Progress in the remediation of hazardous heavy metal-polluted soils by natural zeolite

Nano silica's role in regulating heavy metal uptake in Calendula officinalis

BACKGROUND

Soil contamination with heavy metals poses a significant threat to plant health and human well-being. This study explores the potential of nano silica as a solution for mitigating heavy metal uptake in Calendula officinalis.

RESULTS

Greenhouse experiments demonstrated, 1000 mg•kg- 1 nano silica caused a 6% increase in soil pH compared to the control treatment. Also in 1000 mg. kg- 1 nano silica, the concentrations of available Pb (lead), Zn (zinc), Cu (copper), Ni (nickel), and Cr (chromium) in soil decreased by 12%, 11%, 11.6%, 10%, and 9.5%, respectively, compared to the control. Nano silica application significantly reduces heavy metal accumulation in C. officinalis exposed to contaminated soil except Zn. In 1000 mg.kg- 1 nano silica shoots Zn 13.28% increased and roots Zn increased 13% compared to the control treatment. Applying nano silica leads to increase the amount of phosphorus (P) 25%, potassium (K) 26% uptake by plant, In 1000 mg.kg - 1 treatment the highest amount of urease enzyme activity was 2.5%, dehydrogenase enzyme activity, 23.6% and the highest level of alkaline phosphatase enzyme activity was 13.5% higher than the control treatment.

CONCLUSION

Nano silica, particularly at a concentration of 1000 mg.kg - 1, enhanced roots and shoots length, dry weight, and soil enzyme activity Moreover, it increased P and K concentrations in plant tissues while decreasing heavy metals uptake by plant.

Reducing toxic element leaching in mine tailings with natural zeolite clinoptilolite

The use of natural zeolite clinoptilolite to reduce the leaching rate of potentially toxic elements such as Cd, Pb, and Mn in soil from mine tailings was studied. Soil from the surroundings of the mine El Bote in Zacatecas, Mexico, was analyzed, and the zeolite was characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, and nitrogen physisorption. An ammonium-exchange method for the zeolite was employed. Leaching experiments using packed columns with polluted soil and zeolite mixtures were carried out and the effect of the pH of the carrier solutions was studied. Incorporation of zeolite in the soil achieved a beneficial increase in pH, from 5.03 to 6.95. The concentration of Cd and Mn was reduced when zeolite was present in the column and the ammonium-modified zeolite with ammonia also enhanced the concentration reduction of metallic species in leachates in a range of 28 to 68%. The first-order model best fits the experimental data, suggesting that the leaching rate is controlled by concentration difference between the liquid and the soil matrix. These results demonstrate the potential for using natural zeolite clinoptilolite to reduce the leaching rate of potentially toxic elements in soil from mine tailings.

Insight into chromium adsorption from contaminated soil using Mg/Al LDH-zeolite

The study investigated the effectiveness of Mg/Al LDH-zeolite (MALZ) in immobilizing exchangeable Cr (e-Cr) within the soil. The research systematically evaluated various variables affecting the immobilization of e-Cr in contaminated soil (CS), including soil pH levels (ranging from 5.0 to 9.0), different weight ratios of MALZ (1 %, 3 %, and 5 %), durations of differing incubation periods (15, 30 and 45 days), and different SM content levels (30 %, 50 %, and 70 %). The initial concentration of Cr in the CS was maintained at 50 mg/kg. The investigation findings revealed that the optimal conditions for immobilizing the e-Cr were a soil pH of 5.0, an MALZ weight ratio of 3 %, an incubation period of 30 days, and an SM level of 70 %. Under these ideal conditions, the percentage of e-Cr within the CS decreased significantly, from 87.49 % (45.64 mg/kg) in the control treatment (CT) to just 19.82 % (10.08 mg/kg) when incubated with MALZ. The primary mechanisms responsible for immobilizing the e-Cr onto MALZ included pore filling, reduction processes, co-precipitation, organic interactions and electrostatic attractions leading to the formation of carbonate-bound complexes such as Cr(VI)-carbonate, Cr(III)-carbonate, and organic complexes. Surface functional groups on MALZ, housing iron and aluminium oxyhydroxides and silicon and oxygen elements, expedited these procedures. This study provided a valuable understanding of the mitigation of soils contaminated with chromium and contributed to understanding the relations between MALZ and the e-Cr in the soil. The discoveries carry substantial consequences for the advancement of efficient remediation technologies.

A green approach to efficient soil decontamination using supercritical carbon dioxide and ethanol

As the need for global decommissioning and site remediation of aging and shut-down nuclear power plants continues to grow, it becomes increasingly crucial to efficiently treat contaminated soil while minimizing waste generation. This study explores an innovative soil decontamination approach that utilizes supercritical carbon dioxide (SCCO₂) as the primary solvent, along with ethanol as a co-solvent and specific additives, including a chelate ligand (catechol ligand) and a co-ligand (NEt₄PFOSA). The advantages of SCCO₂, such as its penetration and solubility, coupled with its ability to separate from radioactive waste, are harnessed in this research. The study demonstrates that the combination of SCCO₂, ethanol, and additives significantly enhances decontamination efficiency, particularly for cesium (Cs), strontium (Sr), and uranium (U) contamination. Results indicate that decontamination efficiency varies with soil particle size, with smaller particles presenting greater challenges. This study presents a promising eco-friendly soil decontamination technology using SCCO₂ containing ethanol and specific additives to efficiently reduce radioactive contamination in soil.

Organic amendments perform better than inorganic amendments in reducing the absorption and accumulation of cadmium in lettuce

The main purpose of applying organic or inorganic amendments is to guarantee crop safe production in heavy metal contaminated soil. However, previous studies showed that the effects of organic or inorganic composite amendments on the cadmium (Cd) concentration of lettuce (Lactuca sativa var. ramosa Hort) were inconsistent. Accordingly, a sixty-day pot experiment was carried out to examine the impacts of the inorganic materials (lime, L and zeolite, Z), organic materials (biochar, B and compost, C), and their combination on the immobilization of Cd in soil and its uptake by lettuce. The objective was to identify the most suitable soil amendment combination that promotes safe lettuce production. The results revealed that the combined application of BC, LZC, and LBC significantly increased the plant height by 11.09-28.04% and fresh weight by 183.47-207.67%. This improvement can be attributed to enhanced soil quality, such as increased dissolved organic carbon (DOC) by 70.19-80.42%, soil respiration (SR) by 29.04-38.46%, and soil microbial carbon content (SMC) by 36.94-46.63%. Compared to inorganic fertilizers and their combination with organic amendments, organic amendments had a significant impact on reducing shoot Cd concentration by 33.93%-56.55%, while increasing the activity of catalase by 138.87-186.86%. And soil available Cd measured by diffusive gradients in thin-films (DGT-Cd) decreased 24.73-88.13% in all treatments. Correlation analysis showed that plant Cd concentration was significantly correlated with soil pH, SR, cation exchange capacity (CEC), DOC and SMC. These results demonstrated that organic amendments, especially the combination of biochar and compost, have greater potential than inorganic amendments and inorganic-organic combinations for realizing safe production of lettuce and improving soil quality in the Cd moderately contaminated acid farmland.

Determination of the Basic Geotechnical Parameters of Blast-Furnace Slag from the Kremnica Region

A decisive aspect of site evaluation for construction is the presence of anthropogenic materials occurring in the geological environment. The geotechnical properties of blast-furnace slag were investigated as a potential substitute for aggregates in the construction industry. The basic geotechnical parameters of the slag were determined, which are critical for evaluating its stability, environmental impact, and usability in geotechnical construction. The research focused on monitoring the physical and mechanical properties of the two samples, and also included mineralogical analysis. The obtained results demonstrated that the slag belongs to the category of poorly graded gravel, G2/GP, and gravel with an admixture of fine-grained soil, G3/G-F. In addition, other important parameters, such as the water disintegration of the slag aggregate, the minimum and maximum bulk densities, the California bearing ratio (CBR), the oedometric modulus (Eoed), and shear tests (the angle of internal friction φ and cohesion c), were determined. The results from this paper provide important information for the proper management of blast-furnace slag so to minimize its environmental impact and achieve sustainability in the mining industry. At the same time, it enables a better understanding of the use of slag as a substitute for aggregates in geotechnical tasks. Despite its local importance in relation to the investigated case, the presented study has significant educational and scientific value for the construction sector, where it is necessary to evaluate anthropogenic activities and materials.

Safe Production Strategies for Soil-Covered Cultivation of Morel in Heavy Metal-Contaminated Soils

Morel is a popular edible mushroom with considerable medicinal and economic value which has garnered global popularity. However, the increasing heavy metal (HM) pollution in the soil presents a significant challenge to morels cultivation. Given the susceptibility of morels to HM accumulation, the quality and output of morels are at risk, posing a serious food safety concern that hinders the development of the morel industry. Nonetheless, research on the mechanism of HM enrichment and mitigation strategies in morel remains scarce. The morel, being cultivated in soil, shows a positive correlation between HM content in its fruiting body and the HM content in the soil. Therefore, soil remediation emerges as the most practical and effective approach to tackle HM pollution. Compared to physical and chemical remediation, bioremediation is a low-cost and eco-friendly approach that poses minimal threats to soil composition and structure. HMs easily enriched during morels cultivation were examined, including Cd, Cu, Hg, and Pb, and we assessed soil passivation technology, microbial remediation, strain screening and cultivation, and agronomic measures as potential approaches for HM pollution prevention. The current review underscores the importance of establishing a comprehensive system for preventing HM pollution in morels.

Synthesis of Polyvinyl Alcohol/Coal Fly Ash Hybrid Nano-Fiber Membranes for Adsorption of Heavy Metals in Diesel Fuel

Some studies have shown that the heavy metal emissions (HMEs) emitted from diesel engines can seriously threaten human health. HMEs are mainly related to the content of heavy metal ions in diesel fuel. Therefore, in order to reduce HMEs from diesel engines, a nano-fiber membrane filtration technology for diesel fuel was investigated. Herein, coal fly ash (CFA) from coal-fired power plants combined with polyvinyl alcohol (PVA) was successfully fabricated into nano-fibrous membranes using green electrospinning technology. In order to evaluate the adsorption properties, various hybrid membranes with different mixing ratios (PVA/CFA = 10/0, 10/1, 10/3, 10/5, and 10/7 by weight) were fabricated. The results show that eight metal ions with different concentrations are found in the diesel fuel, including Pb, Cu, Zn, Al, Fe, Cr, Ba, and Ni. All PVA/FA membranes have different adsorption capacities for metal ions, following the order: Cu > Fe > Pb > Al > Zn > Cr > Ba > Ni. In addition, the adsorption capacity of CFA3 (PVA/CFA = 10/3) is the largest. The super lipophilicity of the PVA/FA membranes also provide more adsorption sites for the contact of HMs with the membranes. The above research results provide guidance for development of ultra-fine filters in the future.

Restoration effect of sulfhydryl-modified sepiolite on cadmium in contaminated soil and its effect on the growth of spinach (Spinacia oleracea L)

Coal mining has produced a large amount of coal gangue. It makes the soil around the mining area seriously polluted by heavy metals, affects the growth of crops, and endangers human health. Therefore, there is an urgent need to develop new materials for remediation of Cd in soil. In this study, mercaptosilane-modified sepiolite (Q-Sep) was used as a basic passivator, and it was pretreated with acid (H-Q-Sep) and high temperature (R-Q-Sep) respectively. By analyzing the forms of Cd and pH values in soil after adding modified sepiolite, we compared the remediation effects of two modified methods on Cd in soil. The enrichment of spinach (Spinacia oleracea L) to Cd and changes in physiological and biochemical indexes of spinach were determined, and the effect of modified sepiolite on the growth of spinach was judged. The experimental results showed that the addition of modified sepiolite could significantly increase the soil pH values (p < 0.05); the content of exchangeable Cd in soil decreased by 60.4%; and the maximum increase of residual state was 32.9%. The absorption of Cd in soil by spinach decreased, and root length, plant height, and biomass of spinach all increased. It was proved that the addition of modified sepiolite can improve the productivity of soil, reduce toxicity of heavy metals in soil, and promote growth of plants. As a result, the addition of H-Q-Sep and R-Q-Sep can effectively repair Cd in gangue filled soil, which provides a certain theoretical basis for the passivation remediation of Cd in soil.

High performance self-assembled nano-chlorapatite in the presence of lactonic sophorolipid for the immobilization of cadmium in polluted sediment

A novel lactonic sophorolipid (LS) self-assembled nano-chlorapatite (LS-nClAP) was prepared for the immobilization of severe cadmium (Cd) in sediment. The experimental results indicated that the introduction of LS not only improved the dispersed performance of chlorapatite, but also brought massive hydroxyl and carboxyl groups, which significantly improved the immobilization efficiency of Cd and reduced its eco-toxicity in sediment. LS can significantly increase the effective utilization rate of phosphorus in chlorapatite, and reduce the content of available phosphorus (AP) by half after remediation compared with ClAP. Additionally, the participation of LS possessed a significant impact on the enzyme activities in the sediment, especially for urease, which was closely related to the effective stability of Cd and the introduction of LS. All experimental results of this study provided new insights into the possible effects of Cd immobilization by chlorapatite in contaminated sediments, demonstrating great application potential for sediment remediation in the future.

Mobility of arsenic and vanadium in waterlogged calcareous soils due to addition of zeolite and manganese oxide amendments

Addition of manganese(IV) oxides (MnO₂ ) and zeolite can affect the mobility of As and V in soils due to geochemical changes that have not been studied well in calcareous, flooded soils. This study evaluated the mobility of As and V in flooded soils surface-amended with MnO₂ or zeolite. A simulated summer flooding study was conducted for 8 weeks using intact soil columns from four calcareous soils. Redox potential was measured in soils, whereas pH, major cations, and As and V concentrations were measured biweekly in pore water and floodwater. Aqueous As and V species were modeled at 0, 4, and 8 weeks after flooding (WAF) using Visual MINTEQ modeling software with input parameters of redox potential, temperature, pH, total alkalinity, and concentrations of major cations and anions. Aqueous As concentrations were below the critical thresholds (<100 μg L^-1 ), whereas aqueous V concentrations exceeded the threshold for sensitive aquatic species (2-80 μg L^-1 ). MnO₂ -amended soils were reduced to sub-oxic levels, whereas zeolite-amended and unamended soils were reduced to anoxic levels by 8 WAF. MnO₂ decreased As and V mobilities, whereas zeolite had no effect on As but increased V mobility, compared to unamended soils. Arsenic mobility increased under anoxic conditions, and V mobility increased under oxic and alkaline pH conditions. Conversion of As(V) to As(III) and V(V) to V(IV) was regulated by MnO₂ in flooded soils. MnO₂ can be used as an amendment in immobilizing As and V, whereas the use of zeolite in flooded calcareous soils should be done cautiously.

Molecular Sieve, Halloysite, Sepiolite and Expanded Clay as a Tool in Reducing the Content of Trace Elements in Helianthus annuus L. on Copper-Contaminated Soil

The aim of this study was to determine the effect of copper soil contamination on the trace element content of sunflower aerial parts and in roots. Another aim was to assess whether the introduction of selected neutralizing substances (molecular sieve, halloysite, sepiolite and expanded clay) into the soil could reduce the impact of copper on the chemical composition of sunflower plants. Copper soil contamination with 150 mg Cu²⁺ kg^-1 of soil and 10 g of each adsorbent per kg of soil were used. Soil contamination with copper caused a significant increase in the content of this element in the aerial parts (by 37%) and roots (by 144%) of sunflower. Enriching the soil with the mineral substances reduced the amount of copper in the aerial parts of sunflower. Halloysite had the greatest effect (35%), while expanded clay had the smallest effect (10%). An opposite relationship was found in the roots of this plant. In copper-contaminated objects, a decrease in the content of cadmium and iron and an increase in the concentrations of nickel, lead and cobalt in the aerial parts and roots of sunflower were observed. The applied materials reduced the content of the remaining trace elements more strongly in the aerial organs than in the roots of sunflower. Molecular sieve had the greatest reducing effect on the content of trace elements in sunflower aerial organs, followed by sepiolite, while expanded clay had the least impact. The molecular sieve also reduced the content of iron, nickel, cadmium, chromium, zinc and, especially, manganese, whereas sepiolite reduced the content of zinc, iron, cobalt, manganese and chromium in sunflower aerial parts. Molecular sieve contributed to a slight increase in the content of cobalt, while sepiolite had the same effect on the content of nickel, lead and cadmium in the aerial parts of sunflower. All materials decreased the content of chromium in sunflower roots, molecular sieve-zinc, halloysite-manganese, and sepiolite-manganese and nickel. The materials used in the experiment, especially the molecular sieve and to a lesser extent sepiolite, can be used effectively to reduce the content of copper and some other trace elements, particularly in the aerial parts of sunflower.

Insights into the heavy metal adsorption and immobilization mechanisms of CaFe-layered double hydroxide corn straw biochar: Synthesis and application in a combined heavy metal-contaminated environment

Biochar is an emerging eco-friendly and high-efficiency heavy metal (HM) adsorbent that exhibits satisfactory HM remediation effects in both water and soil environments. However, few studies have investigated the mechanisms and application of biochar in the remediation of combined HM-contaminated environments. Therefore, in the present study, a novel corn straw biochar-loaded calcium-iron layered double hydroxide composite (CaFe-LDH@CSB) was synthesized via the coprecipitation method and applied as a remediation adsorbent to remove HMs in both water and soil environments. The results indicated that the HM adsorption mechanism of CaFe-LDH@CSB in the aquatic phase involved a chemical endothermic adsorption process of functional group-complexed monolayers, dominated by precipitation, ion exchange, complexation and π bond interactions. The maximum adsorption capacity for Cd(II), Pb(II), Zn(II) and Cu(II) in the aqueous phase reached 24.58, 240.96, 57.57 and 39.35 mg g^-1, respectively. In addition, application of CaFe-LDH@CSB in the combined HM-contaminated soil treatment helped to increase the soil pH, which increased by 5.1-17.9% in low-contamination (LC) soil and by 7.0-13.9% in high-contamination (HC) soil. Moreover, application of CaFe-LDH@CSB effectively decreased the acid-soluble fraction of HMs and increased the HM residual fraction. The immobilization mechanism of CaFe-LDH@CSB in the soil was concluded to involve pore filling, functional group action and electrostatic interactions. Overall, this study provided a novel LDH biochar composite that can be effectively applied in the remediation of combined HM-contaminated water and soil environments.

Self-regulated immobilization behavior of multiple heavy metals via zeolitization towards a novel hydrothermal technology for soil remediation

More efficient soil remediation technologies are highly anticipated to treat large quantities of heavy metal-polluted urban sites nowadays. Herein, a novel hydrothermal technology of converting heavy metal-polluted soils into zeolites for in-situ immobilizing heavy metals was proposed. The zeolites (analcime and cancrinite) could be synthesized hydrothermally with certain Na/Si and Al/Si ratios. The formed zeolites could manage to change their species and structure during zeolitization to accommodate different heavy metals in soil according to their size and charge. Since smaller-size Cu²⁺ was introduced, analcime and some cancrinite possessing small cages could be formed adaptively to immobilize the Cu²⁺ by replacing Na⁺ and forming Cu²⁺-OH and Cu²⁺-O. Whereas, cancrinite with large channels managed to form to immobilize the larger-size Cd²⁺ by forming Cd²⁺-O. Interplanar spacing variation of zeolites also corresponded to their structural change for accommodating different heavy metals. Leaching results showed the amounts of Cu and Cd leached from the synthesized zeolites were reduced to 0.005% and 0.05% respectively, reflecting a more stable immobilization of smaller heavy metals by small cages, in agreement with the results of distribution coefficient (K_d). Negligible effect of pH environment on the leaching rates further confirmed the stable structural immobilization of heavy metals by zeolites.

Evaluation of Heavy Metals in Soil Wastewater Stream

Environmental pollution is one of the main factors that significantly affect biological systems and human health. Soil pollution with heavy metals is an increasingly pressing problem worldwide. In general, heavy metals are stable and do not decompose, unlike other organic pollutants. The quantity of them is natural components of soil crust, the remaining come from human activities, which may result from the extensive use of sewage. In the present study, a methodology aimed at simultaneous quantification of 16 heavy elements in soil of 3 different regions was developed. The concentration of 16 soil heavy metals (Se, Cd, V, Be, As, Mn, Co, Zn, Fe, Cr, Pb, Ni, Cu, Mo, Hg, and Ti) was measured in 11 sampling along Riyadh, Qassim, and Medina, Kingdom of Saudi Arabia from 3 sites soil treated with sewage water. These chemical minerals were identified in the samples using an ICPE-9000 spectrometer. The assessment of heavy metal contamination was derived using enrichment factors (EF), the pollution load index (PLI), and geoaccumulation index (I_geo). This study revealed that the soil is predominantly polluted by Cd, As, and Mo of Riyadh and Medina and As, Mo, and Cd of Qassim region at site B and site C, respectively. As recorded, the highest concentration value of 5000 mg/kg for Fe at site (B) followed by Cu. The I_geo value of Cd is 1.1520 in Medina region. The I_geo value of Se is 3.2395 in Medina region, while its cumulative geographical index decreased in the regions of Riyadh and Qassim, which amounted to 2.6114 and 2.1699, respectively. The I_geo values of the rest of the minerals in the three regions studied indicated that the soil is unpolluted, while it was slightly to moderately polluted for both Mo and Hg in most regions studied. The minerals in the soil at all sites studied were less than the general average concentration. With the exception of mercury, molybdenum, arsenic, cadmium, and selenium, whose concentration exceeded the permissible and recommended limits. The increasing order of concentration of minerals was Be < V < Cd < Hg < Mo < Co < Zn < Pb < Cr < Se < As < Ni < Ti < Mn < Cu < Fe at all sites, respectively.

The Role of Zeolite Structure in Its β-cyclodextrin Modification and Tetracycline Adsorption from Aqueous Solution: Characteristics and Sorption Mechanism

Modification of zeolites with organic compounds is of increasing interest due to their significant potential in removing emerging pollutants from water. In this work, zeolites from fly ash with three different structure types, NaX (faujasite), NaA (Linde A) and NaP1 (gismondine), were modified with β-cyclodextrin (β-CD), and their adsorption efficacy towards tetracycline (TC) antibiotic in aqueous solutions have been studied. To assess the effect of modification on the zeolites, they were subjected to chemical, mineralogical and surface analyses using X-ray diffraction (XRD), thermogravimetry (TG), scanning electron microscope (SEM), N2 adsorption/desorption isotherm, Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The maximum adsorption capacity for NaX-CD, NaA-CD and NaP1-CD was around 48, 60, and 38 mg/g, respectively. The fastest adsorption rate was observed for NaP1-CD, which achieved adsorption equilibria after 200 min, while for NaX-CD and NaA-CD it was established after around 24 h. The kinetic data were best described by the Elovich model, followed by pseudo-second order, while the Sips and Redlich-Peterson models were the most suitable to describe the adsorption isotherms. Based on the adsorption data as well as FTIR and XPS results, TC adsorption efficacy is strongly related to the amount of CD attached to the mineral, and hydrogen bonding formation probably plays the major role between CDs and adsorbate.

Insight into functional microorganisms in wet–dry conversion to alleviate the toxicity of chromium fractions in red soil

Soil contamination with potentially toxic element such as chromium (Cr) poses a threat to the environment and human health. The environmental toxicity of Cr is related not only to the total Cr content but also to the distribution of Cr fractions. In this study, laboratory simulation experiments were conducted to explore the characteristics of Cr fractions and responses of the functional microbial community during dynamic leaching and static drying processes. The results showed that acid-soluble Cr and reducible Cr transformed into other relatively stable fractions under dry conditions, and ammonium nitrogen promoted the transformation. Nitrate-nitrogen was significantly positively correlated with Cr fractions in the wet stage (p < 0.05), while ammonium nitrogen showed the same relation in the dry process. Analysis of the microbial community showed that the bacterial and fungal genera Flavihumibacter, Altererythrobacter, Methylobacillus, Flavisolibacter, Lysobacter, and Cladosporium were related to the Cr fractions (acid-soluble Cr, reducible Cr, and oxidizable Cr) under wet conditions, while the microbial genera Ellin6067, MND1, and Ramlibacter were related to Cr fractions under dry conditions. Moreover, the proliferation of the functional microbial genera Methylobacillus, Ellin6067, and MND1 related to Cr fractions in the wet–dry conversion process alleviated the environmental toxicity of Cr. These findings provide useful information for the remediation of Cr-contaminated soils by monitoring the distribution fractions of Cr and the functional microbial community under wet–dry conditions.

Biosorption efficiency of nickel by various endophytic bacterial strains for removal of nickel from electroplating industry effluents: an operational study

Realising the hazardous effect of nickel on human health, microbes and plants are effectively used for bioremediation. The endophytic microorganisms have an important role in the phytoremediation of nickel using Vigna radiata. Therefore, in order to harness the potential of microbial strains, the present study was designed to examine the metal biosorption ability of endophytic bacterial strains isolated from plants growing in nickel-contaminated soil. A total of six endophytic nickel resistance bacteria were isolated from the plant Vigna radiata. The metal tolerant bacterial strains were identified following 16 S rRNA gene sequence analysis. Nickel biosorption estimation and plant growth-promoting (PGP) activities of isolated strains were performed and found high nickel biosorption efficiency of 91.3 ± 0.72% at 600 mg L^-1 using Bacillus safensis an isolated endophytic strain from Vigna radiata. Furthermore, high indole acetic acid (IAA) and exopolysaccharide (EPS) production were obtained in all the strains as compared to without nickel-containing medium used as control. Moreover, the production of high EPS suggests improved biosorption ability of isolated endophytic strains. In addition, a kinetic study was also performed to evaluate different adsorptions isotherms and support the nickel biosorption ability of endophytic strains. The treatment of nickel electroplating industrial effluent was also demonstrated by isolated endophytic strains. Among six (6) strains, B. cereus showed maximum 57.2 ± 0.62% biosorption efficiency of nickel which resulted in the removal of 1003.50 ± 0.90 mg L^-1 of nickel from the electroplating industry effluents containing initial 1791 ± 0.90 mg L^-1 of nickel. All other strains were also capable of significant nickel biosorption from electroplating industry effluents as well. Thus, isolated endophytic nickel tolerant strains can be further used at large-scale biosorption of nickel from electroplating industry effluent.

Application of different alkaline materials as polluted soil amendments: A comparative assessment of their impact on trace element mobility and microbial functions

Treatment with chemical amendments is among the best techniques to remediate soils highly polluted with trace elements. The use of waste-derived products has several advantages in this regard, mainly in terms of reducing process costs and conserving natural resources. In this study, the performance of the synthetic zeolite NaP1 derived from coal combustion fly ash (SZ) and the by-product generated from the processing of aluminum salt slags (BP) was evaluated with this aim in comparison to calcite (CC). For this purpose, mine soils polluted with Zn, Cd, and Pb were amended under controlled laboratory conditions with different doses (0%, 1%, 2%, 5%, and 10%) of SZ, BP, or CC, and their impact on trace element mobility and microbial functions was evaluated. Specifically, the mobile and mobilizable trace element pools, basal soil respiration, and different enzyme activities were analyzed. Both SZ and BP performed better than CC in the immobilization of trace elements, reaching, respectively, mobility decreases up to 89-94% and 66-87% when applied at a dose of 10%. These amendments reduced the mobile trace element pool by precipitating them as acid-soluble precipitates and/or retaining them in the reducible fraction of soils. The alkaline nature of these materials and the concomitant increase in soil pH caused by their application mainly accounted for this behavior. Additionally, soil microbial functionality improved after amendment, especially in the case of SZ, as shown by dehydrogenase and alkaline phosphatase activities, which significantly increased (p < 0.05) up to 536% and 48%, respectively. Therefore, applying SZ or BP as soil amendments can significantly decrease the mobile trace element contents of heavily polluted soils without negatively affecting soil quality, thus facilitating plant growth to revegetate and reclaim degraded spaces.

Effects and mechanisms of Cd remediation with zeolite in brown rice (Oryza sativa)

The effects of zeolite amendment on the Cd absorption and accumulation of two varieties of rice, ZY-18 (Zheyou 18, hybrid indica) and DL-5 (Donglian 5, conventional indica), planted in a slightly Cd-contaminated paddy soil, and the associated mechanisms were studied based on a pot experiment. The results showed that zeolite addition significantly reduced the Cd availability in the soil by the increasing of soil pH and the Cd sorption capacity of soil, as well as the reduction of Eh, dissolved organic carbon (DOC) and the water-soluble Fe/Mn in the soil. The decrease in the amount of Fe and Cd in iron plaques on rice roots treated with zeolite resulted in the reduction of Cd uptake by rice roots. Zeolite application decreased the Cd contents in the roots, stems and leaves, and finally the brown rice through the inhibition of uptake and translocation of Cd by rice plant. The Cd concentration in rice grains of ZY-18 was more decreased than that of DL-5, which was attributed to its higher pH and lower Eh, DOC, and Cd availability in the soil, higher mounts of iron plaque and Cd in plaques on rice roots, and lower root-to-grain transfer factor (TF_root-grain) of ZY-18.

Zeolite-supported manganese oxides decrease the Cd uptake of wheat plants in Cd-contaminated weakly alkaline arable soils

Cd pollution in arable soils has posed serious threats to food safety and human health. Mn oxides and Mn oxide-based materials have been widely applied to the removal of heavy metals for their high adsorption capacity, especially in water treatment. However, the performance and stability of Mn oxide-based materials and the underlying mechanism of Cd immobilization in upland soils remain unclear. Here, zeolite-supported Mn oxides were used as amendment to investigate their impact on the availability of soil Cd in wheat pot experiments. The decrease in soil available Cd content (by 44.3%) and increase in soil available Mn content (by 61.9%) significantly inhibited Cd accumulation in wheat plant tissues under the application of zeolite-supported Mn oxides. The exchangeable Cd was transformed to more stable fractionation of Fe-Mn oxide bound Cd, and the maximum decrease of Cd content in wheat grains, straw and roots reached 65.0%, 11.7% and 55.3%, respectively. Besides, zeolite-supported Mn oxides exhibited high chemical stability and stable Cd immobilization performance in two successive years of wheat pot experiments. These findings improve our understanding of Mn oxide-based materials for soil remediation and indicate that zeolite-supported Mn oxides have great potential for the remediation of Cd-contaminated alkaline upland soils.

Organic and inorganic amendments for the remediation of nickel contaminated soil and its improvement on Brassica napus growth and oxidative defense

In-situ stabilization has been considered an effective way to remediate metal contaminated soil. Thus, pot experiments were undertaken to investigate the effectiveness of multiple stabilization agents such as biochar (BC), mussel shell (MS), zeolite (ZE) and limestone (LS) on the immobilization of Ni, physicochemical features and enzyme activities in polluted soil. Results showed that the sole application of Ni adversely affected the rapeseed growth, photosynthetic pigments, and antioxidative defense. However, the addition of amendments to the contaminated soil significantly reduced Ni bioavailability. The XRD analysis confirmed the formation of Ni related ligands and FTIR showed the presence of hydroxyl, carboxyl and sulfur functional groups, as well as complexation and adsorption of Ni on amendments. Among multiple amendments, biochar significantly enhanced plant biomass attributes and total chlorophyll content. Moreover, addition of amendments also strengthened the antioxidant defense by decreasing Ni induced oxidative stress (H₂O₂ and O₂^.-), increased macronutrient availability, reduced Ni uptake and improved soil health. The qPCR analysis showed that the Ni transporters were significantly suppressed by amendments, which is correlated with the lower accumulation of Ni in rapeseed. The present study showed that immobilizing agents, especially biochar, is an effective amendment to immobilize Ni in soil, which restricts its entry into the food chain.

Mineral Inactivation of Zinc in Polluted Soil—Sustainability of Zeolite, Bentonite and Blends

The study outlines a novel and traceable procedure for inactivating zinc polluted soil (an Anthrosols) adjacent to a former zinc (Zn) ore mine “Orzel Biały” in Bytom (Poland), where the total content of Zn amounted to 3988.0 mg kg−1. This pollution level initiated an inactivation process involving two natural mineral sorbents, i.e., zeolite (Z) and bentonite (B), as well as their five blends (ZeoBen) expressed as ZB: (1) ZB15/85, (2) ZB30/70, (3) ZB50/50, (4) ZB70/30 and (5) ZB85/15. Next, phosphorus (P) as triple superphosphate (TSP, 46% P2O5) was added to individual ZB at rates: 0.25%, 0.5%, 1.0% and 2.0%. All sorbents were added to the Zn polluted soil at 0%, 0.25%, 0.5%, 1.0% and 2.0% (dry weight basis). Treatments (1.0 kg of Zn-polluted soil with ZB sorbents) were aged for 115 days. Data revealed that ZB85/15 with prevailing zeolite caused a Znact inactivation of 66–71%, while zeolite induced 54% and 47% for bentonite. Reactive zinc (Znreac) decreased much more (20%) when zeolite was incorporated at the rate 2.5 g·kg−1 soil, and bentonite was (10%) at the same rate. The application of the sorbent ZB50/50 enriched with triple superphosphate (TSP) raised the stabilization degree for both Zn fractions. The efficiency was significant at the TSP rate of 2.0% of the sorbent and at least the sorbent +TSP of 10 g·kg−1 soil. The cation exchange capacity (CEC) of about 2 cmol(+)·kg−1 controlled the activity −0.50 mmol·dm−3 of either γZnreac or γZnact, hence a very low zinc ionic activity. The use of mineral blends with higher sharing of zeolite is promising for remediating metal-polluted lands in the case of zinc.

Comparison of Physicochemical Properties of Fly Ash Precursor, Na-P1(C) Zeolite-Carbon Composite and Na-P1 Zeolite-Adsorption Affinity to Divalent Pb and Zn Cations

Considering the growing needs of environmental remediation, new effective solutions should be sought. Therefore, the adsorbed amounts of heavy metal ions, such as lead(II) and zinc(II), on the surface of high-carbon fly ash (HiC FA), zeolite-–carbon composite (Na-P1(C)) and pure zeolite (Na-P1), were investigated. The applied solids were characterized using the following techniques: XRD, SEM-EDS, TEM, porosimetry, SLS, electrophoresis and potentiometric titration. The heavy metal concentration in the probes was determined by applying ICP-OES spectroscopy. Adsorption/desorption and electrokinetic measurements were performed in the systems containing one or two adsorbates. The obtained results indicated that Pb(II) ions are adsorbed in larger amounts on the investigated solid surface due to the molecular sieving effect. The largest adsorption capacity relative to lead(II) ions was observed for pure Na-P1 zeolite (407 mg/g). The simultaneous presence of Pb(II) + Zn(II) mixed adsorbates minimally affects the amount of adsorbed Pb(II) ions and causes a significant decrease of Zn(II) ion adsorption (in comparison with analogous systems containing single adsorbates). It was also shown that all solids can be efficiently regenerated using hydrochloric acid. Thus, the selected pure zeolite can be successfully applied in soil remediation or other purifying technologies as an effective Pb(II) adsorbent.

Potential of a novel modified gangue amendment to reduce cadmium uptake in lettuce (Lactuca sativa L.)

In this study, the modified gangue (GE) was prepared by calcination at lower temperatures using potassium hydroxide (KOH) as the activating agent. The field emission scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray fluorescence (XRF) methods were employed to analyze the physicochemical characteristics of GE before and after the modification. Besides, the GE and commercial zeolite (ZE) were compared in the remediation of Cd-contaminated soil in field experiments. The results showed that both the GE and ZE had positive effects on the stabilization of Cd, decreasing the available Cd by 21.2-33.9% and 22.1-28.2%, respectively, while no significant difference was observed between the two amendments, indicating that the modification of GE was successful. Moreover, the application of GE decreased the Cd mobilization and uptake in lettuce shoot and root by 54.9-61.5% and 9.3-13.2%, respectively, and at the same time, the bio-available Cd decreased by 20.9-34.5%. Moreover, with the addition of GE, activities of urease and alkaline phosphatase increased in soil, while the peroxidase and superoxide dismutase activities were notably reduced in plants. Therefore, GE could be used as an effective amendment for the alleviation of Cd accumulation and toxicity, and thereby improve food safety.

Influence of biochar and EDTA on enhanced phytoremediation of lead contaminated soil by Brassica juncea

Phytoremediation technology is an eco-friendly technology for the treatment of the polluted environment. Conversely, the natural and synthetic amendments have been revealed to improve the heavy metal phytoextraction from polluted soils with hyperaccumulation and/or non-hyper accumulating plants. This study evaluated the synergistic effect of biochar (BC) and EDTA to enhance phytoextraction of heavy metal lead (Pb) from artificially polluted soil by Brassica juncea. The BC and EDTA amendment enhanced the growth and survival of B. juncea under Pb stress environment. BC and EDTA significantly increased the biomass of B. juncea and significantly increased the total chlorophyll content in the combined amendment of BC and EDTA (22.2 mg/g) compared to the individual amendment of BC (12.8 mg/g) and EDTA (12.2 mg/g) respectively. The combined use of EDTA and biochar showed enhanced Pb uptake (60.2 mg/g) compared to control (10.0 mg/g). The order of Pb uptake was found to be BC + EDTA (60.2 mg/g) ˃ EDTA (23.5 mg/g) ˃ BC (22. 0 mg/g) ˃ control (10.0 mg/g). The maximum activity of SOD (35.2 ± 1.2 U/mg), POD (47.0 ± 1.8 U/mg) and CAT (28.0 ± 1.0 U/mg) was obtained in the mixed application of EDTA and BC. The obtained results revealed that the combined use of BC and EDTA was the most advantageous option for the treatment of Pb contaminated soil as compared to individual amendments.

Cd bioavailability and nitrogen cycling microbes interaction affected by mixed amendments under paddy-pak choi continued planting

Cadmium (Cd) is the most concerning soil pollutant, and a threat to human health, especially in China. The in-situ immobilization of Cadmium by amendments is one of the most widely adopted methods to remedy soil contamination. The study was designed to evaluate the effect of organo-chemical amendments on soil Cd bioavailability and nitrogen cycling microbes under continuous planting of rice (Oryza sativa) and pak choi (Brassica chinensis L.). The experiment was carried out using four amendments, Lime, Zeolite, Superphosphate, and Biochar, at two different ratios; M1: at the ratio of 47:47:5:1, and M2 at the ratio of 71:23:5:1, respectively. Moreover, both M1 and M2 were enriched at four levels (T1: 0.5%; T2: 1%; T3: 2%; T4: 4%). Results showed that compared with CK (Cd enriched soils), the yield of rice under treatments of M1T1 and M2T1 increased by 8.93% and 8.36%, respectively. While the biomass (fresh weight) of pak choi under M1 and M2 amendments increased by 2.52-2.98 times and 0.76-2.89 times respectively, under enrichment treatments T1, T2, and T3. The total Cd concentrations in rice grains treated with M1T3 and M2T3 decreased by 89.25% and 93.16%, respectively, compared with CK. On the other hand, the total Cd concentrations in pak choi under M1T3 and M2T2 decreased by 92.86% and 90.23%, respectively. The results showed that soil pH was the main factor affecting Cd bioavailability in rice and pak choi. The Variance partitioning analysis (VPA) of rice and pak choi showed that soil pH was the most significant contributing factor. In the rice season, the contribution of soil pH (P) on Cd bioavailability was 10.14% (P = 0.102), and in the pak choi season, the contribution of soil pH was 8.38% (P = 0.133). Furthermore, the abundance of ammonia oxidation and denitrifying microorganisms had significantly correlation with soil pH and exchange Cd. In rice season, when the enrichment level of amendments increased from 0.5% (T1) to 2% (T3), the gene abundance of AOA, AOB, nirK, nirS and nosZ (І) tended to decrease. While in pak choi season, when the enrichment level increased at the level of 0.5% (T1), 1% (T2), and 2% (T3), the gene abundance of AOB, nirS, and nosZ (І) increased. Additionally, the gene abundance of AOA and nirK showed a reduction in the pak choi season contrasting to rice. And the mixed amendment M2 performed better at reducing Cd uptake than M1, which may have correlation with the ratio of lime and zeolite in them. Finally, we conclude that between these two amendments, when applied at a moderate level M2 type performed better than M1 in reducing Cd uptake, and also showed positive effects on both gene abundance and increase soil pH.

Remediation of Cu-polluted soil with analcime synthesized from engineering abandoned soils through green chemistry approaches

Due to the large output and potential ecological risks, disposal of engineering abandoned soils (EAS) has become an enormous challenge for human society. Herein, EAS has been transformed into microporous analcime (ANA) zeolite material through a fast, energy-efficient, and straightforward conversion process. The as-synthesized ANA has been employed to remediate Cu-polluted soil, which shows a significant ecological restoration function in a vegetable pot experiment. With 25 g/kg ANA into Cu contaminated soil (total Cu concentration: 200 ppm), the Cu accumulation concentration in vegetables has been decreased from 5.60 down to 1.80 mg/kg (approaching the background Cu level 1.70 mg/kg in vegetables). Detailed mechanism study combining with DFT calculations reveals that the Cu²⁺ in soil has been captured both inside the ANA pore channels and on the surface via ion-exchange and surface adsorption mechanism. The whole process, including ANA synthesis and Cu polluted soil remediation, has been optimized to show a valuable conceptual model to recycle EAS resource and in-situ remediate Cu polluted soil.

Transcription profiling-guided remodeling of sulfur metabolism in synthetic bacteria for efficiently capturing heavy metals

Heavy metal contamination is becoming a global problem threatening human health. Heavy metal removal by engineered microbes by cellular adsorption and uptake is a promising strategy for treatment of heavy metal contamination. However, this strategy is confronted with limited heavy metal-capturing elements. In this study, we performed a transcription profiling-guided strategy for construction of heavy metal-capturing synthetic bacteria. Transcription profiling of a heavy metal-tolerating Cupriavidus taiwanensis strain revealed up-regulation of sulfur metabolism-related operons (e.g., iscSAU and moaEDAB) by Pb²⁺ and Cd²⁺. A synthetic Escherichia coli strain, EcSSMO, was constructed by design of a synthetic sulfur metabolism operon (SSMO) based on iscSAU/moaEDAB. Biochemical analysis and X-ray photoelectron spectroscopy (XPS) revealed that the synthetic bacteria had remodeled sulfur metabolism and enhanced heavy metal-tolerating capacity, with higher surviving EcSSMO cells than the surviving control cells Ec0 (not containing SSMO) at 50 mg/L of Pb²⁺ and Cd²⁺ (>92 % versus <10 %). Moreover, EcSSMO exhibited much higher heavy metal-capturing capacity than Ec0, removing>90 % of Pb²⁺ and Cd²⁺ at 5 mg/L of Pb²⁺ and Cd²⁺, and >40 % of both heavy metals even at 50 mg/L of Pb²⁺ and Cd²⁺. This study reveals emphasizes feasibility of transcription profiling-guided construction of synthetic organisms by large-scale remodeling metabolic network.

Simultaneous immobilization of As and Cd in a mining site soil using HDTMA-modified zeolite

Arsenic (As) and cadmium (Cd) co-contamination has been a typical problem in Chinese agricultural land adjacent to historical metal mining and smelting activities. Remediation of As and Cd in soil has encountered many difficulties owing to the distinct nature of the two metal(loid)s. In this study, we developed a remediation scheme by adding a hexadecyltrimethylammonium (HDTMA)-modified zeolite to a mining site soil and evaluated the immobilization effect. The result of the increased surface zeta potential indicates that the HDTMA modification conferred the zeolite with adsorbability towards As through the cationic surfactant head. The addition of the highest dosage of HDTMA-modified zeolite (10%) to the contaminated soil greatly improved soil organic matter by 1.4 times, partly due to the elevated C loading on the zeolite from HDTMA. Sequential extraction results show that the addition of HDTMA-modified zeolite not only increased the residual fraction of As (by 2.7-5.9%) but also reduced the toxicity-related fraction (by 2.3-2.7%) when compared to the unmodified zeolite and blank treatments. The oxidizable fractions of Cd in the modified zeolite treatment were significantly higher than that in the blank soil. Besides, the exchangeable fractions of Cd were all significantly reduced in the zeolite treatments. Enzyme activity assays show that the HDTMA-modified zeolite treatment could greatly improve soil microbial environment. The physiologically based extraction test (PBET) also proved that the bioavailability of As and Cd was reduced after the modified zeolite treatment.

Restoring the plant productivity of heavy metal-contaminated soil using phosphate sludge, marble waste, and beneficial microorganisms

Assisted natural remediation (ANR) has been highlighted as a promising, less expensive, and environmentally friendly solution to remediate soil contaminated with heavy metals. We tested the effects of three amendments (10% compost, C; 5 or 15% phosphate sludge, PS5 and PS15; and 5 or 15% marble waste, MW5 and MW15) in combination with microorganism inoculation (rhizobacteria consortium alone, mycorrhizae alone, and the two in-combination) on alfalfa in contaminated soil. Plant concentrations of Zn, Cu, and Pb were measured, along with proline and malondialdehyde production. The microbiological and physicochemical properties of the mining soil were evaluated. Application of the amendments allowed germination and promoted growth. Inoculation with the rhizobacteria consortium and/or mycorrhizae stimulated plant growth. PS and MW stimulated the production of proline. Inoculation of alfalfa with the rhizobacteria-mycorrhizae mixture and the application of MW allowed the safe cultivation of the legume, as shown by the low concentrations of metals in plant shoots. Zn and Pb concentrations were below the limits recommended for animal grazing and accumulated essentially in roots. Soil analyses showed the positive effect of the amendments on the soil physicochemical properties. All treatments increased soil pH (around 7), total organic carbon, and assimilable phosphorus content. Notably, an important decrease in soluble heavy metals concentrations was observed. Overall, our findings revealed that the applied treatments reduced the risk of metal-polluted soils limiting plant growth. The ANR has great potential for success in the restoration of polymetallic and acidic mining soils using the interaction between alfalfa, microorganisms, and organo-mineral amendments.

Hierarchical zeolites comprising orthogonally stacked bundles of zeolite nanosheets for catalytic and adsorption applications

The synthesis of hierarchical MFI zeolites comprising orthogonally stacked bundles of zeolite nanosheets using a new type of triblock structure-directing agents (SDAs) was reported. The textural properties, including the degree of nanosheet branching and the spacing between adjacent nanosheets, could be controlled by changing the length of the linkers in the triblock SDAs. The hierarchical pure-silica silicalite-1 materials exhibited high and stable catalytic activity for the vapor-phase Beckmann rearrangement of cyclohexanone oxime with high selectivity of ε-caprolactam. On the other hand, the hierarchical ZSM-5 materials showed high adsorption capacity of Pb²⁺ ion following a Langmuir-type adsorption behavior. After being deposited with Pd nanoparticles, the hierarchical Pd/ZSM-5 nanocomposites exhibited high activity in the aqueous-phase hydrogenation of phenol to cyclohexanone at room temperature. The results show promise of the disclosed hierarchical zeolites for catalytic and adsorption applications.

Effect of multi-walled carbon nanotubes on extractability of Sb and Cd in contaminated soil

The interaction between multi-walled carbon nanotubes (MWCNTs) and soil heavy metals was rarely studied. With the convenience of detecting multiple metal elements by ICP-AES, this paper examined the potential effectiveness of MWCNTs on extractability of antimony (Sb) and cadmium (Cd) in contaminated soil. Three-step sequential extraction procedure, toxicity characteristic leaching procedure, bioaccessibility and CaCl₂ single extraction were employed to evaluate Sb and Cd speciations and their extractabilities. According to our results, only at low Sb content level of 100 mg/kg, antimony bioavailability reduced with MWCNTs addition of 0.3% and 0.9% by 22.97% and 20.74%, respectively, which might due to the increase of adsorption point, nevertheless, the excess Sb(OH)₆^- was not adsorbed more efficiently. Secondly, due to the difference in effective specific surface area, only under the condition of high content level and MWCNTs addition of 0.1%, the mild acid-soluble fraction increased at most by 15.40% for Sb and 9.40% for Cd, respectively. However, in terms of TCLP-extractable Sb and Cd and CaCl₂-extractable Sb and Cd, no significant, continuous, regular extractability pattern were found. Overall, MWCNTs were selective on extractability of soil heavy metals due to mechanisms of physical adsorption. This paper provides data reference for the interaction between MWCNTs and soil heavy metals extractability.

Comparative Study of Mercury(II) Removal from Aqueous Solutions onto Natural and Iron-Modified Clinoptilolite Rich Zeolite

The contamination of soil and water bodies with mercury from anthropogenic sources such as mining and industry activities causes negative effect for living organisms due to the process of bioaccumulation and biomagnification through the food chain. Therefore, the need for remediation of contaminated areas is extremely necessary and very desirable when it is cost-effective by using low-cost sorbents. This paper compares the sorption abilities of natural and iron-modified zeolite towards Hg(II) ions from aqueous solutions. The influence of pH, solid/liquid ratio (S/L), contact time, and initial concentration on the sorption efficiency onto both zeolites was investigated. At the optimal pH = 2 and S/L = 10, the maximum amount of sorbed Hg(II) is 0.28 mmol/g on the natural zeolite and 0.54 mmol/g on the iron-modified zeolite. It was found that rate-controlling step in mass transfer is intraparticle diffusion accompanied by film diffusion. Ion exchange as a main mechanism, accompanied with surface complexation and co-precipitation were included in the Hg(II) sorption onto both zeolite samples. This is confirmed by the determination of the amount of sorbed Hg(II) and the amount of released exchangeable cations from the zeolite structure as well as by the scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS) of saturated zeolite samples. In a wide pH range, 4.01 ≤ pH ≤ 11.08, the leaching of Hg(II) was observed in the amount of only 0.28–0.78% from natural zeolite and 0.07–0.51% from iron-modified zeolite indicating that both zeolites could be used for remediation purposes while the results suggest that modification significantly improves the sorption properties of zeolite.

Zeolite for Potential Toxic Metal Uptake from Contaminated Soil: A Brief Review

Soil pollution is an increasingly urgent problem for the global environment. Soil can be contaminated with potential toxic metals from many anthropogenic activities, besides fossil fuel combustion and crude oil production, ranging from industry to mining and agriculture. Many technologies have been analysed to solve this type of environmental pollution and methods involving the use of minerals (e.g., clay minerals, zeolites, and natural silica adsorbents) are widely described in the literature. This article provides a summary of studies concerning the use of zeolites in soil remediation. A considerable number of these experiments were conducted using natural zeolites, while fewer concerned the utilization of synthetic zeolites. The mechanism controlling the successful application of these minerals was analysed through referring to global data published on this topic over the last few decades. This review also briefly discusses the limitations on zeolite applications and the drawbacks of the approaches analysed.

Evaluation of Different Types and Amounts of Amendments on Soil Cd Immobilization and its Uptake to Wheat

Using amendments is a cost-effective method to soil cadmium (Cd) remediation, whereas knowledge about how different amendments and rates affect remediation efficiency remains limited. This study aimed to evaluate the impacts of different types and amounts of amendments on soil Cd immobilization and its uptake by plants. Biochar (BC), zeolite (ZE), humic acid (HA), superphosphate (SP), lime (L), and sodium sulfide (SS) were applied at three rates (low, medium, and high) ranging from 0.5 to 5%. The concentration of CaCl₂-extractable Cd was considerably affected by the amendments, except HA, and the high doses achieved better immobilization effects than the low doses did. The addition of amendments decreased weak acid soluble Cd by 4.1-44.0% but slightly increased the fractions of oxidizable and residual Cd. These amendments (except BC and HA dose of 1%) decreased Cd accumulation in grains by 1.3-68.8% and (except SP) in roots by 16.3-65.5% compared with the control. The SP efficiently immobilized Cd but posed a potential soil acidification risk. Moreover, SS treatment increased the soil electrical conductivity (EC) value and restricted the growth of wheat, possibly due to high-salt stress. BC, ZE, and L exerted significant effects on the reduction in available Cd as the application rate increased. These amendments enhanced Cd immobilization mainly by changing Cd availability in soil and influencing its redistribution in different fractions in soil and root uptake by plants. This study concluded that BC-5%, ZE-1%, and L-0.5% can be used for Cd immobilization in acidic or neutral soils.

Zeolite and bentonite as nickel sequestrants in carbonation lime coming from the sugar industry

A laboratory trial was performed to test the sequestration capacity of two minerals (bentonite and zeolite) at three initial concentrations (2.5, 5 and 10%) in order to counter water-soluble nickel (Ni) exceeding the Italian legal limit (10 μg L^-1) in carbonation lime disposed of in a field and sampled for an 85-day lab study. The results show a noticeable reduction in water-soluble and bioavailable Ni in lime after the addition of sequestrants, especially at the dose of 5% bentonite or zeolite, thereby indicating a "ceiling effect" of the sequestrant, i.e., an increasing dose could reduce the adsorption capacity and be less effective. The alkaline pH and the presence of organic matter could be the main factors affecting the good performance of sequestrant addition, causing an increase in the negative charge of the organic and mineral colloids and the formation of unavailable Ni precipitates. The 85-day experiment seems to be sufficient to reach an adsorption equilibrium for water-soluble nickel, while for the bioavailable form a longer period appears to be necessary.

Assessing Chromium Contamination in Red Soil: Monitoring the Migration of Fractions and the Change of Related Microorganisms

The improper stacking of chromium (Cr) slag poses a great threat to the environment and human health. The toxicity of Cr in soil is not only related to its total amount, but also to its fractions. A simulated experiment was conducted in laboratory to assess the environmental risk of Cr fractions migration and distribution in red soil. The results showed the content of acid-soluble and reducible Cr significantly decreased (P < 0.05) in top layer but increased in middle and substratum layers over time. This indicated that acid-soluble and reducible Cr migrated downward with time and the relative mobility of acid-soluble Cr (0.038 mg/kg·d·m) was higher than that of reducible Cr (0.028 mg/kg·d·m). Furthermore, correlation analysis between microbial community and chromium fraction showed the relative abundance of Lysobacter, Flavihumibacter, Flavisolbacter, and Altererythrobacter was significantly (P < 0.05) correlated with acid-soluble and reducible fractions. Thus, these microorganisms might be evaluators to assess the migration of acid-soluble and reducible fractions in red soil. In summary, this study provided a new comprehension on remediation of Cr-contaminated soil by monitoring the migration of acid-soluble and reducible fractions and the changes of related microbial groups.

Assessment of Sewage Water Treatment Using Grinded Bauxite Rock as a Robust and Low-Cost Adsorption

The shortage of water resources in Saudi Arabia is becoming an increasingly serious problem. Management of sewage water is an attractive option to reduce the contamination of water resources such as ground water. This work aims to use bauxite rock as a low-cost adsorbent/coagulant for sewage water treatment in a simple and rapid technique. Different doses (1, 2, 3, 4 and 5 g/l) of the grinded bauxite was used as an adsorbent/coagulant. The results revealed that, at rock doses of 1, 2, 3, 4, and 5 g/l, the COD concentration was decreased from 326 to 134, 98, 83, 70, and 65 mg/l, respectively, while the BOD concentration was lowered from 243 to 196, 104, 71, 60, 51, and 47 for the same rock doses. This was reflected on the turbidity of the treated effluent from each treatment step. Also, the FC counts were reduced to 2 log units. Furthermore, the dose of 3 g/l of the grinded bauxite rock was found to be the least economic dose for the treatment of sewage water. Bauxite mineral has been used explicitly in a high-performance, very affordable method for wastewater treatment.

From microporous to mesoporous mineral frameworks: An alliance between zeolite and chitosan

Microporous and mesoporous minerals are key elements of advanced technological cycles nowadays. Nature-driven microporous materials are known for biocompatibility and renewability. Zeolite is known as an eminent microporous hydrated aluminosilicate mineral containing alkali metals. It is commercially available as adsorbent and catalyst. However, the large quantity of water uptake occupies active sites of zeolite making it less efficient. The widely-used chitosan polysaccharide has also been used in miscellaneous applications, particularly in medicine. However, inferior mechanical properties hampered its usage. Chitosan-modified zeolite composites exhibit superior properties compared to parent materials for innumerable requests. The alliance between a microporous and a biocompatible material with the accompaniment of negative and positive charges, micro/nanopores and proper mechanical properties proposes promising platforms for different uses. In this review, chitosan-modified zeolite composites and their applications have been overviewed.

Implications of spatial heterogeneity of tailing material and time scale of vegetation growth processes for the design of phytostabilisation

Phytostabilisation projects for tailing dams depend on processes occurring at spatial scales of 10⁶ m² and at decadal time scales. Most experiments supporting the design and monitoring of such projects have much smaller spatial and time scales. Usually, they are only designed for one single scale. Here, we report the results of three coupled experiments performed at pot, lysimeter and field plot scales using six sampling periodstimes from 3 to 20 months. The work explicitly accounts for the sampling times when evaluating the effects of amendments on the performance of plants grown in tailing substrates. Two treatments with potentially complementary roles were applied: zeolites to decrease availability of Cd, Cu, Pb and Zn and green fertilizer to increase the availability of nutrients. Zeolites have a positive influence on plant development, especially in the early stages. Analyses of the pooled datasets for all sampling times revealed the possibility of predicting plant physiological variables, such as protein concentrations, pigments and oxidative stress enzyme activities, as a function of the factors extracted by principal component analysis from the metal concentrations in plants, phosphorus concentrations in plants, and sampling times. Two potentially general methodological rules were extracted: account for the spatial geochemical variability of tailings, and cover the broadest possible range of time scales by experiments. The proposed experimental methodology can be of general use for the design of tailing dam remediation technologies with improvements involving the set of measured variables and sampling frequency and by carefully relating the costs to the institutional aspects of tailing dam management.

Heavy Metals Detection in Zeolites Using the LIBS Method

In this study, a possibility of laser-induced breakdown spectroscopy (LIBS) for the analysis of zeolites containing copper, chromium, cobalt, cadmium, and lead in the concentration range of 0.05–0.5 wt.% is discussed. For the LIBS analysis, microporous ammonium form of Y zeolite with the silicon to aluminum molar ratio of 2.49 was selected. Zeolites, in the form of pressed pellets, were prepared by volume impregnation from the water solution using Co(CH3COO)2.4H2O, CuSO4.5H20, K2Cr2O7, PbNO3, and CdCl2 to form a sample with different amounts of heavy metals—Co, Cu, Cr, Pb, and Cd. Several spectral lines of the mentioned elements were selected to be fitted to obtain integral line intensity. To prevent the influence of the self-absorption effect, non-resonant spectral lines were selected for the calibration curves construction in most cases. The calibration curves of all elements are observed to be linear with high regression coefficients. On the other hand, the limits of detection (LOD) were calculated according to the 3σ/S formula using the most intensive spectral lines of individual elements, which are 14.4 ppm for copper, 18.5 ppm for cobalt, 16.4 ppm for chromium, 190.7 ppm for cadmium, and 62.6 ppm for lead.