Micro-dynamic process of cadmium removal by microbial induced carbonate precipitation.

Microbially induced carbonate precipitation (MICP) is a technique used extensively to address heavy metal pollution but its micro-dynamic process remains rarely explored. In this study, A novel Cd-tolerant ureolytic bacterium DL-1 (Pseudochrobactrum sp.) was used to study the micro-dynamic process. With conditions optimized by response surface methodology, the removal efficiency of Cd2+ could achieve 99.89%. Three components were separated and characterized in the reaction mixture of Cd2+ removal by MICP. The quantitative-dynamic distribution of Cd2+ in different components was revealed. Five synergistic effects for Cd2+ removal were found, including co-precipitation, adsorption by precipitation, crystal precipitation on the cell surface, intracellular accumulation and extracellular chemisorption. Importantly, during Cd2+ removal by MICP, the phenomenon that crystalline nanoparticles adhere to the cell surface, but without any micrometer-sized precipitation encapsulated bacterial cells was observed. This indicated that the previously studied model of bacterial cells as nucleation sites for metal cation precipitation and crystal growth is oversimplified. Our findings provided valuable insights into the mechanism of heavy metals removal by MICP, and a more straightforward method for studying biomineralization-related dynamic process.

Effects of environmental factors on soil bacterial community structure and diversity in different contaminated districts of Southwest China mine tailings.

A mass of tailings left by mineral exploitation have caused serious environmental pollution. Although many studies have shown that soil microorganisms have the potential to remediate environmental pollution, the interaction mechanism between microorganisms and the surrounding environment of tailings is still unclear. In this study, 15 samples around pyrite mine tailing were collected to explore the ecological effects of environmental factors on bacterial community. The results showed that most of the samples were acidic and contaminated by multiple metals. Cadmium (Cd), copper (Cu), nickel (Ni) migrated and accumulated to into downstream farmlands while chromium (Cr) was the opposite. Proteobacteria, Chloroflex and Actinobacteria were the dominant phyla. Soil pH, total phosphorus (TP), total nitrogen (TN), available potassium (AK), available phosphorus (AP), the bacteria abundance and diversity all gradually increased with the increase of the distance from the tailing. Invertase, acid phosphatase, total organic carbon (TOC), pH, TP and Cr were the main influencing factors to cause the variation of bacterial community. This work could help us to further understand the changes in soil microbial communities around pollution sources.

Evaluation of phytoremediation potential of native dominant plants and spatial distribution of heavy metals in abandoned mining area in Southwest China.

A field investigation on the content of heavy metals in soils and dominant plants was conducted in three sites (A<0.5 km, B<1.0 km, C<1.5 km) with different distances of mine tailings. The spatial distribution of heavy metals and the accumulation in plants were compared, and the candidate species for ecosystem restoration were selected. The results indicated that the soil was polluted by chromium (Cr), Cadmium (Cd), copper (Cu), nickel (Ni) in varying degrees, which is 2.07, 2.60, 1.79, and 4.49 times higher than the Class-Ⅱ standard in China. The concentrate of Ni, Cd, and Zinc (Zn) increased, while Cr, Lead (Pb), and Cu decreased with the distance from the mine tailings. 73 species (34 families) were found and mainly herbaceous plants. The concentrate of Cd, Cu, Cr, and Ni in 29 dominant plants were measured and 66.67%, 21.43%, 100%, 47.62% plants exceeded the normal concentration range. Based on the comparative analysis of heavy metal content, bioconcentration factor, and translocation factor in plants, Polygonum capitatum has good phytoextraction ability, Boehmeria nivea, Chrysanthemum indicum, Miscanthus floridulus, Conyza canadensis, Rubus setchuenensis, Senecio scandens, and Arthraxon hispidus showed remarkable phytostabilization abilities of Cr, Cd, Ni, and Cu, which can be used as potential phytoremediation candidate.

Multiple heavy metals immobilization based on microbially induced carbonate precipitation by ureolytic bacteria and the precipitation patterns exploration.

Biomineralization to immobilize the toxic metal has great potential for the bioremediation of multiple heavy metal contamination. In this study, the efficiency of Microbially Carbonate Induced Precipitation (MICP) for several common heavy metals (Cu, Zn, Ni, Cd) in mining areas as well as their precipitation patterns were researched. After urease activity and precipitation ability comparison, Sporosarcina kp-4 and kp-22 were selected for subsequent studies. The removal of Cd was mainly based on the formation of cadmium carbonate induced by bacteria activity, while the removal of Cu was depended on the pH increase generated by the same process. Precipitation contributed to Zn and Ni removal was more complex, which was also based on the MICP process. Removal rates of Cu, Zn, Ni, and Cd (the concentration of all metals was 160 mg/L) reached 75.10%, 98.03%, 59.46% and 96.18%, respectively, within 2 h. For the immobilization of Cu, Zn, Ni and Cd at 160 mg/L, the optimal dosages of bacterial cultured solution were about 0.25 mL, 0.8 mL, 0.5 mL and 0.8 mL, respectively. Minimum inhibitory concentrations (MIC) revealed the toxicity of these heavy metals for MICP bacteria was arrange as: Cd > Zn > Ni > Cu. Our study confirmed that urease-producing bacteria could coprecipitate multiple heavy metals even without the ability tolerate them, and the MICP process was an effective biological approach that was worth investigating further to immobilize multiple heavy metals in ecological restoration.

Immobilization of cadmium by Burkholderia sp. QY14 through modified microbially induced phosphate precipitation.

Microbially induced phosphate precipitation (MIPP) is an advanced bioremediation technology to immobilize heavy metals. An indigenous bacterium QY14 with the function of mineralization isolated from Cd contaminated farmland soil was identified as Burkholderia ambifaria. The minimum inhibitory concentration value for QY14 was 550 mg/L for soluble Cd concentration. This study found that the addition of 10 mM Ca2+ during MIPP process could significantly increase the removal ratio of Cd, and the maximum removal ratio of Cd with 10 mM Ca2+ and without Ca2+ in solution was 99.97% and 76.14%, respectively. The increase of acid phosphatase activity and the formation of precipitate containing calcium caused by 10 mM Ca2+ addition contributed the increase of Cd removal efficiency. The results of SEM-EDS, FTIR and XRD showed that Cd was removed by forming Cd containing hydroxyapatite (Cd-HAP). In addition, the dissolution experiment showed the Cd release ratio of Cd-HAP (0.01‰ at initial pH 3.0 of solution) was lower than Cd-absorbed HAP, indicating that Cd was more likely removed by the formation of Ca10-xCdx(PO4)6(OH)2 solid solution. Our findings revealed MIPP-based bioremediation supplied with 10 mM Ca2+ could increase the Cd removal and could potentially be applied for Cd remediation.

24-Epibrassinolide combined with heavy metal resistant bacteria enhancing phytoextraction of Amaranthus hypochondriacus L. in Cd-contaminated soil.

The remediation efficiency of phytoextraction on heavy metal could be influenced by metal bioavailability and plant growth. Hence, we applied a synergistic intensification system with plant hormone (24-Epibrassinolide, EBR) and metal-resistant bacterium (Serratia sp. CTZ4) to enhance Cd extraction of Amaranthus hypochondriacus L. in contaminated soil. Results demonstrated that the combination of CTZ4 and EBR promoted soil microecology through decreasing soil pH, improving soil enzymatic activity (dehydrogenase, invertase, acid phosphate, urease). Besides, microbial community structure was evaluated to understand the diversity and relative abundance of microbe in soil after remediation. Moreover, the maximum extraction of Cd was 5.91 mg kg-1 and increased about 60.16 % to CK. Meanwhile, the antioxidant system (SOD, CAT activities) of plant was improved significantly as well as plants biomass increasing by 46.02 % with the combination of EBR and CTZ4. Thus, our results proved that the utilization of EBR and CTZ4 is an alternated method to enhance phytoextraction efficiency of A. hypochondriacus in Cd-contaminated soil.

Ecological responses of soil microbial abundance and diversity to cadmium and soil properties in farmland around an enterprise-intensive region.

Microorganisms play a vital role in soil biochemical process in contaminated managed ecosystems. In the present study, a field investigation was conducted in farmland around an industrial intensive region contaminated with cadmium, and the changes of microbial assemblages in contaminated soils were assessed by 16S rRNA sequencing and the further statistical analysis. The results revealed obvious variations in microbial richness between referenced and contaminated soils, with Proteobacteri, Chloroflexi, Actinobacteria, Acidobacteria and Nitrospirae dominating the studied communities around the industrial intensive region. Redundancy analysis and Spearman correlation heatmap revealed that about 68.95 % of overall variation in microbial community composition was explained by soil physiochemical properties and Cd existence, among which pH, soil total phosphorus, total nitrogen, organic carbon (OC) and available Cd were identified as dominant factors. No significant difference was found in the similarities and Beta-diversity analysis among different groups. In conclusion, this study revealed the ecological effects of physiochemical parameters and Cd stress on the diversity and abundance of microbial communities, and these findings provided the detailed and integrated correlation between the main factors and microbial indexes in Cd contaminated farmland around the industrial intensive region.