Search Results (40)

Soil improvement is one of the fundamental practices in civil engineering, with a long-standing history. In today’s context, the rapidly increasing demand for construction driven by urbanization has further emphasized the necessity and significance of soil stabilization techniques. This study aims to determine the optimum parameters for improving sandy soils by incorporating sodium alginate (SA) as a biopolymer additive into the microbial calcium carbonate precipitation (MICP) process. Sand types S1, S2, and S3, each with distinct particle size distributions, were selected, and the specimens were prepared at medium relative density. Three distinct approaches, MICP, SA, and MICP + SA, were tested for comparison. Additionally, two different improvement methods, injection and mixing, were applied to investigate their effects on the geotechnical properties of the soils. In this context, hydraulic conductivity, unconfined compressive strength (UCS), and calcite content tests, as well as scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses, were performed to assess the changes in soil behavior. SA contributed positively to the overall efficiency of the MICP process. The study highlights SA-assisted MICP as an alternative that enhances the microstructural integrity of treated soils and responds to the environmental limitations of conventional methods through sustainable innovation. Full article

The carbon emissions from the cement industry account for approximately 8% of global carbon emissions, which exerts significant pressure on the environment. In this paper, the microbial-induced calcium carbonate precipitation (MICP) technology was introduced into the carbonization modification research of recycled hardened cement powder (RHCP), and the carbon sequestration performance of RHCP under different pressures was studied. The physicochemical properties of the carbonated products were characterized by microscopic testing methods, and the carbon sequestration mechanism under different pressures was obtained. Subsequently, carbonated RHCP (C-RHCP) was tested as a partial cement substitute for solidified sludge to evaluate its mechanical and durability properties. The results show that when the pressures were 0.3 and 0.5 MPa, the carbon sequestration capacity of RHCP was relatively good, reaching 59.14 and 59.82 g/kg, respectively. Since the carbon sequestration amounts under the two pressures were similar, and considering the energy consumption, in this study, a reaction pressure of 0.3 MPa was selected to prepare C-RHCP. Compared with pure cement, the 28-day unconfined compressive strength (UCS) of the sludge cured with 30% C-RHCP increased by 12.08%. The water stability coefficient of the solidified sludge in the C-RHCP group was greater than 1 after soaking for 7, 14, and 21 days, while the water stability coefficient of the cement group decreased to 0.92 at 14 days. After 20 freeze–thaw cycles, the mass losses of the cement group, the RHCP group, and the C-RHCP group were 31.43%, 38.99%, and 33.09%, respectively. This research not only provides an environmentally friendly strategy for the resource utilization of RHCP but also pioneers a new synergistic model that combines microbial mineralization with the modification of industrial solid waste. It demonstrated significant scientific value and engineering application prospects in reducing carbon emissions in the cement industry and promoted sustainable geotechnical engineering practices based on the “waste–waste” principle. Full article

Microbial-induced calcium carbonate precipitation is an efficient and environmentally friendly soil stabilization technology. To explore the mineralization performance of carbonate precipitation, this study selects three factors, including the type of cementing solution (TCS), the cementing solution concentration (CSC), and the ratio of bacteria to cementing solution (B/C ratio), to investigate their effects on microbial mineralization. This study reveals the influence of each factor on the amount and rate of carbonate precipitation and analyzes the changes in the characteristics of carbonate precipitation crystals, such as the crystal diameter. The experimental results show that (1) the mineralization effect of magnesium ions and calcium ions results in higher precipitation amounts and rates than copper ions, with less environmental pollution. The concentration of the grout solution is positively correlated with the precipitation amount and negatively correlated with the precipitation rate, while the B/C ratio shows the opposite trend. (2) The crystal diameter of CaCO₃ between crystals reduces as the B/C ratio decreases and the CSC increases. (3) The characteristics of MgCO₃ crystals are mainly affected by the CSC. Both excessively high and low concentrations lead to an increase in crystal diameter. (4) The characteristics of CuCO₃ crystals are relatively stable, with smaller crystal particles maintained at around 1 μm. This study can provide a reference for the reinforcement of different types of soils, offering optimal reinforcement solutions based on the required crystal size, carbonate generation amount, and generation rate. It reduces resource waste and unnecessary chemical use, providing a theoretical foundation for sustainable soil remediation and ecological construction. Full article

To enhance the improvement effect of Enzyme-Induced Carbonate Precipitation (EICP) technology more effectively, an abundant renewable resource—lignin—was introduced as an additive during the EICP modification process of silty clay. The mechanical properties of the improved soil specimens were analyzed from a macroscopic point of view by using unconsolidated undrained (UU) triaxial tests and unconfined compressive strength (UCS) tests to determine the optimal lignin content and curing time. The micro-mechanism of the improved soil specimens was elucidated from the microscopic point of view by combining scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests. The experimental results showed that lignin synergized with EICP could effectively improve the mechanical properties of the soil, and the mechanical properties of the co-consolidated soil specimens were better than those of the single consolidated and untreated soil specimens as a whole. The single EICP-consolidated soil specimen had undergone brittle damage; lignin could enhance the toughness of the soil and weaken its brittle characteristics. With the increase of lignin content, the mechanical indicators of co-consolidated soil specimens showed the trend of increasing and then decreasing, and reached the optimum at 0.75%. Moreover, the addition of lignin significantly increased the cohesive force, while the friction angle was less affected. With extended curing time, the mechanical indicators of the co-consolidated soil specimens increased overall, and tended to stabilize after 7 days of curing, hence selecting 7 days as the optimal curing time. From the microscopic point of view, lignin provides nucleation sites for the calcium carbonate precipitates generated by EICP, and the joint action of the two can fill the soil pores and cement the soil particles, thereby improving the overall strength of the soil. The results of the study can provide a theoretical basis and practical reference for the construction of foundation projects in silty clay areas. Full article

Recently, the ability of microbial-induced calcium carbonate precipitation (MICP) to remediate heavy metals has been widely explored. Sporosarcina pasteurii was selected to remediate heavy metal-contaminated solution and aged refuse, exploring the feasibility of Sporosarcina pasteurii bioremediation of heavy metals and analyzing the changes in heavy metal forms before and after bioremediation, as well as the mechanism of remediation. The results showed that Sporosarcina pasteurii achieved remediation rates of 95%, 84%, 97%, and 98% for Cd, Pb, Zn, and Cr (III) in contaminated solution, respectively. It also achieved remediation rates of 74%, 84%, and 62% for exchangeable Cd, Pb, and Zn in aged refuse, respectively. The content of exchangeable Cr (III) before bioremediation was almost zero. The content of heavy metals with exchangeable form and carbonate-bounded form in aged refuse decreased after bioremediation, while the content of heavy metals with iron–manganese oxide binding form and residual form increased. Simultaneously, the presence of Fe and Al components in aged refuse, as well as the precipitation of calcium carbonate produced during the MICP process, jointly promotes the transformation of heavy metals into more stable forms. Full article

This review reports a critical study on the effect of magnetic fields on the precipitation process of calcium carbonate scale from hard water. Indeed, the harmful consequences of the water scaling phenomenon urged researchers to find effective solutions. One of the interesting antiscaling processes is the magnetic treatment of water, which triggers a reduction in the precipitation of calcium carbonate on the walls when in contact with hard water. In the present review, we discuss selected examples related to this process in a combined analysis of the latest advances and the mechanism of action of the magnetic field. Despite the diversity of studies investigating this phenomenon, the effectiveness of this treatment remains a controversial issue, and it is not possible to obtain a clear explanation of the phenomenon. This review proposes, finally, interesting hypotheses which can effectively explain the effect of magnetic treatment on the behavior of hard waters and the precipitation of calcium carbonate, which include magnetohydrodynamics and the hydration effect. Full article

Fly ash washing wastewater was carbonated with carbon dioxide (CO₂) to remove calcium (Ca) by forming a calcium carbonate (CaCO₃) precipitate. An investigation of the factors affecting carbonation showed that Ca removal was highly dependent on the initial pH of the wastewater. The Ca removal was 10%, 61%, 91% and more than 99% at initial wastewater pH levels of 11.8, 12.0, 12.5 and 13.0, respectively. The optimal conditions for carbonation were initial pH of 13.0, carbonation time of 30 min and CO₂ flow rate of 30 mL/min. The Ca concentration in the wastewater decreased to <40 mg/L, while 73 g of CaCO₃ precipitate was produced per liter of wastewater. However, heavy metals, specifically Pb and Zn, co-precipitated during carbonation, which resulted in a CaCO₃ product that contained as much as 0.61 wt% of Pb and 0.02 wt% of Zn. Activated carbon modified by a quaternary ammonium salt was used to selectively adsorb the Pb and Zn first. The Pb- and Zn-free water was then carbonated. By combining adsorption with carbonation, the Ca concentration in the treated wastewater was decreased to about 28 mg/L, while the Na, Cl and K were retained. The wastewater thus treated was ready for NaCl and KCl recovery. In addition, the precipitate had a Ca content of more than 38 wt% and almost no heavy metals. The average particle size of the precipitate was 47 μm, with a uniform cubic shape. The quality of the precipitate met the requirements for the industrial reuse of CaCO₃. In summary, adsorption and carbonation combined were able to remove pollutants from wastewater while recovering useful resources. Full article

Mining and industrial operations are often associated with metalloid and heavy metal contamination of terrestrial and aquatic ecosystems. Heavy metals can weaken the soil’s purification ability to remediate and can accumulate in the human body through crops grown in contaminated soil. In this study, a stabilization method was applied for the remediation of arsenic (As) and heavy metal (Pb and Zn) contaminated soil. Scallop shells (SLS) and starfish (Asterias amurensis, ASF), commonly regarded as waste resource materials, are selected as stabilizers. Proper recycling/reuse measures are required to limit uncontrolled disposal of SLS and ASF, prevent environmental degradation of coastal areas, and take advantage of their high calcium carbonate contents. The stabilizers were processed through −#10 mesh (0.2 mm) and −#20 mesh (0.85 mm) sieves. In addition, calcined stabilizers were produced by calcining SLS and ASF at 900 °C to compare stabilization efficiency based on the presence/absence of high-temperature heat treatment. Each of the three types of processed stabilizers was added to contaminated soil at 2 to 10 wt.%, and the mixtures were subjected to wet curing for 28 days. Extraction with 0.1 N HCl was applied for stabilization efficiency assessment. Crops were cultivated in the stabilized soil to evaluate As and heavy metal immobilization capacity. Analysis by X-ray diffraction (XRD) established that calcite (CaCO₃) was observed in the natural materials and quicklime (CaO) in the calcined materials. The stabilization efficiency assessment results showed that treatment with SLS and ASF effectively reduced the elution of Pb and Zn. SLS was effective in immobilizing As, but the application of natural ASF increased the leachability of As due to the presence of organic matter. However, applying calcined ASF effectively immobilized As because the organic matter was removed at high temperatures. When the transition of As and heavy metals to crops was evaluated, Pb concentrations that exceeded the criterion for leafy vegetables were detected in the lettuce grown in contaminated soil. However, Pb was not detected in the lettuce grown in SLS- and ASF-treated soil, confirming the stability of heavy metal immobilization. Scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDX) analysis showed that the pozzolanic reaction is related to heavy metal immobilization, and Ca–As precipitation is related to the immobilization of As. The results of this study verified that SLS and ASF effectively immobilize As and heavy metals (Pb and Zn) around mines and that they can be used safely in agricultural soil. Full article

The employment of Microbially Induced Calcium Carbonate Precipitation (MICP) is of increasing interest as a technique for environmentally sustainable soil stabilisation. Recent advancements in synthetic biology have allowed for the conception of a pressure-responsive MICP process, wherein bacteria are engineered to sense environmental loads, thereby offering the potential to stabilise specific soil regions selectively. In this study, a 2D smart bio-geotechnical model is proposed based on a pressure-responsive MICP system. Experimentally obtained pressure-responsive genes and hypothetical genes with different pressure responses were applied in the model and two soil profiles were evaluated. The resulting model bridges scales from gene expression within bacteria cells to geotechnical simulations. The results show that both strata and gene expression–pressure relationships have a significant influence on the distribution pattern of calcium carbonate precipitation within the soil matrix. Among the evaluated experimental genes, Gene A demonstrates the best performance in both of the two soil profiles due to the effective stabilisation in the centre area beneath the load, while Genes B and C are more effective in reinforcing peripheral regions. Furthermore, when the hypothetical genes are utilised, there is an increasing stabilisation area with a decreased threshold value. The results show that the technique can be used for soil reinforcement in specific areas. Full article

Potable water production from seawater generates brines that can produce stress in ecosystems, but they are also a potential source of metal and minerals. In our multi-mineral modular seawater brine mining process under development, calcium removal with minimal magnesium removal was the first stage. Even though calcium removal from reverse osmosis brine has been widely studied, there is no relevant research on its precipitation by carbonates at a near-ambient temperature (a range of 15–35 °C) and its selectivity over other minerals, as well as studies on operating conditions for selective precipitation considering the presence of antiscalants. We studied its reaction kinetics and equilibrium and conducted an ex ante life cycle assessment (LCA). The control of pH levels together with the Ostwald ripening process were very important factors to obtain a selective CaCO₃ precipitation. The first-order average kinetic constant of the precipitation at 35 °C was 0.582 ± 0.141 h⁻¹. The presence of minor ions and an antiscalant did not influence the precipitation, obtaining 85–90% on average for the %Ca²⁺ precipitation while the Mg²⁺ co-precipitation was lower than 5–7%. A lab-scale plant, tested in continuous (5 L/h synthetic brine) and in batch (15 L) modes, showed that the latter performs better and could be of interest at a larger scale. The ex ante LCA for the batch (100 L) showed that the main environmentally impactful factors were the thermostatic heating and the addition of a precipitant (Na₂CO₃), but these could be mitigated at the industrial level. Full article

In this study, the influence of acid solutions on the production of precipitated calcium carbonate (PCC) using seashells was investigated. In terms of the Ca dissolution efficiency and atmosphere for dissolving CO₃²⁻, the results indicate that HCl, HNO₃, CH₃COOH, and HCOOH at 1.0 M were the most ideal among the acid solutions. The use of weak acids resulted in the low degree of dissolution of Al and Fe. These impurities could be mostly removed through the pH adjustment process, leading to PCC with a purity of 99% or more. Further, CH₃COOH and HCOOH exhibited low CaCO₃ carbonation efficiency owing to the hydrogen bonding of the carboxyl group and its hindering effect on the growth of CaCO₃ particles. In addition, in the presence of the carboxyl group, the morphology tended to be oval, and the particle size was small. Particularly, when CH₃COOH was used, the combined effect of the low initial Ca ion concentration and slow CO₂ dissolution rate resulted in minimal changes during the carbonation time and the smallest particle size. However, variations in the degree of Ca concentration with a change in the acid solution concentration influenced the dominance of nucleation and particle growth, leading to variations in the particle size. The results of this study revealed that when manufacturing PCC using seashells, the appropriate acid solution must be selected to obtain the required PCC properties. Full article

In order to promote the improvement of rural living environments, the treatment of rural domestic sewage has attracted much attention in China. Meanwhile, the rural regions’ sewage discharge standards are becoming increasingly stringent. However, the standard compliance rate of the total phosphorus (TP) is very low, and the TP has become the main limiting pollutant for the water pollutant discharge standards of rural domestic sewage treatment facilities. In this study, oyster shell waste was employed as a calcium source, and agricultural waste–rice husk was used as a carbon source to synthesize calcium-modified biochar adsorbent materials (Ca-BC) by a simple one-step pyrolysis method. The resultant Ca-BC adsorbent materials demonstrated efficient phosphate (P) adsorption from aqueous solutions over a wide pH range (3–11) and adsorption selectivity. Ca-BC’s adsorption capacity for P increased with the pyrolysis temperature, increasing from 700 °C to 900 °C, which was attributed to the higher specific surface area and calcium oxide content at higher pyrolysis temperatures. The Ca-BC sample, which was made from oyster shells and rice husks with a mass ratio of 2:1 and a pyrolysis temperature of 900 °C, had a maximum adsorption capacity of 196.2 mg/g. The Langmuir model and pseudo-second-order model were the best at describing the adsorption process, and the predominant sorption mechanism for P is the precipitation of calcium oxide or calcium hydroxide with phosphate to create hydroxyapatite. Ca-BC can effectively remove P from rural domestic sewage. The removal rate of the total phosphorus (TP) in rural domestic sewage is 93.9–99.4%. After the adsorption treatment, the discharge of the TP in the rural sewage met the second-grade (TP < 3 mg/L) or even the first-grade (TP < 2 mg/L) Discharge Standard of Water Pollutants for Centralized Rural Sewage Treatment Facilities (DB33/973-2021). This study provides an experimental basis for efficient P removal by Ca-BC adsorbent materials and suggests possible applications in rural domestic sewage. Full article

In order to promote the improvement of the rural living environment, the treatment of rural domestic sewage has attracted much attention in China. Meanwhile, the rural regions’ sewage discharge standards are becoming increasingly stringent. However, the standard compliance rate of total phosphorus (TP) is very low, and TP has become the main limiting pollutant for the water pollutants discharge standards of rural domestic sewage treatment facilities. In this study, waste eggshell (E) was employed as a calcium source, and waste peanut shell (C) was employed as a carbon source to prepare calcium-modified biochar adsorbent materials (E-C). The resulting E-C adsorbent materials demonstrated efficient phosphate (P) adsorption from aqueous solutions over the initial pH range of 6–9 and had adsorption selectivity. At an eggshell and peanut shell mass ratio of 1:1 and a pyrolysis temperature of 800 °C, the experimental maximum adsorption capacity was 191.1 mg/g. The pseudo second-order model and Langmuir model were best at describing the adsorption process. The dominant sorption mechanism for P is that Ca(OH)₂ is loaded on biochar with P to form Ca₅(PO₄)₃OH precipitate. E-C was found to be very effective for the treatment of rural domestic sewage. The removal rate of TP in rural domestic sewage was 91–95.9%. After adsorption treatment, the discharge of TP in rural sewage met the second-grade (TP < 3 mg/L) and even first-grade (TP < 2 mg/L). This study provides an experimental basis for efficient P removal by E-C adsorbent materials and suggests possible applications in rural domestic sewage. Full article

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. Full article

The characteristics and sources of the hydrochemical composition of natural water are mainly influenced and limited at the basin scale by what factors, this has become a focal issue of environmental change in the middle latitude areas and even globally. In this study, three large drainage systems (Junggar, Yili, and Erlqis) in the north of the Tianshan Mountains were selected to study the hydrochemistry of different river basins and understand the relevant causes and attributions of different water bodies in arid environments in the Central Asian Orogenic Belt (CAOB). Natural water samples from the three drainage systems and their hydrochemical data, combined with literature data of the north and south Tianshan Mountains and surrounding areas, were systematically collected and comprehensively compared with other higher, middle, and low latitude watersheds with different climate conditions. The results show that the total dissolved solid concentrations in the CAOB drainage systems are generally higher than those of rivers in Chinese monsoon and humid regions, and also higher than the world average level. The relative concentrations of different ions are similar to those of rivers in the monsoon region of eastern China and most rivers in the world dominated by carbonate weathering. The ionic compositions of surface water bodies from different sub-basins in the study area are distributed near the Ca apex in the piper diagram, while those of phreatic and confined groundwater samples tend to the Na apex. The compositional differences in the anions are not like those in the cations, rarely distinguishable for these different water types. In a sub-drainage basin scale from I to VIII sub-basins in the study area, major ion concentrations and distributions in these basins are evidently heterogeneous. Almost all the cation and anion concentrations span > 1 order of magnitude, especially sodium and chlorine; however, the calcium and alkalinity concentrations and distributions are relatively homogeneous for some basins in the Junggar and Yili drainage systems. The relative homogeneity between anions and the relative heterogeneity between cations can be recognized in the ion chemistry of natural water samples between different types, indicating that the composition and distribution of cations in water is more conducive than those of anions to identifying the differences and commonalities between different regions. Atmospheric precipitation plays a key role on the hydrogeochemical process in the north of the Tianshan Mountains, and the elevation differentiation of chemical weathering and ion concentrations are closely related to it. The positive correlation between total solute flux and runoff cannot be explained by the dilution effect, indicating that hydroclimatic factors such as runoff and aridity have a strong effect in limiting the hydrochemistry of natural water bodies in the study area. Full article