Applying the first microcapsule-based self-healing microbial-induced calcium carbonate materials to prevent the migration of Pb ions.

Lead (Pb) accumulation can lead to serious threats to surrounding environments and damage to the liver and kidneys. In the past few years, microbial-induced carbonate precipitation (MICP) technology has been widely applied to achieve Pb immobilization due to its environmentally friendly nature. However, harsh pH conditions can cause the instability of the carbonate precipitation to degrade or dissolve, increasing the potential of Pb2+ migration into nearby environments. In this study, microcapsule-based self-healing microbial-induced calcium carbonate (MICC) materials were applied to prevent Pb migration. The highest sporulation rate of 95.8% was attained at 7 g/L yeast extract, 10 g/L NH4Cl, and 3.6 g/L Mn2+. In the germination phase, the microcapsule not only prevented the bacterial spores from being threatened by the acid treatment but secured their growth and reproduction. Micro analysis also revealed that cerussite, calcite, and aragonite minerals were present, while extracellular polymeric substances (EPSs) were identified via Fourier transform infrared spectroscopy (FTIR). These results confirm their involvement in combining Pb2+ and Ca2+. The immobilization efficiency of above 90% applied to MICC materials was attained, while it of below 5% applied to no MICC use was attained. The findings explore the potential of applying microcapsule-based self-healing MICC materials to prevent Pb ion migration when the calcium carbonate degrades under harsh pH conditions.

Applying a nanocomposite hydrogel electrode to mitigate electrochemical polarization and focusing effect in electrokinetic remediation of a Cu- and Pb-contaminated loess.

Inappropriate handling of copper (Cu) and lead (Pb)-containing wastewater resulting from metallurgical and smelting industries in Northwest China encourages their migration to surrounding environments. Their accumulation causes damage to liver and kidney function. The electrokinetic (EK) technology is considered to be an alternative to traditional remediation technologies because of its great maneuverability. The EK remediation is accompanied by the electrode polarization and the focusing effect toward affecting removal efficiency. In this study, a nanocomposite hydrogel (NCH) electrode was proposed and applied to the EK remediation of Cu- and Pb-contaminated loess. The mechanical, adsorption capacity, adsorption kinetics, and electrochemical properties of the NCH electrode were investigated in detail, followed by microscopic analyses of Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and Raman spectrometer. Results showed that the enhancement of the mechanical properties of the NCH electrode was attributed to the crosslinks of graphene nanoparticles, calcium alginate, and hydrogen bonds, while the Cu or Pb adsorption by the NCH electrode was in a chemisorption manner. The second layer formation might address the increase in adsorption capacity with increasing temperature. These results highlight the relative merits of the NCH electrode and verify the potential of applying the NCH electrode to the EK remediation of Cu- and Pb-contamianted loess.

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+.

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-Peff 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 CO32- 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 CO32- 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 copper in loess soil using microbial-induced carbonate precipitation: Insights from test tube experiments and one-dimensional soil columns.

Biomineralization as an alternative to traditional remediation measures has been widely applied to remediate copper (Cu)-contaminated sites due to its environmental-friendly nature. Immobilizing Cu is, however, a challenging task as it inevitably causes inactivation of ureolytic bacteria. In the present work, a series of test tube experiments were conducted to derive the relationships of Cu immobilization efficiency versus pH conditions. The Cu speciation transformation that is invisible in the test tube experiments was investigated via numerical simulations. Apart from that, the one-dimensional soil column tests, accompanied by the X-ray diffraction (XRD) and Raman spectroscopy analysis, mainly aimed not only to investigate the variations of Cu immobilization efficiency with the depth but to reveal the underlying mechanisms affecting the Cu immobilization efficiency. The results of the test tube experiments highlight the necessity of narrowing pH ranges to as close as 7 by introducing an appropriate bacterial inoculation proportion. The coordination adsorption of Cu, while performing the one-dimensional soil column tests, is encouraged by alkaline environments, which differs from the test tube experiments where Cu2+ is capsulized by carbonate precipitates to prevent their migration. The findings highlight the potential of applying the microbial-induced carbonate precipitation (MICP) technology to Cu-rich water bodies and Cu-contaminated sites remediation.

Effect of seepage conditions on the microstructural evolution of loess across north-west China.

Loess features metastable microstructure and is deemed susceptible to chemical contaminant permeation. However, studies on the loess permeability evolution under water and chemical environments are remarkably limited. In this study, the response of the loess to the water and sodium sulfate seepages was analyzed using the temporal relationship of cations concentration, X-ray diffraction and fluorescence (XRD and XRF), mercury intrusion porosimetry (MIP), and scanning electron microscope (SEM) tests. The permeability evolution characteristics were identified, and its underlying mechanisms were revealed from aspects of the diffuse double layer (DDL) theory and physiochemical actions. The discharge of Mg2+ and precipitation of calcium carbonate, referred also to as the dedolomitization, degraded the macro permeability when subjected to the water seepage test. The salt-induced swelling, induced by the intrusion of Na+ into the DDL, caused an increase in the micropore fraction under the sodium sulfate seepage test, thereby increasing the macro permeability.

Effects of the Urease Concentration and Calcium Source on Enzyme-Induced Carbonate Precipitation for Lead Remediation.

Heavy metal contamination during the rapid urbanization process in recent decades has notably impacted our fragile environments and threatens human health. However, traditional remediation approaches are considered time-consuming and costly, and the effect sometimes does not meet the requirements expected. The present study conducted test tube experiments to reproduce enzyme-induced carbonate precipitation applied to lead remediation under the effects of urease concentration and a calcium source. Furthermore, the speciation and sequence of the carbonate precipitation were simulated using the Visual MINTEQ software package. The results indicated that higher urease concentrations can assure the availability of CO3 2- during the enzyme-induced carbonate precipitation (EICP) process toward benefiting carbonate precipitation. The calcium source determines the speciation of carbonate precipitation and subsequently the Pb remediation efficiency. The use of CaO results in the dissolution of Pb(OH)2 and, therefore, discharges Pb ions, causing some difficulty in forming the multi-layer structure of carbonate precipitation and degrading Pb remediation. The findings of this study are useful in widening the horizon of applications of the enzyme-induced carbonate precipitation technology to heavy metal remediation.

Effects of bacterial inoculation and calcium source on microbial-induced carbonate precipitation for lead remediation.

Heavy metal contamination has caused serious threats to surrounding fragile environments and human health. While the novel microbial-induced carbonate precipitation (MICP) technology in the recent years has been proven effective in improving material mechanical and durability properties, the mechanisms remedying heavy metal contamination still remain unclear. In this study, the potential of applying the MICP technology to the lead remediation under the effects of urease activity and calcium source was explored. The values of OD600 corresponding to the ureolytic bacterial activity, electrical conductivity (EC), urease activity (UA) and pH were applied to monitor the degree of urea hydrolysis. Further, the carbonate precipitations that possess different speciations and cannot be distinguished through test tube experiments were reproduced using the Visual MINTEQ software package towards verifying the validity of the proposed simulations, and revealing the mechanisms affecting the lead remediation efficiency. The findings summarised in this work give deep insights into lead-contaminated site remediation engineering.

Revealing the Enhancement and Degradation Mechanisms Affecting the Performance of Carbonate Precipitation in EICP Process.

Given that acid-rich rainfall can cause serious damage to heritage buildings in NW China and subsequently accelerate their aging problem, countermeasures to protect their integrity and also to preserve the continuity of Chinese culture are in pressing need. Enzyme-induced carbonate precipitation (EICP) that modifies the mechanical properties of the soil through enhancing the interparticle bonds by the precipitated crystals and the formation of other carbonate minerals is under a spotlight in recent years. EICP is considered as an alternative to the microbial-induced carbonate precipitation (MICP) because cultivating soil microbes are considered to be challenging in field applications. This study conducts a series of test tube experiments to reproduce the ordinary EICP process, and the produced carbonate precipitation is compared with that of the modified EICP process subjected to the effect of higher MgCl2, NH4Cl, and CaCl2 concentrations, respectively. The modified EICP, subjected to the effect of higher MgCl2 concentrations, performs the best with the highest carbonate precipitation. The enhancement mechanism of carbonate precipitation is well interpreted through elevating the activity of urease enzyme by introducing the magnesium ions. Furthermore, the degradation of carbonate precipitation presents when subjected to the effect of higher NH4Cl concentration. The decreasing activity of urease enzyme and the reverse EICP process play a leading role in degrading the carbonate precipitation. Moreover, when subjected to the effect of higher CaCl2 concentrations, the slower rate of urea hydrolysis and the decreasing activity of urease enzyme are primarily responsible for forming the "hijacking" phenomenon of carbonate precipitation. The findings of this study explore the potential use of the EICP technology for the protection of heritage buildings in NW China.