Immobilizing of lead and copper using chitosan-assisted enzyme-induced carbonate precipitation

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.

Biopolymer-enzyme-induced carbonate precipitation (EICP) for the green solidification/stabilization of graphite tailings: Mechanical, leaching, and microstructural characterization

The rapidly increasing global demand for graphite has increased the generation of graphite tailings. However, graphite tailings are rich in harmful substances, such as heavy metals, which pose serious threats to the environment and human health. Traditional recycling methods of graphite tailings face challenges, including high costs, substantial CO2 emissions, and limited effectiveness in heavy-metal stabilization, which inhibit their environmental sustainability in large-scale applications. In this study, a novel ecofriendly strategy was proposed for the green solidification and stabilization (S/S) of graphite tailings. Chitosan (CTS), a biopolymer, was introduced during the enzyme-induced carbonate precipitation (EICP) of graphite tailings. The potential of CTS-EICP in binding the loose particle structure of graphite tailings and inhibiting the release of heavy-metal pollutants was discussed from dual dimensions of mechanical strength and environmental effects. Results revealed that the unconfined compressive strength (UCS), split tensile strength (STS), and calcium carbonate generation rate of CTS-EICP-treated graphite tailings significantly improved compared with those of EICP-treated tailings. The effects were optimal when the CTS content was 0.15 %, reaching 897.8 kPa, 258.1 kPa, and 8.03 %, respectively. After CTS-EICP treatment, the pH of the tailing leachate stabilized at 7.90-8.26 and the fixation rate of heavy-metal ions was 92.61 %-100 %. CTS promoted the formation of carbonate crystals via urease stabilization mechanism, which were embedded in the three-dimensional network formed via the crosslinking of CTS molecules and yielded a multilayer composite barrier structure of "tailings-CTS-carbonate-CTS-tailings." CTS-EICP provided a new perspective for employing biopolymer-EICP synergistic remediation interactions in multidimensional green S/S of heavy metal-containing tailings.

Flowing-water remediation simulation experiments of lead-contaminated soil using UCB technology

The flowing-water remediation of contaminated soil was investigated. Urease combined with biochar (UCB) technology was used to handle the Pb2+-contaminated sand column. The results showed that with the continuous increase of pore volume, the concentration of Pb2+ in the leachate undergoes three stages: slow growth, rapid growth, and steady state. With increasing seepage velocity, the concentration of Pb2+ in leachate increased slightly. The residual amount of each section of the sand column gradually decreased with increasing migration distance. The comparative results indicated that the UCB technology had a good solidification effect on Pb2+. This was due to urease-induced CaCO3 precipitation, cementation, and adsorption of Pb2+. Biochar provided more nucleation sites for urease, and some Pb2+ was adsorbed on its surface or diffused into the pores of biochar, or ions exchanged with functional groups on the surface of biochar, which effectively stabilized the free Pb2+.

Effects of enzyme-induced carbonate precipitation (EICP) with different urease sources on the zinc remediation

Enzyme-induced carbonate precipitation (EICP) has been studied in the remediation of heavy metals in recent years. This study aims to investigate the impact of EICP with jack bean urease (JU) and sword bean urease (SU) on the Zn2+ remediation. The results show that relatively high concentration of organic molecules in SU can protect urease from deactivation and absorb Zn2+. For the treatment subject to 1 mmol/L of Zn2+ without calcium chloride added, the final NH4+ conversion efficiency and Zn2+ immobilization percentage for the SU group are 95 % and 95.1 % higher than those for JU group, respectively. For the treatment with calcium chloride added, SU group can produce enough CO32-, contributing to over 99 % formation of CaCO3. The CaCO3 can decrease Zn2+ concentration through the physical absorption. The highest Zn2+ immobilization percentage (98.9 %) can be achieved using SU with 0.5 mol/L calcium chloride. The XRD, FTIR and SEM-EDS analysis also show that CaCO3, ZnCO3, and Zn(OH)2 can be generated, and the organic molecules in SU and CaCO3 can adsorb Zn2+. SU is recommended as the catalyzer to immobilize Zn2+. The mechanism of Zn2+ immobilization can be understood as bio-mineralization to form ZnCO3 and Zn(OH)2, as well as adsorption by organic molecules in SU and CaCO3.

Effects of enzyme-induced carbonate precipitation technique on multiple heavy metals immobilization and unconfined compressive strength improvement of contaminated sand

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.

Effect of biopolymer chitosan on manganese immobilization improvement by microbial‑induced carbonate precipitation

Microbially induced carbonate precipitation (MICP), as an eco-friendly and promising technology that can transform free metal ions into stable precipitation, has been extensively used in remediation of heavy metal contamination. However, its depressed efficiency of heavy metal elimination remains in question due to the inhibition effect of heavy metal toxicity on bacterial activity. In this work, an efficient, low-cost manganese (Mn) elimination strategy by coupling MICP with chitosan biopolymer as an additive with reduced treatment time was suggested, optimized, and implemented. The influences of chitosan at different concentrations (0.01, 0.05, 0.10, 0.15 and 0.30 %, w/v) on bacterial growth, enzyme activity, Mn removal efficiency and microstructure properties of the resulting precipitation were investigated. Results showed that Mn content was reduced by 94.5 % within 12 h with 0.15 % chitosan addition through adsorption and biomineralization as MnCO3 (at an initial Mn concentration of 3 mM), demonstrating a two-thirds decrease in remediation time compared to the chitosan-absent system, whereas maximum urease activity increased by ∼50 %. Microstructure analyses indicated that the mineralized precipitates were spherical-shaped MnCO3, and a smaller size and more uniform distribution of MnCO3 is obtained by the regulation of abundant amino and hydroxyl groups in chitosan. These results demonstrate that chitosan accelerates nucleation and tunes the growth of MnCO3 by providing nucleation sites for mineral formation and alleviating the toxicity of metal ions, which has the potential to upgrade MICP process in a sustainable and effective manner. This work provides a reference for further understanding of the biomineralization regulation mechanism, and gives a new perspective into the application of biopolymer-intensified strategies of MICP technology in heavy metal contamination.

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.

Study on Cu- and Pb-contaminated loess remediation using electrokinetic technology coupled with biological permeable reactive barrier

Although the electrokinetic (EK) remediation has drawn great attention because of its good maneuverability, the focusing phenomenon near the cathode and low removal efficiency remain to be addressed. In this study, a novel EK reactor was proposed to remediate Cu and Pb contaminated loess where a biological permeable reactive barrier (bio-PRB) was deployed to the middle of the EK reactor. For comparison, three test configurations, namely, CG, TG-1, and TG-2, were available. CG considered the multiple enzyme-induced carbonate precipitation (EICP) treatments, while TG-1 considered both the multiple EICP treatments and pH regulation. TG-2 further considered NH4+ recovery based on TG-1. CG not only improved Cu and Pb removals by the bio-PRB but also depressed the focusing phenomenon. TG-1 causes more Cu2+ and Pb2+ to migrate toward the bio-PRB and aggravates Cu and Pb removals by the bio-PRB, depressing the focusing phenomenon. TG-2 depressed the focusing phenomenon the most because Cu2+ and Pb2+ can combine with not only CO32- but PO43-. The removal efficiency of Cu and Pb is 34% and 36%, respectively. A NH4+ recovery of about 100% is attained.

Evaluating gas breakthrough pressure and gas permeability in a landfill cover layer for mitigation of hazardous gas emissions

The construction of an engineered cover layer over landfills is a common method applied to reduce the emission of hazardous gases into the atmosphere. Landfill gas pressures can reach 50 kPa or even higher in some cases, thus posing a serious threat to nearby properties and human safety. As such, the evaluation of gas breakthrough pressure and gas permeability in a landfill cover layer is of great necessity. In this study, the loess soil that is often applied as a cover layer in landfills in northwestern China was used to conduct gas breakthrough, gas permeability, and mercury intrusion porosimetry (MIP) tests. Resultantly, the smaller the capillary tube diameter, the higher the capillary force, and the more significant the capillary effect. Gas breakthrough could be attained with no difficulty, provided that the capillary effect was minimal or approached zero. A good fit between the experimental gas breakthrough pressure-intrinsic permeability relationship and a logarithmic equation was found. The mechanical effect blew up the gas flow channel. In the worst-case scenario, the mechanical effect could lead to the overall failure of a loess cover layer in a landfill. A new gas flow channel was formed between the rubber membrane and the loess specimen as a result of the interfacial effect. Although both the mechanical and interfacial effects can elevate the gas emission rate, the latter did not play a role in the improvement of the gas permeability; therefore, misleading interference took place in the evaluation of the gas permeability, and an overall failure of the loess cover layer. To tackle this problem, the point at which the large- and small-effective stress asymptotes cross on the volumetric deformation-P_eff diagram may be applied to give early warning signals of the potential overall failure of the loess cover layer in landfills in northwestern China.

Investigating immobilization efficiency of Pb in solution and loess soil using bio-inspired carbonate precipitation

Lead (Pb) metal accumulation in surrounding environments can cause serious threats to human health, causing liver and kidney function damage. This work explored the potential of applying the MICP technology to remediate Pb-rich water bodies and Pb-contaminated loess soil sites. In the test tube experiments, the Pb immobilization efficiency of above 85% is attained through PbCO₃ and Pb(CO₃)₂(OH)₂ precipitation. Notwithstanding that, in the loess soil column tests, the Pb immobilization efficiency decreases with the increase in depth and could be as low as approximately 40% in the deep ground. PbCO₃ and Pb(CO₃)₂(OH)₂ precipitation has not been detected as the majority of Pb²⁺ combines with -OH (hydroxyl group) when subjected to 500 mg/kg Pb²⁺. The alkaline front promotes the chemisorption of Pb²⁺ with CO₃^2- reducing the depletion of quartz mineral close to the surface. However, OH^- is in shortage in the deep ground retarding the Pb immobilization. The Pb immobilization efficiency thus decreases with the increase in depth. Quartz and albite minerals, when subjected to 16,000 mg/kg Pb²⁺, appear not to intervene in the chemisorption with Pb²⁺ where the chemisorption of Pb²⁺ with CO₃^2- plays a major role in the Pb immobilization. Compared to the nanoscale urease applied to the enzyme-induced carbonate precipitation (EICP) technology, the micrometer scale ureolytic bacteria penetrate into the deep ground with difficulty. The 'size' issue remains to be addressed in near future.

Immobilizing lead and copper in aqueous solution using microbial- and enzyme-induced carbonate precipitation

Inappropriate irrigation could trigger migration of heavy metals into surrounding environments, causing their accumulation and a serious threat to human central nervous system. Traditional site remediation technologies are criticized because they are time-consuming and featured with high risk of secondary pollution. In the past few years, the microbial-induced carbonate precipitation (MICP) is considered as an alternative to traditional technologies due to its easy maneuverability. The enzyme-induced carbonate precipitate (EICP) has attracted attention because bacterial cultivation is not required prior to catalyzing urea hydrolysis. This study compared the performance of lead (Pb) and copper (Cu) remediation using MICP and EICP respectively. The effect of the degree of urea hydrolysis, mass and species of carbonate precipitation, and chemical and thermodynamic properties of carbonates on the remediation efficiency was investigated. Results indicated that ammonium ion (NH4+) concentration reduced with the increase in lead ion (Pb2+) or copper ion (Cu2+) concentration, and for a given Pb2+ or Cu2+ concentration, it was much higher under MICP than EICP. Further, the remediation efficiency against Cu2+ is approximately zero, which is way below that against Pb2+ (approximately 100%). The Cu2+ toxicity denatured and even inactivated the urease, reducing the degree of urea hydrolysis and the remediation efficiency. Moreover, the reduction in the remediation efficiency against Pb2+ and Cu2+ appeared to be due to the precipitations of cotunnite and atacamite respectively. Their chemical and thermodynamic properties were not as good as calcite, cerussite, phosgenite, and malachite. The findings shed light on the underlying mechanism affecting the remediation efficiency against Pb2+ and Cu2+.