Enzyme-induced carbonate precipitation (EICP) has been studied in remediation of heavy metal contaminated water or soil in recent years. This paper aims to investigate the immobilization mechanism of Zn2+, Ni2+, and Cr(VI) in contaminated sand, as well as strength enhancement of sand specimens by using EICP method with crude sword bean urease extracts. A series of liquid batch tests and artificially contaminated sand remediation experiments were conducted to explore the heavy metal immobilization efficacy and mechanisms. Results showed that the urea hydrolysis completion efficiency decreased as the Ca2+ concentration increased and the heavy metal immobilization percentage increased with the concentration of Ca2+ and treatment cycles in contaminated sand. After four treatment cycles with 0.5 mol/L Ca2+ added, the immobilization percentage of Zn2+, Ni2+ and Cr(VI) were 99.99 %, 86.38 %, and 75.18 %, respectively. The microscale analysis results presented that carbonate precipitates and metallic oxide such as CaCO3, ZnCO3, NiCO3, Zn(OH)2, and CrO(OH) were generated in liquid batch tests and sand remediation experiments. The SEM-EDS and FTIR results also showed that organic molecules and CaCO3 may adsorb or complex heavy metal ions. Thus, the immobilization mechanism of EICP method with crude sword bean urease can be considered as biomineralization, as well as adsorption and complexation by organic matter and calcium carbonate. The unconfined compressive strength of EICP-treated contaminated sand specimens demonstrated a positive correlation with the increased generation of carbonate precipitates, being up to 306 kPa after four treatment cycles with shear failure mode. Crude sword bean urease with 0.5 mol/L Ca2+ added is recommended to immobilize multiple heavy metal ions and enhance soil strength.

Ecological restoration of bauxite residue has received extensive attention, and organic matter plays a crucial role in the soil formation process of bauxite residue. However, the interaction between organic matter and alkaline minerals in bauxite residue is not well understood. In this work, molecular spectroscopic techniques combined with isothermal titration calorimetry (ITC) were employed to investigate the interactions between humic acid (HA) and four representative alkaline minerals in bauxite residue (calcite, garnet, sodalite, and cancrinite). The results show that the adsorption processes of HA onto calcite and garnet were primarily governed by monolayer surface adsorption and controlled by surface reactions, which were different for sodalite and cancrinite. Both garnet and cancrinite had strong binding affinities with fluorescent HA, while cancrinite only bound with a small fraction of HA. In contrast, the bindings of calcite and sodalite with fluorescent HA were weak. The ITC results indicate distinct thermodynamic properties of different alkaline minerals in the interaction with HA. The molar enthalpy of calcite was - 45.88 kJ/mol, which was much higher than those of garnet, sodalite, and cancrinite, suggesting that calcite exhibited a relatively uniform interaction mechanism with HA dominated by enthalpy change, while the others showed heterogeneous entropy-driven mechanisms. The findings contribute to a better understanding on the microscale connections between organic matter and alkaline minerals in bauxite residue.

This study investigates the co-precipitation of calcium and barium ions in hypersaline wastewater under the action of Bacillus licheniformis using microbially induced carbonate precipitation (MICP) technology, as well as the bactericidal properties of the biomineralized product vaterite. The changes in carbonic anhydrase activity, pH, carbonate and bicarbonate concentrations in different biomineralization systems were negatively correlated with variations in metal ion concentrations, while the changes in polysaccharides and protein contents in bacterial extracellular polymers were positively correlated with variations in barium concentrations. In the mixed calcium and barium systems, the harvested minerals were vaterite containing barium. The increasing concentrations of calcium promoted the incorporation and adsorption of barium onto vaterite. The presence of barium significantly increased the contents of O-CO, N-CO, and Ba-O in vaterite. Calcium promoted barium precipitation, but barium inhibited calcium precipitation. After being treated by immobilized bacteria, the concentrations of calcium and barium ions decreased from 400 and 274 to 1.72 and 0 mg/L (GB/T15454-2009 and GB8978-1996). Intracellular minerals were also vaterite containing barium. Extracellular vaterite exhibited bactericidal properties. This research presents a promising technique for simultaneously removing and recycling hazardous heavy metals and calcium in hypersaline wastewater.

While the extensive utilization of disposable plastic straws has resulted in significant environmental issues such as microplastics and soil and ocean pollution, the quest for alternative straws for versatile use remains a formidable challenge. Here, drawing inspiration from naturally water-resistant materials such as bones and sea urchins, we have developed seaweed-based straws with significantly improved water resistance and mechanical strength via in-situ mineralization of CaCO3 on their surfaces. Specifically, the COO- groups on the G (α-L-guluronate) blocks of alginate were employed to establish a robust cross-linked network, while the COO- groups on the M (β-D-mannuronate) blocks attracted free Ca2+ through electrostatic forces, thereby promoting CaCO3 nucleation. This effectively prevents COOH groups from hydrating, reducing swelling, and results in the fabrication of nano- to micron-sized CaCO3 particles that reinforce the structure without compromising the cross-linked network. Compared with the control group, the S5% sample (prepared with 5 % Na2CO3 solution) exhibited a 102 % increase in water contact angle, a 35 % decrease in swelling degree, and a 35.5 % and 37.5 % increase in ultimate flexural and tensile stress, respectively. Furthermore, the potential use of these straws as a waste for heavy metal adsorption was investigated, addressing environmental concerns while demonstrating economic feasibility.

The vast application and deep integration of plastic commodity with our human lives raise a great concern about the ubiquitous microplastics (MPs) in nature, yet the environmental behavior of MPs remain unclear. As a main type and candidate of MPs, pristine polypropylene MPs (PP-MP-Pris), as well as the influence of ultraviolet (UV) irradiation on the degree of aging and surface characteristics, were characterized quantitatively by Fourier infrared spectroscopy, scanning electron microscopy, contact angle meter, automatic specific surface area and pore analyzer and laser particle analyzer, with natural aged PP-MPs (PP-MP-Age) as comparison. The carbonyl index (CI) of UV aged PP-MPs (PP-MP-U) was increased with extension of exposure time, while biofilm with abundant functional groups and the maximum CI value were the characteristics of PP-MP-Age. Moreover, the adsorption capacity of PP-MP-U for crystal violet (CV) was increased and reached the maximum after 30 days, while that of PP-MP-Age was weakened, probably due to the enhanced hydrophilicity and the shedding of calcium carbonate (CaCO3) during the natural aging process, which was demonstrated by hydrochloric acid treatment, indicating the vital involvement of CaCO3. Moreover, the better fitting to PSO kinetics and Freundlich isotherm models indicated that the multilayered and non-homogeneous surface adsorption was acted as the rate-controlling step. Furthermore, the positive values of ΔGθ, ΔHθ and ΔSθ indicated that the adsorption was a non-spontaneous, endothermic process with increased degree of the freedom on the interface of PP-MPs and CV solution. The presence of divalent salts inhibited CV adsorption, demonstrating that electrostatic attraction played a major role in CV capture. The hydrophobic interaction, micropore filling, hydrogen bonding, and π - π conjugation were possible involved. This study is of great significance for better understanding the complex pollution of MPs and its potential environmental risks in the future.

Caudofoveata are molluscs that protect their vermiform body with a scleritome, a mosaic of unconnected blade/lanceolate-shaped aragonite sclerites. For the species Falcidens gutturosus and Scutopus ventrolineatus we studied the crystallographic constitution and crystal orientation texture of the sclerites and the scleritome with electron-backscatter-diffraction (EBSD), laser-confocal-microscopy (LCM) and field-emission electron microscopy (FE-SEM) imaging. Each sclerite is an aragonite single crystal that is completely enveloped by an organic sheath. Adjacent sclerites overlap laterally and vertically are, however, not connected to each other. Sclerites are thickened in their central portion, relative to their periphery. Thickening increases also from sclerite tip towards its base. Accordingly, cross-sections through a sclerite are straight at its tip, curved and bent towards the sclerite base. Irrespective of curved sclerite morphologies, the aragonite lattice within the sclerite is coherent. Sclerite aragonite is not twinned. For each sclerite the crystallographic c-axis is parallel to the morphological long axis of the sclerite, the a-axis is perpendicular to its width and the b-axis is within the width of the sclerite. The single-crystalinity of the sclerites and their mode of organization in the scleritome is outstanding. Sclerite and aragonite arrangement in the scleritome is not given by a specific crystal growth mode, it is inherent to the secreting cells. We discuss that morphological characteristics of the sclerites and crystallographic preferred orientation (texture) of sclerite aragonite is not the result of competitive growth selection. It is generated by the templating effect of the organic substance of the secreting cells and associated extracellular biopolymers.

Carbide slag (CS) is a kind of solid waste generated by the hydrolysis of calcium carbide for acetylene production. Its major component is Ca(OH)2, which shows great potential in CO2 mineralization to produce CaCO3. However, the types of impurities in CS and their mechanisms for inducing the morphological evolution of CaCO3 are still unclear. In this work, the influence of impurities in CS on the morphology evolution of CaCO3 was investigated. The following impurities were identified in the CS: Al2O3, MgO, Fe2O3, SiO2 and CaCO3. Ca(OH)2 was used to study the influence of impurities (Al2O3 and Fe2O3) on the evolution of CaCO3 morphology during CS carbonation. Calcite (CaCO3) was the carbonation product produced during CS carbonation under varying conditions. The morphology of calcite was changed from cubic to rod-shaped, with increasing solid-liquid ratios. Moreover, rod-shaped calcite was converted into irregular particles with increasing CO2 flow rate and stirring speed. Rod-shaped calcite (CaCO3) was formed by CS carbonation at a solid-liquid ratio of 10:100 under a stirring speed of 600 rpm and a CO2 flow rate of 200 ml/min; and spherical calcite was generated during Ca(OH)2 carbonation under the same conditions. Al2O3 impurities had negligible effects on spherical CaCO3 during Ca(OH)2 carbonation. In contrast, rod-shaped CaCO3 was generated by adding 0.13 wt% Fe2O3 particles, similar to the content of Fe2O3 in CS. Rod-shaped calcite was converted into particulate calcite with increasing Fe2O3 content. The surface wettability and surface negative charge of Fe2O3 appeared to be responsible for the formation of rod-shaped CaCO3. This study enhances our understanding and utilization of CS and CO2 reduction and the fabrication of high-value rod-shaped CaCO3.

Enzyme-induced calcium carbonate precipitation (EICP) and microbially-induced calcium carbonate precipitation (MICP) techniques represent emerging trends in soil stabilization. However, the impact of soil density on biomineralization, particularly in historical earthen sites, remains unclear. This study compares the consolidation effects of EICP and MICP on cylindrical samples (10 cm × 5 cm) with three densities (1.5 g/cm3, 1.6 g/cm3, and 1.7 g/cm3) derived from the soil near the UNESCO World Cultural Heritage Site of Suoyang Ancient City, Gansu Province, China. Results showed that calcium carbonate production increased across all densities through bio-cementation, with higher densities producing more calcium carbonate. MICP-treated specimens exhibited larger increases in calcium carbonate production compared to those treated with EICP. Specimens with a density of 1.7 g/cm³ showed a wave velocity increase of 3.26% (EICP) and 7.13% (MICP), and an unconfined compressive strength increase of 8% (EICP) and 26% (MICP). These strength increases correlated with the generation of calcium carbonate. The findings suggest that biomineralization can be effectively utilized for in situ consolidation of earthen sites, emphasizing the importance of considering soil density in biologically-based conservation technologies. Furthermore, MICP shows potential advantages over EICP in providing stronger, compatible and more sustainable soil reinforcement.

Microbially induced carbonate precipitation (MICP) has been used to cure rare earth slags (RES) containing radionuclides (e.g. Th and U) and heavy metals with favorable results. However, the role of microbial extracellular polymeric substances (EPS) in MICP curing RES remains unclear. In this study, the EPS of Lysinibacillus sphaericus K-1 was extracted for the experiments of adsorption, inducing calcium carbonate (CaCO3) precipitation and curing of RES. The role of EPS in in MICP curing RES and stabilizing radionuclides and heavy metals was analyzed by evaluating the concentration and morphological distribution of radionuclides and heavy metals, and the compressive strength of the cured body. The results indicate that the adsorption efficiencies of EPS for Th (IV), U (VI), Cu2+, Pb2+, Zn2+, and Cd2+ were 44.83%, 45.83%, 53.7%, 61.3%, 42.1%, and 77.85%, respectively. The addition of EPS solution resulted in the formation of nanoscale spherical particles on the microorganism surface, which could act as an accumulating skeleton to facilitate the formation of CaCO3. After adding 20 mL of EPS solution during the curing process (Treat group), the maximum unconfined compressive strength (UCS) of the cured body reached 1.922 MPa, which was 12.13% higher than the CK group. The contents of exchangeable Th (IV) and U (VI) in the cured bodies of the Treat group decreased by 3.35% and 4.93%, respectively, compared with the CK group. Therefore, EPS enhances the effect of MICP curing RES and reduces the potential environmental problems that may be caused by radionuclides and heavy metals during the long-term sequestration of RES.

Biocementation, driven by ureolytic bacteria and their biochemical activities, has evolved as a powerful technology for soil stabilization, crack repair, and bioremediation. Ureolytic bacteria play a crucial role in calcium carbonate precipitation through their enzymatic activity, hydrolyzing urea to produce carbonate ions and elevate pH, thus creating favorable conditions for the precipitation of calcium carbonate. While extensive research has explored the ability of ureolytic bacteria isolated from natural environments or culture conditions, bacterial synergy is often unexplored or under-reported. In this study, we isolated bacterial strains from the local eutrophic river canal and evaluated their suitability for precipitating calcium carbonate polymorphs. We identified two distinct bacterial isolates with superior urea degradation ability (conductivity method) using partial 16 S rRNA gene sequencing. Molecular identification revealed that they belong to the Comamonas and Bacillus genera. Urea degradation analysis was performed under diverse pH (6,7 and 8) and temperature (15 °C,20 °C,25 °C and 30 °C) ranges, indicating that their ideal pH is 7 and temperature is 30 °C since 95% of the urea was degraded within 96 h. In addition, we investigated these strains individually and in combination, assessing their microbially induced carbonate precipitation (MICP) in silicate fine sand under low (14 ± 0.6 °C) and ideal temperature 30 °C conditions, aiming to optimize bio-mediated soil enhancement. Results indicated that 30 °C was the ideal temperature, and combining bacteria resulted in significant (p ≤ 0.001) superior carbonate precipitation (14-16%) and permeability (> 10- 6 m/s) in comparison to the average range of individual strains. These findings provide valuable insights into the potential of combining ureolytic bacteria for future MICP research on field applications including soil erosion mitigation, soil stabilization, ground improvement, and heavy metal remediation.