Chitosan enhances calcium carbonate precipitation and solidification mediated by bacteria

Force mechanism analysis of composite microbial dust suppressants based on extracellular polymeric substances (EPS) mode components

As an important potential dust suppression method, the slow onset time is one of the key factors that restrict the effect of microbial dust suppressant. In the early stage, we have confirmed that extracellular polymeric substances (EPS) can improve the dust suppression effect by wetting coal dust and increasing Ca2+ nucleation sites. Therefore, in this study, chitosan (CTS) and bovine serum albumin (BSA) in different ratios (CTS: BSA = 1:1, 1:2, 2:1) as model molecules of EPS were combined with Bacillus subtilis to prepare efficient and fast microbial dust suppressants. Furthermore, the interaction forces were analyzed through molecular dynamics simulation. Results showed that adding CTS and BSA would improve the dust suppression effect, and the dust suppression effect was the best when the ratio of CTS: BSA was 1:2. In addition, the contact angle decreased as the BSA content increased. The Fourier transform infrared spectroscopy (FTIR) results showed that when the ratio of CTS to BSA was 1:2, the dust suppressants were easier to interact with coal dust by the key functional groups and form calcite type CaCO3. The molecular dynamics simulation results showed that the main interaction was Van der Waals force. In addition, the interaction force was strongest when CTS: BSA was 1:2, increasing by 137% compared with the microbial dust suppressants without CTS or BSA. This study provides theoretical support for the development of efficient and rapid microbial dust suppressants.

Effects of trehalose and sodium alginate on microbially induced carbonate precipitation

The process of altering the microbial-induced carbonate precipitation (MICP) by adding additives has been extensively studied. The impact of polysaccharides, as an important component of bacteria, still requires deeper exploration on MICP. This work thus focuses on two types of sugars, sodium alginate (SA) and trehalose (Tre), to explore their effects on biomineralization of carbonate induced by Bacillus pumilus Z6. The results show that B. pumilus Z6 can raise the environmental pH and increase the supersaturation of carbonate and bicarbonate ions through carbonic anhydrase. The presence of organic functional groups and the negative carbon isotope signatures in minerals provide evidence of microbial involvement. Tre and SA do not change the mineral phase, which mainly consists of hollow rice-like granular vaterite and irregular calcite. Tre is conducive to the formation of calcite, whereas the carboxyl groups in SA contribute to the stability of vaterite. Both Tre and SA enhance the removal rate of calcium ions; however, SA is more effective for this purpose. Furthermore, mineralization experiments with calcium alginate gel tablets indicate that SA can attract calcium carbonate to nucleate on its surface. This research offers significant insights into biomineralization processes and introduces novel perspectives for advancing MICP technology.

Understanding microbial biomineralization at the molecular level: recent advances

Microbial biomineralization is a phenomenon involving deposition of inorganic minerals inside or around microbial cells as a direct consequence of biogeochemical cycling. The microbial metabolic processes often create environmental conditions conducive for the precipitation of silicate, carbonate or phosphate, ferrate forms of ubiquitous inorganic ions. Till date the fundamental mechanisms underpinning two of the major types of microbial biomineralization such as, microbially controlled and microbially induced remains poorly understood. While microbially-controlled mineralization (MCM) depends entirely on the genetic makeup of the cell, microbially-induced mineralization (MIM) is dependent on factors such as cell morphology, cell surface structures and extracellular polymeric substances (EPS). In recent years, the organic template-mediated nucleation of inorganic minerals has been considered as an underlying mechanism based on the principles of solid-state bioinorganic chemistry. The present review thus attempts to provide a comprehensive and critical overview on the recent progress in holistic understanding of both MCM and MIM, which involves, organic-inorganic biomolecular interactions that lead to template formation, biomineral nucleation and crystallization. Also, the operation of specific metabolic pathways and molecular operons in directing microbial biomineralization have been discussed. Unravelling these molecular mechanisms of biomineralization can help in the biomimetic synthesis of minerals for potential therapeutic applications, and facilitating the engineering of microorganisms for commercial production of biominerals.

An experimental study on the curing of desert sand using bio-cement

In order to promote the development and utilization of desert sand, this study is based on researching the most suitable ratio of bio-cement, analyzing the shear strength and permeability of improved desert sand by combining bio-cement and fly ash, and clarifying the applicability of tap water in bio-cement. The relationship between the two and the microstructural properties was investigated using the results of the straight shear test and the permeability test. The results showed that the urease solution prepared with tap water had a more pronounced temperature resistance. The urea concentration and the corresponding pH environment had a direct effect on the urease activity. The calcium carbonate yield was positively correlated with the calcium concentration, and the urea concentration was higher in the ranges of 1.0-1.5 mol/L. As the enzyme-to-gel ratio decreased, the calcium carbonate precipitate produced per unit volume of urease solution gradually converged to a certain value. The shear strength (increased by 37.9%) and permeability (decreased by about 8.9-68.5%) of the modified desert sand peaked with the increase in fly ash content. The microscopic test results indicated that the fly ash could provide nucleation sites for the bio-cement, effectively improving the mechanical properties of the desert sand. The crystal types of calcium carbonate in the modified desert sand were calcite and aragonite, which were the most stable crystal types. This study provides innovative ideas for interdisciplinary research in the fields of bioengineering, ecology and civil engineering.

Analysis of unconfined compressive strength and environmental impact of MICP-treated lead-zinc tailings sand instead of sand as embankment material

Tailings can be used as embankment materials instead of sand. However, they contain large amounts of heavy metal pollutants, which can lead to groundwater pollution. In this study, (lead-zinc) Pb-Zn tailings with five particle sizes and Sporosarcina pasteurii were used as test materials. Combined with the unconfined compressive strength (UCS) and leaching of heavy metal pollutants from Pb-Zn tailings, the feasibility of applying microbial induced carbonate precipitation (MICP)-treated Pb-Zn tailings to embankment materials was analysed from the perspective of strength and environmental performance. The results showed that the UCS and carbonate content of the specimens made of Pb-Zn tailings treated using MICP decreased with a decrease in the number of Pb-Zn tailing particles. The pH value of the leaching solution after MICP treatment of Pb-Zn tailings sand was stable at 7.83-8.03, and the fixation rate of metal ions was 90.28 %-100 %. FTIR, X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy tests showed that after the Pb-Zn tailings with particle sizes less than 100 mesh were treated using MICP, the number of carbonate crystals, crystal uniformity, and crystal overlap on the surface of the sample were considerably higher than those of the tailings with particle sizes greater than 250 mesh. The compressive strength and environmental performance of Pb-Zn tailings with particle sizes less than 100 mesh treated using MICP are good, and they are more suitable for embankment materials.

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.

Insight into biomolecular interaction-based non-classical crystallization of bacterial biocement

In an attempt to draw a correlation between calcium carbonate (CaCO3) precipitation and biomacromolecules such as extracellular polymeric substances and enzyme activity in biomineralizing microbe, this report aims to elucidate the ureolytic and ammonification route in Paenibacillus alkaliterrae to explore the possible role of organic biomolecule(s) present on cell surface in mediating nucleation and crystallization of biogenic CaCO3. After 168 h of biomineralization in ureolysis and ammonification, 2.2 g/l and 0.87 g/l of CaCO3 precipitates were obtained, respectively. The highest carbonic anhydrase activity (31.8 µmoles/min/ml) was evidenced in ammonification as opposed to ureolysis (24.8 µmoles/min/ml). Highest urease activity reached up to 9.26 µmoles/min/ml in ureolytic pathway. Extracellular polymeric substances such as polysaccharides and proteins were found to have a vital role not only in the nucleation and crystal growth but also in addition direct polymorphic fate of CaCO3 nanoparticles. EPS production was higher during ammonification (3.1 mg/ml) than in ureolysis (0.72 mg/ml). CaCO3 nanoparticle-associated proteins were found to be 0.82 mg/ml in ureolysis and 0.56 mg/ml in ammonification. After 30 days of biomineralization, all the polymorphic forms stabilized to calcite in ureolysis but in ammonification vaterite predominated. In our study, we showed that organic template-mediated prokaryotic biomineralization follows the non-classical nucleation and varying proportions of these organic components causes selective polymorphism of CaCO3 nanoparticles. Overall, the findings are expected to further the fundamental understanding of enzymes, EPS-driven non-classical nucleation of CaCO3, and we foresee the design of fit-for-purpose futuristic biominerals arising from such renewed understanding of biomineralization. KEY POINTS: • Organic-inorganic interface of cell surface promote crystallization of biominerals • Carbohydrate and proteins in the interface results selective polymorphism of CaCO3 • Calcite stabilized at 30 days in ureolysis, vaterite-calcite mix in ammonification.

Experimental Study on the Effect of an Organic Matrix on Improving the Strength of Tailings Strengthened by MICP

In order to improve the effect of microbial-induced calcium carbonate precipitation (MICP) in tailings reinforcement, sodium citrate, an organic matrix with good water solubility, was selected as the crystal form adjustment template for inducing calcium carbonate crystallization, and the reinforcements of tailings by MICP were conducted in several experiments. The effects of sodium citrate on the yield, crystal form, crystal appearance, and distribution of calcium carbonate were analyzed by MICP solution test; thus, the related results were obtained. These showed that the addition of a proper amount of organic matrix sodium citrate could result in an increment in the yield of calcium carbonate. The growth rate of calcium carbonate reached 22.6% under the optimum amount of sodium citrate, and the crystals of calcium carbonate were diverse and closely arranged. Based on this, the MICP reinforcement test of tailings was carried out under the action of the optimum amount of sodium citrate. The microscopic analysis using CT and other means showed that the calcium carbonate is distributed more uniformly in tailings, and the porosity of samples is significantly reduced by layered scanning analysis. The results of triaxial shear tests showed that adding organic matrix sodium citrate effectively increased the cohesion, internal friction angle, and peak stress of the reinforced tailings. It aims to provide a novel idea, a creative approach, and a method to enhance the reinforcement effect of tailings and green solidification technology in the mining environment.

A Study on Bio-Stabilisation of Sub-Standard Soil by Indigenous Soil Urease-Producing Bacteria

Sub-standard soils are of great concern worldwide due to diverse economic losses and the possibility of severe environmental hazards ranging from catastrophic landslides, building collapse, and erosion to loss of lives and properties. This study explored the potential of urease-producing bacteria, <i>Bacillus cereus</i> and <i>Bacillus paramycoides</i>, to stabilise sub-standard soil bio-stabilisation. The maximum urease activity measured by <i>B. cereus</i> and <i>B. paramycoides</i> was 665 U/mL and 620 U/mL, respectively. <i>B. cereus</i> and <i>B. paramycoides</i> precipitated 943 ± 57 mg/L and 793 ± 51 mg/L of CaCO₃ at an optical density (425 nm) of 1.01 and 1.09 and pH 8.83 and 8.59, respectively, after 96 hours of incubation. SEM microstructural analysis of the precipitated CaCO₃ revealed crystals of various sizes (2.0–23.0 µm) with different morphologies. XRD analysis confirmed that the precipitated CaCO₃ comprised calcite and aragonite crystals. SEM analysis of the microstructure of organic and sandy clay soils treated with <i>B. cereus</i> and <i>B. paramycoides</i> showed the formation of bio-precipitated calcium carbonate deposits on the soil particles (biocementing soil grains), with <i>B. cereus</i> precipitating more CaCO₃ crystals with a better biocementing effect compared to <i>B. paramycoides</i>. Overall, the experimental results attributed CaCO₃ formation to bacterial-associated processes, suggesting that soil ureolytic bacteria are potentially useful to stabilise sub-standard soil.

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

Enzyme-induced carbonate precipitation (EICP) is considered as an environmentally friendly method for immobilizing heavy metals (HMs). The fundamental of the EICP method is to catalyze urea hydrolysis using the urease, discharging CO₃^2- and NH₄⁺. CO₃^2- helps to form carbonates that immobilize HMs afterwards. However, HMs can depress urease activity and reduce the degree of urea hydrolysis. Herein, the potential of applying the chitosan-assisted EICP method to Pb and Cu immobilization was explored. The chitosan addition elevated the degree of urea hydrolysis when subjected to the effect of Cu²⁺ toxicity where the protective effect, flocculation and adsorption, and the formation of precipitation, play parts in improving the Cu immobilization efficiency. The use of chitosan addition, however, also causes the side effect (copper-ammonia complex formation). Two calcium source additions, CaCl₂ and Ca(CH₃COO)₂, intervened in the test tube experiments not only to prevent pH from raising to values where Cu²⁺ complexes with NH₃ but also to separate the urease enzyme and Cu²⁺ from each other with the repulsion of charges. The FTIR spectra indicate that the chitosan addition adsorbs Cu²⁺ through its surface hydroxyl and carboxyl groups, while the SEM images distinguish who the mineral are nucleating with. The findings shed light on the potential of applying the chitosan-assisted EICP method to remedy lead- and copper-rich water bodies.

On the physicochemical properties, setting chemical reaction, and in vitro bioactivity of aragonite–chitosan composite cement as a bone substitute

A chitosan gel additive modulates the initial vaterite dissolution–recrystallisation in injectable aragonite-based composite cement and promotes its in vitro bioactivity.

Effect of Jute Fibres on the Process of MICP and Properties of Biocemented Sand

There has been increasing interest, in the past decade, in bio-mediated approaches to soil improvement for geotechnical applications. Microbially induced calcium carbonate precipitation (MICP) has been investigated as a potentially sustainable method for the strengthening and stabilisation of soil structures. This paper presents the results of a study on the effect of jute fibres on both the MICP process and properties of biocemented sand. Ureolytic Sporosarcina pasteurii has been used to produce biocemented soil columns via MICP in the laboratory. Results showed that columns containing 0.75% (by weight of sand) untreated jute fibres had unconfined compressive strengths approximately six times greater on average compared to biocemented sand columns without jute fibres. Furthermore, efficiency of chemical conversion was found to be higher in columns containing jute fibres, as measured using ion chromatography. Columns containing jute had calcimeter measured CaCO₃ contents at least three times those containing sand only. The results showed that incorporation of jute fibres into the biocemented sand material had a beneficial effect, resulting in stimulation of bacterial activity, thus sustaining the MICP process during the twelve-day treatment process. This study also explores the potential of jute fibres in self-healing MICP systems.

Experimental Study of EICP Combined with Organic Materials for Silt Improvement in the Yellow River Flood Area

Enzyme-induced carbonate precipitation (EICP) is an emerging biogeotechnical technique that uses free urease to improve soil. Despite its advantages of eliminating complex microbial cultures and reducing reaction byproducts, its efficiency is considered lower than that of microbial induced calcite precipitation (MICP) due to the lack of nucleation sites that induce calcium carbonate deposition. To enhance the strengthening efficiency of EICP for fine-grained soils, an improved EICP method that involves adding an appropriate mass concentration of organic materials (skim milk powder, glutinous rice powder, and brown sugar) into urease solution was proposed and applied to reinforce silt in the Yellow River flood area of China. The preferred concentration and ratio of cementation solution and the optimum concentration of each of the organic materials were determined. Then, the reinforcement effect of the improved EICP at the optimum concentration was compared with the control group, and the reinforcement mechanism for this method was discussed. The results show that after the organic material inclusions, soil strength can be enhanced by 33% compared with EICP-treated soil and is nearly four times higher than that of untreated soil. The superiority of this method over traditional EICP and MICP mainly stems from its ability to provide templates and nucleation sites for calcium carbonate deposition and to improve the size, morphology, and structure of calcium carbonate crystals.

Permanent Antimicrobial Poly(vinylidene fluoride) Prepared by Chemical Bonding with Poly(hexamethylene guanidine)

Biofouling is one of the major obstacles in the application of poly(vinylidene fluoride) (PVDF) membrane in water and wastewater treatment. Developing antimicrobial PVDF could kill the attached microbe in the initial stage, thus theoretically inhibiting the formation of biofilm and delaying the occurrence of biofouling. However, the leaching of the antimicrobial component and deterioration of antimicrobial properties remain a concern. In this work, an antimicrobial PVDF (PVDF-g-AGE-PHMG) was developed by chemical bonding PVDF with poly(hexamethylene guanidine hydrochloride) (PHMG). The obtained PVDF-g-AGE-PHMG was blended with pristine PVDF to prepare an antimicrobial PVDF membrane. The results of Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) confirmed that PHMG was successfully grafted into the PVDF membrane. The morphologies, membrane porosity, water contact angles, antimicrobial properties, mechanical properties, and thermostability of the as-prepared membranes were investigated. When the content of PVDF-g-AGE-PHMG reached 10.0 wt %, the inhibition rates of both antimicrobial PVDF membrane against Escherichia coli and Staphylococcus aureus were above 99.99%. Due to the increased hydrophilicity, excellent antimicrobial activity, nonleaching of antimicrobial component, good mechanical properties, and thermostability, the as-prepared PVDF membrane has promising applications in the field of water treatment.

Microbial Induced Carbonate Precipitation Using a Native Inland Bacterium for Beach Sand Stabilization in Nearshore Areas

Microbial Induced Carbonate Precipitation (MICP) via urea hydrolysis is an emerging sustainable technology that provides solutions for numerous environmental and engineering problems in a vast range of disciplines. Attention has now been given to the implementation of this technique to reinforce loose sand bodies in-situ in nearshore areas and improve their resistance against erosion from wave action without interfering with its hydraulics. A current study has focused on isolating a local ureolytic bacterium and assessed its feasibility for MICP as a preliminary step towards stabilizing loose beach sand in Sri Lanka. The results indicated that a strain belonging to Sporosarcina sp. isolated from inland soil demonstrated a satisfactory level of enzymatic activity at 25 °C and moderately alkaline conditions, making it a suitable candidate for target application. Elementary scale sand solidification test results showed that treated sand achieved an approximate strength of 15 MPa as determined by needle penetration device after a period of 14 days under optimum conditions. Further, Scanning Electron Microscopy (SEM) imagery revealed that variables such as grain size distribution, bacteria population, reactant concentrations and presence of other cations like Mg2+ has serious implications on the size and morphology of precipitated crystals and thus the homogeneity of the strength improvement.