Optimal synthesis of dolochar derived faujasite zeolite X for highly effective Cd(II) removal

Cadmium-induced water pollution is a major environmental issue because of its persistent nature and adverse ecological impacts. Adsorption is a highly favored method due to its versatility and high efficacy in cadmium removal. Hence, the present work aims to develop a low-cost, highly effective adsorbent-dolochar-derived nanoporous zeolite to easily and effectively purify Cd(II) polluted water. The work focuses on the Cd(II) batch adsorption study using the optimal hydrothermal synthesis of a crystalline faujasite Zeolite X (ZX) from dolochar. The synthesis parameters were optimized using Response Surface Methodology, specifically Box Behnken Design (RSM-BBD), to maximize the crystallinity percentage. Variables such as initial Cd(II) concentration, solution pH, dosage, time, and temperature were studied for the Cd(II) batch adsorption study. The optimum conditions for synthesizing ZX include NaOH/Dolochar, crystallization temperature, and crystallization time of 1.375, 100 °C, and 11 h, respectively. The resultant XRD structure exhibited an average crystal size and crystallinity of 0.79 μm and 87.231 %, respectively. The average pore size, micropore volume, micropore area, and total surface area were 3.316 nm, 0.311 cc. g-1, 567.226 m2 g-1, and 583.117 m2 g-1, respectively. The maximum removal was accomplished with optimum conditions of 0.25 g.L-1 dosage, 80 min, at 313.15 K, and 6.5 pH. Adsorption isotherm results agreed with those hypothesized by Freundlich isotherm, with a maximum adsorption capacity of 714.285 mg g-1, and the pseudo-second-order kinetic model describes the adsorption kinetics well. The relevance of the results highlights the importance of using this dolochar-derived nanoporous zeolite as an adsorbent to effectively treat Cd(II) containing wastewater.

Reducing cadmium accumulation in shrimp using Escherichia coli with surface-displayed peptide

Cadmium (Cd) is a hazardous metal that can accumulate in aquatic organisms and endanger human health via the food chain. In this study, genetic engineering was used to display a peptide with Cd-binding potential on the surface of Escherichia coli cells. This whole-cell adsorbent exhibited high affinity for Cd ions (Cd²⁺) in the solution. The Cd²⁺ adsorption capacity of the whole-cell adsorbent was three-fold that of the control cells in a 20 μM Cd²⁺ solution, and 97.2% ± 2.38% of the Cd²⁺ was removed. The whole-cell adsorbent was fed to shrimp (Neocaridina denticulata), and the surface-engineered E. coli successfully colonized the shrimp intestine, which showed significantly less Cd accumulation than the group not fed surface-engineered E. coli. The whole-cell adsorbent evidently protected shrimp from the toxicity of Cd²⁺ by adsorbing it. Moreover, the whole-cell adsorbent mitigated the changes in microbial community structure in the shrimp gut caused by the exposure of Cd²⁺. These findings suggest that this strategy is effective for controlling the contamination of Cd²⁺ in shrimp.

One-pot synthesis of siliceous ferrihydrite - coated halloysite nanorods in alkaline medium: Structure, properties and cadmium adsorption performance

The application of amorphous ferrihydrite (Fh) for Cd(II) removal is restricted by its unstable and easily transformable nature. Although doping with silicates stabilized ferrihydrite, its product siliceous ferrihydrite (SiFh) again suffered from the disadvantage of spontaneous agglomeration. Herein, ferrihydrite was hybridized with halloysite nanotubes (HNTs) to prepare a novel siliceous ferrihydrite - coated halloysite nanorods (SiFh@HNTs) in alkaline medium, to break through the current barriers. The characterization results showed that SiFh@HNTs could simultaneously overcome the defects of easy phase transformation of ferrihydrite and easy aggregation of SiFh nanoparticles (NPs). Meanwhile, the optimal SiFh@HNT₄₀ with halloysite content of 40 % formed a well-developed mesoporous structure and exhibited the desired surface properties: a high specific surface area of 303.4 m²/g, an isoelectric point as low as pH_iep = 4.5, and rich functional Fe - OH groups. The formation mechanism of such excellent sturcture-properties of SiFh@HNT₄₀ were mainly attributed to two factors: the generation of smaller (∼5 nm) SiFh NPs induced by the integration of halloysite-derived SiO₄^4- into ferrihydrite, and the dispersion of SiFh NPs on clay nanotubes. Furthermore, the adsorption capacity of SiFh@HNT₄₀ for Cd(II) was up to 137.8 mg/g at 30 °C and pH 6, which was much higher than that of aggregated ferrihydrite (11.2 mg/g), halloysite (18.8 mg/g) and goethite (49.4 mg/g). The adsorption thermodynamics study revealed the adsorption of Cd(II) on SiFh@HNT₄₀ was clearly chemisorption with a (ΔH_ads)_q of 43.3 kJ/mol. Characterization results of XPS and FTIR confirmed that the rich Fe - OH groups on SiFh@HNT₄₀ was the main adsorption sites, and Cd(II) was specifically adsorbed by inner-sphere surface complexation. In addition, SiFh@HNT₄₀ had application potential in the mixed-metal wastewaters treatment. Cyclic regeneration experiments showed that SiFh@HNT₄₀ had good regeneration performance and could be reused many times.

A novel N-arachidonoyl-l-alanine-catabolizing strain of Serratia marcescens for the bioremediation of Cd and Cr co-contamination

Cadmium (Cd) and chromium (Cr) are widespread contaminants with a high risk to the environment and humans. Herein we isolated a novel strain of Serratia marcescens, namely strain S27, from soil co-contaminated with Cd and Cr. This strain showed strong resistance to Cd as well as Cr. S27 cells demonstrated Cd adsorption rate of 45.8% and Cr reduction capacity of 84.4% under optimal growth conditions (i.e., 30 °C, 200 rpm, and pH 7.5). Microscopic characterization of S27 cells revealed the importance of the functional groups C-O-C, C-H-O, C-C, C-H, and -OH, and also indicated that Cr reduction occurred on bacterial cell membrane. Cd(II) and Cr(VI) bioaccumulation on S27 cell surface was mainly in the form of Cd(OH)₂ and Cr₂O₃, respectively. Further, metabolomic analyses revealed that N-arachidonoyl-l-alanine was the key metabolite that promoted Cd and Cr complexation by S27; it primarily promotes γ-linolenic acid (GLA) metabolism, producing siderophores and coordinating with organic acids to enhance metal bioavailability. To summarize, our results suggest that S27 is promising for the bioremediation of environments contaminated with Cd and Cr in tropical regions.

Adsorption capacity of Penicillium amphipolaria XK11 for cadmium and antimony

Heavy metal pollution is a global problem that affects both the environment and human health. Microorganisms play an important role in remediation. Most studies on the use of microorganisms for heavy metal remediation focus on single heavy metals. In this study, a strain of Penicillium amphipolaria, XK11 with high resistance to both antimony (Sb III) and cadmium (Cd II) was screened from the mineral slag. The strain also had a high phosphate solubilization capacity. The single-factor adsorption experiment results showed that the initial pH (pH₀), adsorption time (T), and initial solution concentration (C₀) all affected the adsorption of Sb and Cd by XK11. When the initial pH₀ (Cd = 6, Sb = 4) and adsorption time (T = 7 d) were constant, XK11 achieved the maximum removal rate of Cd (45.6%) and Sb (34.6%). These results confirm that XK11 has potential as a biomaterial or remediation of Sb and Cd pollution.

Nano-chlorapatite modification enhancing cadmium(II) adsorption capacity of crop residue biochars

Cadmium (Cd) contamination in rivers or lakes has attracted worldwide concerns. Biochar pyrolyzed form crop residues (CR) could adsorb Cd(II) from aquatic environments, while the removal capacity of single CR biochar is relatively low. Nano-chlorapatite (nClAP) modification can enhance metal scavenging ability, but little is known about the behaviors and mechanisms of Cd(II) adsorption by nClAP-modified CR biochars. In this study, the influences of feedstock type, pyrolysis temperature, nClAP modification and aquatic environments on Cd(II) adsorption of biochars derived from rice (RB) and wheat (WB) husks were investigated comprehensively. Results showed that the pristine RB and WB showed low and similar Cd(II) adsorption capacities, while the rise of pyrolysis temperatures from 300 to 600 °C significantly improved the adsorption capacities. The Cd(II) adsorption of both RB and WB was regarded as monolayer chemical processes controlled by chemical precipitation, surface complexation and cation exchange mechanisms. Moreover, the nClAP modification notably enhanced Cd(II) adsorption capacities from 13.2 to 39.9 mg·g^-1 of pristine biochars to 25.2-60.7 mg·g^-1 of modified biochars attributed to the improved contribution of Cd(II)-phosphate precipitation. Among all biochars, the nClAP-modified RB and WB pyrolyzed at 500 °C had the highest Cd(II) adsorption capacities with 60.7 and 48.3 mg·g^-1, respectively. These biochars could maintain good adsorption performances under the neutral-alkaline (pH 6-8), low ionic strength, high dissolved organic matter and all oxidation-reduction potential conditions. In conclusion, this study reveals the importance of nClAP modification to optimize Cd(II) adsorption of CR biochars, which provides a promising future for its practical application in aquatic Cd(II) scavenging.

Vermiculite as a potential functional additive for water treatment bioreactors inhibiting toxic action of heavy metal cations upsetting the microbial balance

A new adsorbent that combines mineral vermiculite with the yeast Saccharomyces cerevisiae, was used for Cd²⁺ removal. The influence of vermiculite presence on the toxic effects of Cd²⁺ to Saccharomyces cerevisiae yeast was evaluated as a function of the microorganisms' respiratory activity (CO₂ production). The Cd²⁺ toxicity increased with prolonged exposure time reaching the LC₅₀ value of 857 and 489 mg L^-1 after 30 and 120 min, respectively. The yeast managed to bioaccumulate 25.0 ± 0.6 mg g^-1 of Cd²⁺ at the initial Cd²⁺ concentration of 741.9 mg L^-1; the maximum Cd²⁺ adsorption capacity of vermiculite reached 25 ± 5 mg g^-1. The addition of the mineral decreased the cations toxic effect; the LC₂₀ value in vermiculite absence attained approximately 200 mg L^-1 after 30 min and decreased to 80 mg L^-1 after 2 h, while in the bio-mineral system it was at the level of 435 ± 50 mg L^-1 without a significant change in time. The mineral provided a superior living environment for the yeast by removing part of the cations, releasing essential microelements and providing a protective, clay hutch-like habitat for the cells.

Aggregation, solubility and cadmium-adsorption capacity of CuO nanoparticles in aquatic environments: Effects of pH, natural organic matter and component addition sequence

Nanoparticles (NPs), heavy metals and natural organic matter may co-exist in the water bodies. Currently, knowledge on their interaction effects on the behaviors and fates of NPs and heavy metal ions is rather limited, which is critical to comprehensively understand their environmental risk. In this study, the aggregation, solubility and Cd-adsorption of CuO NPs co-existing with humic acid (HA) and Cd²⁺ upon different solution pH and contact sequences were determined. In the ternary systems of CuO NPs, HA and Cd²⁺, pH was more important than the contact sequence of the components in affecting the NP aggregation, while the contact sequence was a predominant factor in determining the NP solubility. Pre-equilibration of CuO NPs and HA before addition of Cd²⁺ resulted in the highest solubility and lowest aggregation of the NPs, relative to other sequences of addition of the components. The adsorption capacity of CuO NPs for Cd-ions increased with an increasing pH value from 5 to 9. HA significantly enhanced the Cd-adsorption capacity of CuO NPs at pH 7 and 9, while at pH 5 a non-significant effect was observed. The results are helpful to better estimate the behaviors and fates of CuO NPs and Cd²⁺ when they coexisting in natural waters.

Self-immobilized biochar fungal pellet combined with bacterial strain H29 enhanced the removal performance of cadmium and nitrate

A newly isolated strain Phoma sp. ZJ6, which could form fungal pellet (FP) by self-immobilization, was identified. A novel longan seed biochar embedded in FP (BFP) combined with strain H29 (BFP-H29) effectively improved the Cd(II) removal efficiency and simultaneously removed nitrate. The adsorption process of BFP was well fitted with the pseudo-second-order kinetics model and Langmuir isotherm model, which demonstrated that the adsorption process was favorable and mainly dominated by chemisorption. Compared with single FP, biochar, and strain H29, BFP-H29 significantly enhanced the Cd(II) removal and the removal ratio reached 90.47%. Meanwhile, the simultaneous removal efficiency of the BFP-H29 for nitrate could reach 93.80%. Characterization analysis demonstrated that the primary removal mechanisms of BFP-H29 were precipitation and surface complexation. BFP-H29 had excellent performance in simultaneous removal of Cd(II) and nitrate, indicating its potential as a promising composite in the removal of cadmium and nitrate in wastewater.

Identification of a novel heavy metal resistant Ralstonia strain and its growth response to cadmium exposure

A novel Ralstonia Bcul-1 strain was isolated from soil samples that was closest to Ralstonia pickettii. Broad-spectrum resistance was identified to a group of heavy metal ions and tolerance to concentrations of Cd²⁺ up to 400 mg L^-1. Low concentrations of heavy metal ions did not have distinctive impact on heavy metal resistance genes and appeared to induce greater expression. Under exposure to Cd²⁺, cell wall components were significantly enhanced, and some proteins were also simultaneously expressed allowing the bacteria to adapt to the high Cd²⁺ living environment. The maximum removal rate of Cd²⁺ by the Ralstonia Bcul-1 strain was 78.97% in the culture medium supplemented with 100 mg L^-1 Cd²⁺. Ralstonia Bcul-1 was able to survive and grow in a low nutrient and cadmium contaminated (0.42 mg kg^-1) vegetable soil, and the cadmium removal rate was up to 65.76% in 9th growth. Ralstonia Bcul-1 mixed with biochar could maintain sustainable growth of this strain in the soil up to 75 d and the adsorption efficiency of cadmium increased by 16.23-40.80% as compared to biochar application alone. Results from this work suggests that Ralstonia Bcul-1 is an ideal candidate for bioremediation of nutrient deficient heavy metal contaminated soil.

From hazardous agriculture waste to hazardous metal scavenger: Tobacco stalk biochar-mediated sequestration of Cd leads to enhanced tobacco productivity

Tobacco is a super-enriched plant for heavy metals, and its productivity is sensitively affected by Cd. In this study, tobacco stalk was converted to biochar (TS-biochar) for the sequestration of Cd in soils to enhance the productivity of tobacco. FTIR, SEM-EDX, and XPS characterizations of TS-biochar together with Cd2+ adsorption kinetics revealed that oxy-containing functional groups (‒OH, C˭O, and ‒COOH) in TS-biochar played a crucial role on Cd2+ adsorption. The changes of soil property and Cd speciation by adding TS-biochar in red (acidic) and cinnamon (alkaline) soils was evaluated. Effects of TS-biochar on tobacco growth and development under Cd stress was also investigated. Results indicated that a 2 wt% addition of TS-biochar in red soil could significantly increase the soil pH value (from 5.21 to 7.39) and reduce exchangeable Cd fractions (from 40% to 23%), but those were not obvious in cinnamon soil. Under the stress of Cd, TS-biochar could obviously improve the tobacco dry biomass, and decrease the accumulation of Cd in the middle and upper leaves, thus reducing economic loss. Overall, the application of TS-biochar on Cd contaminated soil can transform bioavailable Cd into low hazardous forms, so as to repair soils and improve the productivity of tobacco.

Anaerobic fermentation treatment improved Cd2+ adsorption of different feedstocks based hydrochars

Hydrothermal carbonization technology has attracted wide attention in recent years owing to its advantages, e.g., high yield and clean production, compared with traditional pyrolysis. Anaerobic fermentation (AF) is a new method to modify carbon materials, which may improve the surface properties of hydrochar (HC). To explore whether AF has effects on different feedstocks based HCs, two kinds of HCs derived from wheat straw and poplar sawdust were treated with AF for different time in this study. By comparing the changes in physicochemical properties of anaerobic fermentative hydrochars (AFHCs), adsorption behaviors of Cadmium (Cd²⁺) on AFHCs were evaluated. The results showed that the surface electrical characteristics, specific surface area, and oxygen-containing functional groups of HCs improved significantly after AF treatment, which confirmed our hypothesis that AF is suitable for improving the adsorption of different feedstocks based HCs. The adsorption capacity of Cd²⁺ on AFHCs was significantly enhanced by a 3.1-3.4 times increase after AF treatment. The effect of AF treatment on wheat straw hydrochar (WHC) was more evident than poplar sawdust hydrochar (SHC). WHCs treated with AF own higher adsorption capacity of Cd²⁺, which was attributed to the higher negative charge, more exchangeable cations, and more oxygen-containing functional groups. The adsorption process was found to be a spontaneous endothermic reaction dominated by chemisorption and controlled by electrostatic attraction, ion exchange, functional groups complexation, and π-bonding coordination. These results were contributed to understanding the modification of HC by AF and its application in heavy metal pollution remediation.

Disulfide polymer grafted polypropylene/polyethylene filter media for selective cadmium removal

Heavy metal pollution caused by stormwater runoff has triggered a demand for effective heavy metal sorbents. Effective heavy metal removal using conventional stormwater runoff treatment processes that employ filtration mechanisms as primary removal mechanisms is difficult. Therefore, we attempt to improve cadmium removal performance by attaching disulfide polymer (DiS-COP) containing soft bases, thiols, onto the surface of polypropylene/polyethylene (PP/PE) fiber media, which is widely used for stormwater runoff treatment. Material characterization demonstrated that DiS-COP was successfully grafted and grown on the surface of PP/PE (Dis-PP/PE). The batch and continuous flow adsorption capacities of Dis-PP/PE were 81.1 mg/g and 2.33 mg/g, respectively, which is 40 times higher than those of pristine PP/PE. Applicability of DiS-PP/PE at pH 6-8 was demonstrated, and effects of calcium and humic acid on cadmium adsorption were investigated. Calcium marginally affected cadmium adsorption, which can be explained using the Hard and soft (Lewis) acids and bases theory (HSAB), but cadmium removal efficiency decreased owing to humic acid (HA)-Cd complex formation and agglomeration in the presence of organic material. In a breakthrough test, the adsorption column exhibited complete cadmium uptake over 24 h until it reached the breakthrough point. Therefore, heavy metal adsorption performance of PP/PE was successfully enhanced by grafting DiS-COP on its surface.

Thermal characteristics and cadmium binding behavior of EC-ELP fusion polypeptides

Elastin-like polypeptides (ELPs) are stimulus-responsive protein-based biopolymers that exhibit phase transition behavior. By joining them to synthetic phytochelatin (EC), EC-ELP fusion proteins with temperature sensitivity and metal-binding functionality were generated to remove heavy metal ions biologically. Three different EC domains (EC₁₀, EC₂₀, EC₃₀) were incorporated into the ELP, and the EC-ELP fusion proteins were expressed in E. coli. Their thermal properties and metal binding abilities were then investigated according to the EC length. In addition, the feasibility of reusing EC-ELPs and the cadmium ion binding affinity of reused EC-ELPs were explored.

Enhancement of cadmium removal by oxygen-doped carbon nitride with molybdenum and sulphur hybridization

Graphitic carbon nitride, as a popular material in the field of environmental remediation, still suffers from unsatisfactory performance for heavy metals adsorption owing to lack of specific adsorption sites. In this study, molybdenum (Mo) and sulphur (S) were simultaneously introduced onto the surface of oxygen-doped graphitic carbon nitride (OCN) for the enhancement of Cd²⁺ adsorption. The synthesized MOS/OCN-1 exhibited substantially increased maximum adsorption capacity of 293.8 mg/g, calculated from Sips isotherm model, which was 8.7 times higher than that for pristine OCN (33.9 mg/g). The adsorption efficiency of MOS/OCN-1 was >94% even under high concentration of coexisting ions (i.e., Ca²⁺, Mg²⁺ and Zn²⁺). MoO₃ and MoS₂ on the surface of OCN were proven to interact with Cd²⁺ by forming CdMoO₄ and CdS species. OCN provided a stable matrix with a large surface area making more active sites exposed, which greatly facilitated Mo(IV) oxidation and Cd²⁺ precipitation. Our findings revealed that as well as the well-known Cd-S interaction, Mo atoms in the hybrid composites also played an important role in Cd²⁺ removal, which opened up the application possibility of OCN with Mo and S hybridization for in-situ Cd²⁺ remediation.

Variations in pH significantly affect cadmium uptake in grafted muskmelon (Cucumis melo L.) plants and drive the diversity of bacterial communities in a seedling substrate

Substrates are fundamental prerequisites for growing grafted seedlings. In this study, substrates with different pH levels (5.0, 5.5, 6.0, 6.5, 7.0, and 8.0) were set up to elucidate the effect of pH on cadmium (Cd) uptake in grafted muskmelon (Cucumis melo L.) plants. Bacterial diversity was also investigated. Results showed that pH and high Cd concentration greatly affected the growth of grafted plants. The chlorophyll content of the muskmelon leaves decreased at 100 μM Cd. The majority of the Cd ions accumulated in the rootstock rather than in the shoot tissue in all of the treatments. The shoots and roots showed the highest Cd content at pH 5.5 and the lowest Cd content at pH 8.0 regardless of the Cd concentration. The operational taxonomic units belonging to Proteobacteria and Bacteroidetes were significantly (p < 0.05) enriched at different substrate pH levels compared with those at pH 5.0. The operational taxonomic units belonging to the phyla Firmicutes, Acidobacteria, and Chloroflexi were significantly decreased. The available nitrogen, phosphorus, Cd, and pH were strongly linked to bacterial community compositions. On the contrary, the available potassium was weakly correlated with the bacterial structure. This study demonstrates that pH greatly affects Cd uptake in grafted muskmelon plants and predicts microbial community structures in breeding substrates with different pH levels. Our results suggest that Cd accumulation in grafted plants can be reduced by setting the appropriate substrate pH. This work can serve as a reference for growing high-quality grafted plants and ensuring food safety in the presence of Cd contamination.

Use of PVA/α-MnO2-stearic acid nanocomposite films prepared by sonochemical method as a potential sorbent for adsorption of Cd (II) ion from aqueous solution

Alpha manganese dioxide (α-MnO₂) nanorods have been covalently functionalized with stearic acid by solvothermal method. The α-MnO₂-stearic acid nanorods were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy. The amount of stearic acid grafted onto α-MnO₂ surface was determined about 41wt% by thermogravimetric analysis. The α-MnO₂-stearic acid nanorods was used as nanofiller for the preparation of poly(vinyl alcohol) (PVA) nanocomposites (NCs). The NCs with 1, 3, and 5wt% of nanofiller were prepared through ultrasound assisted technique as an economical, fast, eco-friendly, and effective method. The PVA/α-MnO₂-stearic acid NCs were characterized by different techniques. The results showed that with increasing the α-MnO₂-stearic acid content, the thermal stability and tensile properties were improved. Besides, the NC 5wt% was used as adsorbent for sorption of Cd (II) ion. The adsorption efficiency of NC 5wt% in different initial concentrations of Cd (II) ion (20-100mgL^-1) was 58-95%. All the isotherm data were well fitted by both Langmuir and Freundlich equations. The adsorption kinetics study demonstrated that pseudo-second-order model was the best fit with the experimental data. The results indicated that prepared NCs are promising adsorbents for the removal of Cd (II) ion from the aqueous solution.