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Given copper and lead mine enrichment and the high yield of North-Western China, the metallurgical industry attains prosperity. Metallurgical industrial waste which contains heavy metals can cause contamination and threaten the fragile loess environment. The disposal of hazardous waste is becoming a challenging and unavoidable task when land contamination consistently raises environmental concerns. Electrokinetic (EK) technology is an increasingly popular alternative to traditional approaches for contaminated soil remediation because of its high time effectiveness and low risk of secondary pollution. This study investigates the effect of degree of compaction, initial water content, and electric field intensity on the percentage removal of heavy metals in the loess. An EK reactor applied to the EK experiments is set up to measure pH, electric conductivity (EC), and electromigration of cation and heavy metal ions. The experimental results indicate that the lower degree of compaction promoted the formation of the agglomerate structure, and therefore, caused the electromigration path wider and straighter towards elevating the percentage removal of heavy metals. Furthermore, the slippage of the Gouy layer aggravated the electromigration of the cations and heavy metal ions when subjected to the higher initial water content, thereby enhancing the percentage removal of heavy metals. Moreover, the electromigration of the cations and heavy metal ions was notably accelerated by the higher electric current resulting from the higher electric field intensity. The removal of heavy metals was improved accordingly. These results, however, also highlighted the deteriorated percentage removal by the precipitation of heavy metal compounds at the cathode, induced by the alkaline front. A dynamic pH regulation during the EK remediation and a permeable reactive barrier to separate the heavy metal ions from the hydroxide ions are deemed as of great necessity for optimising the removal of heavy metals in the loess. [ABSTRACT FROM AUTHOR]

[Display omitted] • Graphene oxide particles provided larger average pore size and pore volume. • Graphene oxide particles also helped to form hydrogen bonds. • Hydrogen bonds promoted the crosslinked structure formation. • The surface functional groups encouraged copper adsorption. • The non-faradaic nature of the proposed hydrogel reduced the polarization effect. Inappropriate handling of heavy metal-containing wastewater resulting from extensive mining and smelting activities causes serious threats to surrounding environments and human health. The electrokinetic (EK) technology has been extensively applied to remediate heavy metal-contaminated sites because of its great manoeuvrability. The electrochemical polarization effect, however, reduces the electron transfer rate and could badly degrade EK removal efficiency. To this end, the present work proposed a novel graphene oxide-alginate composite hydrogel and applied it to copper removal as electrodes in EK remediation. A series of laboratory tests concerning its physical, mechanical, adsorption, and electrochemical properties were conducted. The graphene oxide (GO) particles provide larger average pore size and pore volume than traditional activated carbon electrodes. The surface functional groups, including hydroxyl, carboxyl, and epoxide groups, promote hydrogen bond formation. The GO particles and the calcium alginate are crosslinked with the hydrogen bonds, improving the mechanical properties of the graphene oxide-alginate composite hydrogel. Furthermore, the functional groups also encourage copper adsorption, and a high GO proportion does not necessarily mean an improvement in copper adsorption. The dynamic mechanism of Cu(II) adsorption onto the graphene oxide-alginate composite hydrogel conforms with the pseudo-second-order kinetic model. Moreover, the absence of oxidative and reductive peaks in the cyclic voltammograms is mainly attributed to the non-faradaic nature of the graphene oxide-alginate composite hydrogel, indicating a reduced electrode polarization effect. The Nyquist plot also approves its low resistance nature. The results were then compared against other electrodes to highlight the relative merits of the proposed graphene oxide-alginate composite hydrogel electrode. [ABSTRACT FROM AUTHOR]

When exposed to chemicals, the micro-structural evolution of the loess and the impacts on the macro-mechanical properties are considered crucial for contaminated land reclamation. In this study, the micro-structural characteristics of the loess specimens that are exposed to acetic acid or sodium sulfate, are studied using scanning electron microscopy, X-ray diffraction, and energy dispersive spectroscopy analyses. Further, their macro-mechanical properties are determined by direct shear tests. The corrosion of the cement between particles under acetic acid environments and the salt-induced swelling under saline conditions play an important role in the micro-structural deterioration. The cohesion and friction angle for the wetted loess are about 16 kPa and 19° respectively, while for the acetic acid-contamianted loess, they reduce to 10 kPa and 15° respectively. They, for the sodium sulfate-contaminated loess, reduce further to below 10 kPa and 13° respectively. The macro-mechanical properties show good correspondence with the micro-structural deterioration. • The effect of contamination on the microstructure deterioration was investigated. • The mechanism that leads to the deteriorated microstructure was revealed. • The correspondence with the macroscale mechanical properties was examined. [ABSTRACT FROM AUTHOR]

• The effect of contamination on the microstructural characteristics is investigated. • The change in microstructural characteristics is highlighted via microscale tests. • The mechanical and microstructural properties present a good mutual correspondence. Soil contamination not only can cause environmental problems but also lead to a notable change in the mechanical properties of soil. Loess widely distributed over North-West (NW) China is featured with the metastable structure, and chemical contaminants produced especially during the rapid development of NW China in recent years seriously threaten the fragile loess environments. When exposed to chemical contaminants, the impacts on the microstructural characteristics of the loess and the resultant mechanical properties are deemed critical for land reclamation in NW China. In light of this, the microscale structural characteristics of the loess exposed to acetic acid, phosphoric acid, sodium hydroxide, and sodium sulfate respectively are studied using scanning electron microscopy, X-ray diffraction, and energy dispersive spectroscopy tests. Additionally, their resultant macroscale mechanical properties are determined by direct shear tests. The deterioration mechanism regarding the microscale structural characteristics when exposed to the contaminants is revealed, and the resultant macroscale mechanical properties present a good correspondence with the deteriorated microscale structural characteristics. The findings of this work provide some guideposts for contaminated land reclamation in NW China. [ABSTRACT FROM AUTHOR]

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. • The carbon and nitrogen sources and trace elements affected the sporulation rate. • The microcapsule prevented the spores from affecting by harsh pH conditions. • The microcapsule provided the spores with more sites to combine with Pb2+ and Ca2+. • The microcapsule achieved the self-healing of carbonate precipitation. [ABSTRACT FROM AUTHOR]

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. • The capillary effect is a crucial factor affecting the gas breakthrough pressure. • Both the mechanical and interfacial effects gear up the gas emission rate. • The interfacial effect can mislead the assessment of the failure of a cover layer. • The volumetric deformation goes up with decrease in P eff in an exponential manner. • Such exponential relationship gives early warning of the failure of a cover layer. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • The relationships of copper immobilization efficiency vs. carbonate species are derived. • The test tube experiments highlight the necessity of narrowing pH to as close as 7. • Cu is coordinately adsorbed with minerals in the one-dimensional column tests. • The reduction in immobilization efficiency appears when under pH < 4 or pH > 9. [ABSTRACT FROM AUTHOR]

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 OD 600 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. [Display omitted] • The potential of applying the novel biomineralisation to Pb removal is explored. • The intracellular urease and calcium source elevate the degree of urea hydrolysis. • The multilayer structure depresses the effect of Pb2+ and prevents their migration. [ABSTRACT FROM AUTHOR]