Biomimetic Hydrogel Composites for Soil Stabilization and Contaminant Mitigation

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.

Poly (acrylamide-co-2-acrylamido-2-methylpropane sulfonic acid)-g-carboxymethylcellulose-Ca(II) hydrogel beads for efficient adsorption of Cd(II)

To address Cd(II) contamination in aquatic environments and the limitations of conventional adsorbents, such as poor mechanical strength, low adsorption capacity, and insufficient reusability, a novel hydrogel bead adsorbent with a semi-interpenetrating polymer network (semi-IPN) was developed using a simple physical and chemical crosslinking approach. The hydrogel beads, composed of poly(acrylamide-co-2-acrylamido-2-methylpropanesulfonic acid)-carboxymethyl cellulose-Ca(II) (P(AM-AMPS)-CMC-Ca(II)), exhibited a high compressive strength of 60.0 kPa. Their well-developed porous structure and abundant functional groups significantly enhanced their Cd(II) adsorption potential. Isothermal, kinetic, and thermodynamic adsorption experiments conducted at pH 5.65 indicated that Cd(II) adsorption onto the hydrogel beads followed the Freundlich isotherm model and the pseudo-second-order kinetic model, suggesting a non-spontaneous, endothermic, and heterogeneous multilayer adsorption process. The adsorption mechanism was governed by both physical and chemical interactions, with a maximum adsorption capacity of 275.13 mg/g. Characterization (SEM, FTIR, XPS) confirmed that Cd(II) adsorption was primarily driven by electrostatic attraction, complexation with functional groups, and ion exchange. After five adsorption-desorption cycles using 0.1 mol/L HCl as the desorption agent, the adsorption efficiency remained above 90 %. Overall, the hydrogel beads, with simple preparation method, high strength, and excellent regeneration, could be a promising eco-friendly adsorbent for Cd(II) removal.

Spatiotemporal patterns and co-occurrence patterns of dissimilatory nitrate reduction to ammonium community in sediments of the Lancang River cascade reservoirs

Dissimilatory nitrate reduction to ammonium (DNRA) is an important nitrate reduction pathway in freshwater sediments. Many studies have focused on the DNRA process in various natural habitats. However, the joint operation of cascade reservoirs will affect the physical and chemical properties of sediments, which may change the DNRA process and bacterial community pattern in the surface sediments of cascade reservoirs. Our study was the first to investigate the spatiotemporal distribution patterns of potential DNRA rate, nrfA gene abundances, and DNRA bacterial community diversity in surface sediments of the Lancang River cascade reservoirs. The results of slurry incubation experiments combined with the 15N isotope tracer experiment ascertained that the potential rates of DNRA were 0.01-0.15 nmol-N cm-3 h-1, and qPCR results indicated that the abundance range of nrfA was 1.08 × 105-2.51 × 106 copies g-1 dry weight. High throughput sequencing of the nrfA gene revealed that the relative abundance of Anaeromyxobacter (4.52% on average), Polyangium (4.09%), Archangium (1.86%), Geobacter (1.34%), and Lacunisphaera (1.32%) were high. Pearson and RDA correlation analysis exhibited that nrfA gene abundance was positively correlated with altitude, pH, OC, and sand concentration. Anaeromyxobacter was positively correlated with reservoir age and DNRA potential rate. The deterministic environmental selection process plays a crucial role in the formation of the DNRA bacterial community. Network analysis displayed that the dominant DNRA genus was the key population of the DNRA microbial community in the sediments of Lancang River cascade reservoirs. This study reveals that the variation of DNRA bacterial activity and community structure is largely driven by the construction of cascade reservoirs, and provides a new idea for further understanding the characteristics of the DNRA community in the cascade reservoir ecosystem.

Composite biomediated engineering approaches for improving problematic soils: Potentials and opportunities

Several conventional chemical stabilizers are used for soil stabilization, among which cement is widely adopted. However, the high energy consumption and environmental challenges associated with these stabilizers have necessitated the transition toward the adoption/deployment of eco-friendly approaches for soil stabilization. Biomediated techniques are sustainable soil improvement methods adopting less toxic microorganisms, enzymes, or polymers for cementing soil. However, these processes also have several drawbacks, such as slow hardening, environmental impact, high cost, and lack of compatibility with different types of soils. It is hypothesized that these limitations may be overcome by exploring the prospects and opportunities offered by hybrid technological approaches involving the integration of nontraditional stabilizers and microbial-induced biomineralization processes for improving problematic soils. This paper discusses selected previous studies integrating different technologies and their benefits and challenges. The emerging fungi-based bio-mediation techniques and the possibility of forming sustainable fungal-based biocomposites to improve problematic soils are also highlighted.

Biopolymer-assisted enzyme-induced carbonate precipitation for immobilizing Cu ions in aqueous solution and loess

Wastewater, discharged in copper (Cu) mining and smelting, usually contains a large amount of Cu2+. Immobilizing Cu2+ in aqueous solution and soils is deemed crucial in preventing its migration into surrounding environments. In recent years, the enzyme-induced carbonate precipitation (EICP) has been widely applied to Cu immobilization. However, the effect of Cu2+ toxicity denatures and even inactivates the urease. In the present work, the biopolymer-assisted EICP technology was proposed. The inherent mechanism affecting Cu immobilization was explored through a series of test tube experiments and soil column tests. Results indicated that 4 g/L chitosan may not correspond to a higher immobilization efficiency because it depends as well on surrounding pH conditions. The use of Ca2+ not only played a role in further protecting urease and regulating the environmental pH but also reduced the potential for Cu2+ to migrate into nearby environments when malachite and azurite minerals are wrapped by calcite minerals. The species of carbonate precipitation that are recognized in the numerical simulation and microscopic analysis supported the above claim. On the other hand, UC1 (urease and chitosan colloid) and UC2 (urea and calcium source) grouting reduced the effect of Cu2+ toxicity by transforming the exchangeable state-Cu into the carbonate combination state-Cu. The side effect, induced by 4 g/L chitosan, promoted the copper-ammonia complex formation in the shallow ground, while the acidic environments in the deep ground prevented Cu2+ from coordinating with soil minerals. These badly degraded the immobilization efficiency. The Raman spectroscopy and XRD test results tallied with the above results. The findings shed light on the potential of applying the biopolymer-assisted EICP technology to immobilizing Cu ions in water bodies and sites.

Ultra-Tough Self-Healing Hydrogel via Hierarchical Energy Associative Dissipation

Owing to high water content and homogeneous texture, conventional hydrogels hardly reach satisfactory mechanical performance. Tensile-resistant groups and structural heterogeneity are employed to fabricate tough hydrogels. However, those techniques significantly increase the complexity and cost of material synthesis, and have only limited applicability. Here, it is shown that ultra-tough hydrogels can be obtained via a unique hierarchical architecture composed of chemically coupled self-assembly units. The associative energy dissipation among them may be rationally engineered to yield libraries of tough gels with self-healing capability. Tunable tensile strength, fracture strain, and toughness of up to 19.6 MPa, 20 000%, and 135.7 MJ cm⁻3 are achieved, all of which exceed the best known records. The results demonstrate a universal strategy to prepare desired ultra-tough hydrogels in predictable and controllable manners.

A State-of-the-Art Review of Organic Polymer Modifiers for Slope Eco-Engineering

In slope ecological restoration projects, reinforcing soil and promoting vegetation growth are essential measures. Guest soil spraying technology can be used to backfill modified soil and vegetation seeds onto the slope surface, resulting in successful ecological restoration. The use of organic polymer modifiers to reinforce soil has several benefits, such as high strength, effective results, and low pollution levels. Organic polymer soil modifiers can be divided into two categories: synthetic polymer modifiers and biopolymer modifiers. This paper provides a thorough review of the properties and interaction mechanisms of two types of polymer modifiers in soil consolidation. The properties of organic polymer modifiers make them applicable in soil and vegetation engineering on slopes. These modifiers can enhance soil mechanics, infiltration, and erosion resistance and promote vegetation growth. Therefore, the suitability of organic polymer modifiers for soil and vegetation engineering on slopes is demonstrated by their properties and potential for improvement in key areas. Furthermore, challenges and future prospects for slope protection technology using organic polymer modifiers are suggested.

Comparison of Solidification Characteristics between Polymer-Cured and Bio-Cured Fly Ash in the Laboratory

Fly ash (FA) usually causes air and soil pollution due to wind erosion. However, most FA field surface stabilization technologies have long construction periods, poor curing effects, and secondary pollution. Therefore, there is an urgent need to develop an efficient and environmentally friendly curing technology. Polyacrylamide (PAM) is an environmental macromolecular chemical material for soil improvement, and Enzyme Induced Carbonate Precipitation (EICP) is a new friendly bio-reinforced soil technology. This study attempted to use chemical, biological, and chemical-biological composite treatment solutions to solidify FA, and the curing effect was evaluated by testing indicators, such as unconfined compressive strength (UCS), wind erosion rate (WER), and agglomerate particle size. The results showed that due to the viscosity increase in the treatment solution, with the increase in PAM concentration, the UCS of the cured samples increased first (from 41.3 kPa to 376.1 kPa) and then decreased slightly (from 376.1 kPa to 367.3 kPa), while the wind erosion rate of the cured samples decreased first (from 39.567 mg/(m²·min) to 3.014 mg/(m²·min)) and then increased slightly (from 3.014 mg/(m²·min) to 3.427 mg/(m²·min)). Scanning electron microscopy (SEM) indicated that the network structure formed by PAM between the FA particles improved the physical structure of the sample. On the other hand, PAM increased the nucleation sites for EICP. Due to the stable and dense spatial structure formed by the "bridging" effect of PAM and the cementation of CaCO₃ crystals, the mechanical strength, wind erosion resistance, water stability, and frost resistance of the samples cured by PAM-EICP were increased significantly. The research will provide curing application experience and a theoretical basis for FA in wind erosion areas.

Valorization of tannery solid wastes for sustainable enzyme induced carbonate precipitation process

Biocementation via enzyme induced carbonate precipitation (EICP) is an emerging ground improvement technique that utilizes urease for calcium carbonate precipitation. Usage of expensive laboratory grade chemicals in EICP hinders its implementation at field level applications. In this study, the feasibility of utilizing solid wastes generated from leather industry was investigated for EICP process. Initially, the proteinaceous fleshing waste was used as nitrogen source for production of an extracellular urease from Arthrobacter creatinolyticus MTCC 5604 followed by its subsequent use in EICP with suspended solids of tannery lime liquor, as alternative calcium source. The calcium ion solution was prepared by treating suspended solids of lime liquor with 1 N HCl. The EICP was optimum with 1000 U of urease, 1.0 M urea and 1.0 M CaCl₂.2H₂O for test tube experiments. Sand solidification experiments under optimal conditions with five times addition of cementation solution yielded a maximum unconfined compressive strength (UCS) of 810 kPa with laboratory grade CaCl₂.2H₂O and 780 kPa with calcium from lime liquor. The crystalline phases and morphology of the CaCO₃ precipitate were analyzed by XRD, FTIR and SEM-EDX. The results showed the formation of more stable calcite in EICP with calcium obtained from lime liquor, while calcite and vaterite polymorphs were obtained with CaCl₂.2H₂O. Utilization of fleshing waste and lime liquor in EICP could reduce the pollution load and sludge formation that are generated during the pre-tanning operations of leather manufacturing. The results indicated the viability of process to achieve cost effective and sustainable biocementation for large scale applications.

Enhancing soil aggregation and acetamiprid adsorption by ecofriendly polysaccharides hydrogel based on Ca2+- amphiphilic sodium alginate

Soil aggregation plays an important role in agricultural production activities. However, the structure of soil aggregation is destroyed by the natural environment and unreasonable farming management, resulting in the loss of water, fertilizers and pesticides in soil. At present, hydrogels have been widely reported to promote the formation of soil aggregation. In this paper, amphiphilic calcium alginate (ASA/Ca²⁺) was applied to promote the formation of soil aggregation and enhance pesticide retention. Initially, an ASA was obtained through the one-pot Ugi condensation (a four-component green chemical reaction). Then, ASA/Ca²⁺ hydrogel is prepared by Ca²⁺ cross-linking. The formation of soil aggregation was determined through the Turbiscan Lab Expert stability analyzer, Confocal Laser Scanning Microscope (CLSM), and Transmission Electron Microscope (TEM). And the effect of soil aggregation on acetamiprid environmental behavior was investigated by adsorption kinetics, adsorption isotherms, and leaching. The results shown that the three-dimensional network structure of ASA/Ca²⁺ hydrogel can promote the formation of soil aggregation. Aggregate durability index (ADI) was 0.55 in the presence of ASA/Ca²⁺ hydrogel, indicating that amphiphilic hydrogel can enhance the stability of soil aggregation. The adsorbing capacity of acetamiprid was 1.58 times higher than pure soil, and the release of acetamiprid only about 20% in the presence of ASA/Ca²⁺ hydrogel. These results would be helpful for the formation of soil aggregation and pesticides adsorption on soil aggregation. Thus, ASA/Ca²⁺ hydrogel is likely to improve soil quality, simultaneously it can minimize the mobility of pesticides in the agricultural system.

Advances in Enzyme Induced Carbonate Precipitation and Application to Soil Improvement: A Review

Climate change and global warming have prompted a notable shift towards sustainable geotechnics and construction materials within the geotechnical engineer’s community. Earthen construction materials, in particular, are considered sustainable due to their inherent characteristics of having low embodied and operational energies, fire resistance, and ease of recyclability. Despite these attributes, they have not been part of the mainstream construction due to their susceptibility to water-induced deterioration. Conventional soil improvement techniques are generally expensive, energy-intensive, and environmentally harmful. Recently, biostabilization has emerged as a sustainable alternative that can overcome some of the limitations of existing soil improvement methods. Enzyme-induced carbonate precipitation (EICP) is a particularly promising technique due to its ease of application and compatibility with different soil types. EICP exploits the urease enzyme as a catalyst to promote the hydrolysis of urea inside the pore water, which, in the presence of calcium ions, results in the precipitation of calcium carbonate. The purpose of this paper is to provide a state-of-the-art review of EICP stabilization, highlighting the potential application of this technique to field problems and identifying current research gaps. The paper discusses recent progress, focusing on the most important factors that govern the efficiency of the chemical reactions and the precipitation of a spatially homogenous carbonate phase. The paper also discusses other aspects of EICP stabilization, including the degree of ground improvement, the prediction of the pore structure of the treated soil by numerical simulations, and the remediation of potentially toxic EICP by-products.

Surface rainfall erosion resistance and freeze-thaw durability of bio-cemented and polymer-modified loess slopes

Microbially induced calcite precipitation (MICP) has been shown to mitigate sand erosion; however, few studies have applied MICP on loess soils. In this study, polyacrylamide (PAM) was added to the cementation solution, and combined MICP-PAM treatment was applied to improve the surface erosion resistance of loess-slopes. The freeze-thaw (FT) durability of MICP-PAM treated loess slopes was also studied. The obtained results showed that MICP-PAM treatment improved erosion resistance and addition of 1.5 g/L PAM achieved the best erosion control and highest surface strength. The high erosion resistance of MICP-PAM treated slopes could be attributed to the stable spatial structure of precipitation, and PAM addition conveyed stronger resistance to tension or shear force. With increasing number of FT cycles, the surface strength of MICP-PAM treated loess slopes decreased; however, slopes subjected to 12 FT cycles still only lost little soil. In MICP-PAM treated loess slopes, cracks and pores evolved with increasing number of FT cycles. With increasing number of FT cycles, porosity and fractal dimension increased, pore ellipticity decreased slightly, and the percentage of various pores changed slightly. The number of FT cycles had less effect on MICP-PAM treated loess slopes than on untreated slopes. MICP-PAM treatment significantly mitigated surface erosion of loess-slopes and improved FT weathering resistance, thus presenting promising potential for application in the field. In addition, based on the linear correlations between surface strength and rainfall-erosion resistance, surface strength could be measured to evaluate the rainfall-erosion resistance for MICP-PAM treated slopes in practical engineering applications.

Enhanced rainfall erosion durability of enzymatically induced carbonate precipitation for dust control

Globally, most cities are facing severe challenges caused by dust pollution. Recently, the significant dust control application potential of the environmentally friendly enzymatically induced carbonate precipitation (EICP) has been demonstrated. However, repeated rainfall erosion negatively affects the long-term durability of several EICP treated areas. This study applied EICP and added either polyvinyl acetate (PVAc) or polyethylene glycol (PEG) to the cementation solution. The results showed that both PVAc and PEG could improve the shear resistance and rainfall-erosion resistance of treated dust soils. However, for repeated rainfall erosion, the surface strength and calcium carbonate (CaCO₃) contents of samples still decreased to less than 250 kPa and 1.1%, respectively. Therefore, combined EICP-PVAc-PEG treatment was proposed and the rainfall-erosion durability of treated dust soils was further studied. With the EICP-PVAc-PEG treatment, the dust samples achieved better shear resistance, higher surface strength, and better repeated rainfall-erosion resistance. Considering cost, cementation effects, and the effects of repeated rainfalls, EICP-PVAc-PEG treatment with 50 g/L PVAc and 30 g/L PEG was most suitable for dust control. The combined EICP-PVAc-PEG treatment significantly suppressed the generation of dust and improved the rainfall-erosion durability.

Mineralization crust field experiment for desert sand solidification based on enzymatic calcification

Sandstorms have been recognized as severe natural disasters worldwide and it is of great significance to propose an effective and environmentally friendly method to combat sandstorm. In this study, the enzymatic calcification (EC) treatment technology was used for mineralization crust and desert sand solidification. Both laboratory experiments and field site tests were conducted to demonstrate the feasibility of EC treatment to improve wind-erosion resistance and rainfall-erosion resistance. Results showed that with the concentration of reactants higher than 0.25 M or the ratio of urease solution to the cementation solution above 0.8, the improvement effects of wind-erosion resistance and rainfall-erosion resistance decreased. Therefore, the 0.25 M of reagent concentration and 0.8 of ratio of urease solution to the cementation solution were chosen for subsequent field site test. The two test sites had similar CaCO₃ contents, thus obtaining a similar increasing range of surface strength. However, the test site one had larger surface strengths due to thicker cemented crust layers. Both the two test sites had sufficient wind-erosion resistance because of crust layer. Moreover, rainfalls decreased surface strength; the surface strength recovered to a high level after water evaporation. In addition, the effect of rainfall on thickness of crust layer and CaCO₃ was small. The EC treatment had good ecological compatibility, and the combined EC and grass seed treatment was effective for mitigation of desertification. The results demonstrated that EC treatment significantly improved both wind-erosion and rainfall-erosion resistance, which presents promising potential for anti-desertification.

Application of enzymatic calcification for dust control and rainfall erosion resistance improvement

Globally, most cities are facing severe challenges associated with dust pollution and it is of great significance to propose an effective and environmentally friendly dust control method. This study used enzymatically induced calcite precipitation (EICP) technology for dust control. Moreover, polyvinyl acetate (PVAc) was added to the cementation solution to improve its rainfall erosion resistance. The results showed that the optimum ratio of urease solution to cementation solution differed according to the concentrations of reactants in the cementation solution. Under combined EICP and PVAc (50 g/L) treatment, the stability of the dust-slope significantly improved. Moreover, little dust soil loss was washed out by simulated rainfall because of the more stable spatial structure of CaCO₃ precipitation. Furthermore, PVAc addition increased the surface strength of slopes, while the cemented layer became thinner. With this combined EICP and PVAc (50 g/L) treatment, in a field test, the treated area of the slope had higher surface strengths and stronger erosion resistance than untreated areas. These higher surface strengths were attributed to the smaller particle size, and the stronger cementing effect of grass seeds. These results demonstrated that EICP-PVAc treatment significantly controlled dust and mitigated surface erosion of dust-slopes. This represents promising potential for the prevention of dust pollution.

Study on a new type of environment-friendly polymer and its preliminary application as soil consolidation agent during tree transplanting

Transplanting trees with rhizospheric soil is an important way to facilitate tree survival in the process of landscaping and reforestation. Traditional way to prevent looseness of rhizospheric soil is forming soil balls around the roots with bags, boxes or rope wrapping, which is cumbersome, laborious and easy to break. This study is aimed to develop a new type of degradable environment-friendly polymer as soil consolidation agent to facilitate tree transplanting. In this paper, the KGM/CA/PVA ternary blending soil consolidation agent was prepared by using Konjac glucomannan (KGM), chitosan (CA) and polyvinyl alcohol (PVA) as raw materials. Through the verification and evaluation, the clay and sandy soil can be consolidated and formed into soil balls by the ternary blend adhesive, which was convenient for transportation. The preliminary application of the ternary blend adhesive in the transplanting process of sierra salvia, Japanese Spindle (Euonymus japonicus) and Juniperus sabina ‘Tamaricifolia’ confirmed that the application of soil consolidation agent can effectively solve the problem that the root ball of seedling is easily broken in the process of transplant. And the application of soil consolidation agent has no adverse effect on the growth of transplanted seedlings. The research and development of ternary blending soil consolidation agent and its preliminary application in seedling transplanting will provide a new solution to solve the problem of soil ball breakage in the process of seedling transplanting. This is an important stage in the development of new seedling transplanting technology. Therefore, the research and development of soil consolidation agent is of great significance.

Efficacy of Enzymatically Induced Calcium Carbonate Precipitation in the Retention of Heavy Metal Ions

This study evaluated the efficacy of enzyme induced calcite precipitation (EICP) in restricting the mobility of heavy metals in soils. EICP is an environmentally friendly method that has wide ranging applications in the sustainable development of civil infrastructure. The study examined the desorption of three heavy metals from treated and untreated soils using ethylene diamine tetra-acetic acid (EDTA) and citric acid (C6H8O7) extractants under harsh conditions. Two natural soils spiked with cadmium (Cd), nickel (Ni), and lead (Pb) were studied in this research. The soils were treated with three types of enzyme solutions (ESs) to achieve EICP. A combination of urea of one molarity (M), 0.67 M calcium chloride, and urease enzyme (3 g/L) was mixed in deionized (DI) water to prepare enzyme solution 1 (ES1); non-fat milk powder (4 g/L) was added to ES1 to prepare enzyme solution 2 (ES2); and 0.37 M urea, 0.25 M calcium chloride, 0.85 g/L urease enzyme, and 4 g/L non-fat milk powder were mixed in DI water to prepare enzyme solution 3 (ES3). Ni, Cd, and Pb were added with load ratios of 50 and 100 mg/kg to both untreated and treated soils to study the effect of EICP on desorption rates of the heavy metals from soil. Desorption studies were performed after a curing period of 40 days. The curing period started after the soil samples were spiked with heavy metals. Soils treated with ESs were spiked with heavy metals after a curing period of 21 days and then further cured for 40 days. The amount of CaCO3 precipitated in the soil by the ESs was quantified using a gravimetric acid digestion test, which related the desorption of heavy metals to the amount of precipitated CaCO3. The order of desorption was as follows: Cd > Ni > Pb. It was observed that the average maximum removal efficiency of the untreated soil samples (irrespective of the load ratio and contaminants) was approximately 48% when extracted by EDTA and 46% when extracted by citric acid. The soil samples treated with ES2 exhibited average maximum removal efficiencies of 19% and 10% when extracted by EDTA and citric acid, respectively. It was observed that ES2 precipitated a maximum amount of calcium carbonate (CaCO3) when compared to ES1 and ES3 and retained the maximum amount of heavy metals in the soil by forming a CaCO3 shield on the heavy metals, thus decreasing their mobility. An approximate improvement of 30% in the retention of heavy metal ions was observed in soils treated with ESs when compared to untreated soil samples. Therefore, the study suggests that ESs can be an effective alternative in the remediation of soils contaminated with heavy metal ions.

Chitosan enhances calcium carbonate precipitation and solidification mediated by bacteria

Formation of the biominerals in living organisms is mainly associated with organic macromolecules. These organic materials play an important role in the nucleation, growth, and morphology controls of the biominerals. Current study mimics this concept of organic matrix- mediated biomineralization by using microbial induced carbonate precipitation (MICP) method in combination with the cationic polysaccharide chitosan. CaCO₃ precipitation was performed by the hydrolysis of urea by the ureolytic bacteria Pararhodobacter sp. SO1 in the presence of CaCl₂, with and without chitosan. The crystal polymorphism and morphology of oven-dried samples were analyzed by X-ray diffraction and scanning electron microscopy. The amount of precipitate obtained was higher in the presence of chitosan. The precipitate included both of the CaCO₃ and the chitosan hydrogel. Rhombohedral crystals were dominant in the precipitate without chitosan and distorted crystal agglomerations were found with chitosan. Sand solidification experiments were conducted in the presence of chitosan under different experimental conditions. By adding chitosan, more strongly cemented sand specimens could be obtained than those from conventional method. All of these results confirm the positive effect of chitosan for the CaCO₃ precipitation and sand solidification.

Hydrogel Interferometry for Ultrasensitive and Highly Selective Chemical Detection

Developing ultrasensitive chemical sensors with small scale and fast response through simple design and low-cost fabrication is highly desired but still challenging. Herein, a simple and universal sensing platform based on a hydrogel interferometer with femtomol-level sensitivity in detecting (bio)chemical molecules is demonstrated. A unique local concentrating effect (up to 10⁹ folds) in the hydrogel induced by the strong analyte binding and large amount of ligands, combined with the signal amplification effect by optical interference, endows this platform with an ultrahigh sensitivity, specifically 10^-14 m for copper ions and 1.0 × 10^-11 mg mL^-1 for glycoprotein with 2-4 order-of-magnitude enhancement. The specific chemical reactions between selected ligands and target analytes provide high selectivity in detecting complex fluids. This universal principle with broad chemistry, simple physics, and modular design allows for high performance in detecting wide customer choices of analytes, including metal ions and proteins. The scale of the sensor can be down to micrometer size. The nature of the soft gel makes this platform transparent, flexible, stretchable, and compatible with a variety of substrates, showing high sensing stability and robustness after 200 cycles of bending or stretching. The outstanding sensing performance grants this platform great promise in broad practical applications.

In Situ Real-Time Study on Dynamics of Microbially Induced Calcium Carbonate Precipitation at a Single-Cell Level

Ureolytic microbially induced calcium carbonate precipitation (MICP) is a promising green technique for addressing a variety of environmental and architectural concerns. However, the dynamics of MICP especially at the microscopic level remains relatively unexplored. In this work, by applying a bacterial tracking technique, the growth dynamics of micrometer-sized calcium carbonate precipitates induced by Sporosarcina pasteurii were studied at a single-cell resolution. The growth of micrometer-scale precipitates and the occurrence and dissolution of many unstable submicrometer calcium carbonate particles were observed in the precipitation process. More interestingly, we observed that micrometer-sized precipitated crystals did not grow on negatively charged cell surfaces nor on other tested polystyrene microspheres with different negatively charged surface modifications, indicating that a negatively charged surface was not a sufficient property for nucleating the growth of precipitates in the MICP process under the conditions used in this study. Our observations imply that the frequently cited model of bacterial cell surfaces as nucleation sites for precipitates during MICP is oversimplified. In addition, additional growth of calcium carbonates was observed on old precipitates collected from previous runs. The presence of bacterial cells was also shown to affect both morphologies and crystalline structures of precipitates, and both calcite and vaterite precipitates were found when cells physically coexisted with precipitates. This study provides new insights into the regulation of MICP through dynamic control of precipitation.