA) as the template, calcium carbonate hollow microspheres (CaCO3 HMs) are prepared through the Ostwald ripening process, which can be utilized for the loading of doxorubicin hydrochloride (DOX). The DOX loaded CaCO3 HMs (CaCO3/DOX) are co-encapsulated with ibuprofen (IBU) in the cellulose nanofiber (CNF)/sodium alginate (SA) hydrogels cross-linked by Ca2+. The loading efficiency of DOX in the CaCO3 HMs is 95.7 %, and the loading efficiency of IBU in the hydrogels is 97.0 %. In the weakly alkaline environment (pH ~7.4) that is characteristic of intestinal fluids of human body, the CNF/SA hydrogels are swollen and the encapsulated IBU is first released for pain control, and the release rate of IBU can reach 57.8 %. In the weakly acidic environment (pH ~6.5) that is characteristic of colonic fluids of human body, the CaCO3 HMs are decomposed to release the loaded DOX with a release rate of 50.1 %, which can be used for the treatment of colorectal cancer. The results of release kinetics indicate that the delivery of IBU is governed by first-order model and DOX by zero-order model. The developed sequential delivery system (SDS) can not only enable the release of DOX in colon of human body, but also simultaneously relieve the pain of patients during the chemotherapy of colon cancer.

Efficient recovery of heavy metals and selenium from wastewater using granular sludge: The crucial role of glutathione (GSH)

Microbial technology offers an effective method for treating heavy metals and selenium (Se) in wastewater, yet the recovery of these valuable elements is often overlooked. This study introduces a glutathione (GSH)-enhanced granular sludge technology for the removal and recovery of heavy metals and Se from wastewater. Using the new technology, the removal rates of copper (Cu), cadmium (Cd), and Se from wastewater reached 99.4-99.99%, while the recovery rates reached 73.2-87.9%. Both long-term reactor operation and short-term stimulation experiments indicated that GSH substantially increased the residual fraction of Cu, Cd, and Se in the sludge. This residual fraction was identified as metal selenides (MSe), composed of Cu1.08Se (75.4 ± 1.8%) and CdSe (15.4 ± 1.0%). The increased abundance and significant upregulation of GSH-related genes, including gshA, gshB, and gor, as well as the indispensable roles of GSH, glutathione reductase (GorA), and NADPH in the in vitro synthesis of MSe, demonstrated that the GSH-mediated Painter-type reaction was the primary pathway for MSe synthesis in the sludge. The biosynthesized MSe was efficiently extracted and recovered from the final sludge, and the extract showed high catalytic activity in pollutant degradation. Given the widespread presence of GSH in diverse microorganisms, the GSH-mediated mechanism for MSe synthesis is likely to occur in various environments contaminated with heavy metals and Se.

Quantitative study of competitive and selective immobilization of Pb(II)-Ni(II)-Zn(II)-MB(I) by biogenic monohydrocalcite composite and its potential environmental effects

The study of the competitive and selective immobilization properties and mechanisms of pollutants immobilized by metastable biogenic monohydrocalcite is of great importance for the assessment of the eco-environmental effects and applications of hydrated calcite at the Earth's poles. Microbial culture technology was used to induce the synthesis of biogenic monohydrocalcite (BMHC), and mineral characterization, batch adsorption experiments and chemical analyses were further used to investigate the sequestration characteristics, action mechanism, and environmental effects of BMHC on Pb(II)-Ni(II)-Zn(II)-methylene blue (MB) compound pollution. The results show that BMHC is an organic-inorganic mineral composite (about 3.60 % organic matter, Mg/Ca ≈ 0.07). The adsorption and immobilization processes of Pb(II), Ni(II), Zn(II), and MB(I) by BMHC are all better fitted by the pseudo-second-order kinetic equation. The passivation ability of BMHC for contaminants is ranked as Pb(II) ≫ Zn(II) > Ni(II) > MB(I). BMHC exhibits an excellent selective sequestration capacity of Pb(II) (k ≥ 31.89), which is related to the solubility product of the carbonate minerals, the initial concentration of Pb(II), ion exchange and mineral phase transformation. Based on these results, it is proposed that the synthesis and transformation of monohydrocalcite under global warming at the Earth's poles may influence the biogeochemical cycling of environmental pollutants. The study provides a theoretical basis for the environmental effects and geochemical action of biogenic monohydrocalcite and its applications.

Effects of trehalose and sodium alginate on microbially induced carbonate precipitation

The process of altering the microbial-induced carbonate precipitation (MICP) by adding additives has been extensively studied. The impact of polysaccharides, as an important component of bacteria, still requires deeper exploration on MICP. This work thus focuses on two types of sugars, sodium alginate (SA) and trehalose (Tre), to explore their effects on biomineralization of carbonate induced by Bacillus pumilus Z6. The results show that B. pumilus Z6 can raise the environmental pH and increase the supersaturation of carbonate and bicarbonate ions through carbonic anhydrase. The presence of organic functional groups and the negative carbon isotope signatures in minerals provide evidence of microbial involvement. Tre and SA do not change the mineral phase, which mainly consists of hollow rice-like granular vaterite and irregular calcite. Tre is conducive to the formation of calcite, whereas the carboxyl groups in SA contribute to the stability of vaterite. Both Tre and SA enhance the removal rate of calcium ions; however, SA is more effective for this purpose. Furthermore, mineralization experiments with calcium alginate gel tablets indicate that SA can attract calcium carbonate to nucleate on its surface. This research offers significant insights into biomineralization processes and introduces novel perspectives for advancing MICP technology.

Investigation on the rotary atomization evaporation of high-salinity desulfurization wastewater: Performance and products insights

Spray drying of concentrated wastewater epitomizes a harmonious convergence of technological progress, economic viability, and practicality within the realm of zero liquid discharge. Nevertheless, elevated salinity may influence the atomization and evaporation processes, along with the storage and transportation of evaporation products. This study systematically examines the influence of salinity on rotary atomization evaporation performance and evaporation products of wastewater through a series of experimental investigations. The results indicate that, under identical conditions, the overall droplet size of high-salinity wastewater is approximately 20-50% larger compared to conventional wastewater. Salinity significantly influences the atomization particle size (D32), followed by rotation speed, and then influent flow rate. The high-salinity wastewater droplets manifest a multi-bubble growth pattern with earlier shell expansion, where the reduction of free water dominates the overall process dynamics. Despite the diminished evaporation rate, the total evaporation duration shortens with elevating salinity, reducing flue gas consumption by about 10%. With elevated crystalline salt content, high-salinity wastewater evaporation products exhibit pronounced hygroscopicity, manifesting as a viscous powder with suboptimal flowability (FF 3.57) at a 2 wt% moisture content. This study bridges the gap in rotary spray drying technology for high-salinity wastewater treatment, contributing to sustainable water conservation and energy-efficient management.

Fabrication of CaCO3 Microcubes and Mechanistic Study for Efficient Removal of Pb from Aqueous Solution

Pb(II) contamination in aquatic environments has adverse effects on humans even at a low concentration, so the efficient removal of Pb at a low cost is vital for achieving an environmentally friendly, sustainable, and healthy society. A variety of CaCO3-based functional adsorbents have been synthesized to remove Pb, but the adsorption capacity is still unsatisfactory. Herein, calcite CaCO3 microcubes/parallelepipeds are synthesized via simple precipitation and a hydrothermal approach and found to outperform previously reported nano-adsorbents considerably. The CaCO3 achieves a high removal efficiency for Pb(II) (>99%) at a very low dosage (0.04-0.1 g/L) and an initial Pb(II) concentration of 100 mg/L. The CaCO3 presents an excellent adsorption capacity of 4018 mg/g for Pb(II) removal and depicts good stability over a wide range of pH 6-11. The maximum adsorption kinetics are fitted well by the pseudo-second-order kinetic model, whereas the Freundlich isotherm delineates the adsorption data at equilibrium well, indicating a multilayer adsorption process. The ex situ study confirms that the Pb(II) adsorption mechanism by CaCO3 can be attributed to the rapid metal-ion-exchange reaction between Pb(II) and Ca2+. Furthermore, a red shift in the Fourier Transform Infrared (FTIR) spectroscopy peak from 1386 cm-1 to 1374 cm-1 of CaCO3 after Pb removal indicates the adsorption of Pb onto the surface. This adsorbent provides an opportunity to treat wastewater and can be extended to remove other toxic heavy metals.

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.