The production rate of industrial and agricultural waste is increasing due to population growth. Soil is the most important receiver of industrial and agricultural waste. Contaminants such as heavy metals in various waste after reception by the soil, immediately become part of the cycle that has different impacts on the environment. Geopolymer, as a chemical stabilizer has the potential to stabilize heavy metals in the soil. In this research, several geopolymers for the stabilization of heavy metals in soil were synthesized. Silicon dioxide (SiO2) and aluminosilicate (Al2SiO4) must be used to produce the geopolymers. Rice husk ash was used as the SiO2 source. Also, Iranian zeolite and sepiolite, and red clay soil were utilized as the source of Al2SiO4. The synthesized geopolymers were investigated for the adsorption of nickel and cadmium. Also, batch and column studies of using geopolymers for the chemical stabilization of heavy metals in soil were conducted. The results revealed a high adsorption capacity of the geopolymers. The zeolite, sepiolite, and red clay geopolymer-soil samples adsorbed 100% of the heavy metals (i.e., Ni and Cd) at a concentration of 100 ppm. The zeolite geopolymer adsorbent adsorbed 57% and 96% of Ni and Cd at a concentration of 1000 ppm, respectively. In general, it was concluded that the use of geopolymer compounds in soils with high heavy metal adsorption capacity could be an efficient approach to prevent groundwater resource pollution.

Present study reports the laboratory and field scale application of different organic and inorganic amendments to immobilize cadmium (Cd) and lead (Pb) in a co-contaminated alluvial paddy soil. For that purpose, lime, biochar, Fe-biochar and two composite amendments (CA) composed of biochar, lime, sepiolite and zeolite (CA1: composite amendment 1) and manure, lime and sepiolite (CA2: composite amendment 2) were firstly tested in an incubation experiment to ameliorate Cd and Pb co-contaminated alluvial soil. It was observed that liming and CA2 elevated the soil pH and reduced DTPA extractable Cd and Pb in the incubated soil leading to higher metal immobilization. Therefore, efficiency of lime and CA2 was further investigated in field conditions with mid rice as the test crop to evaluate field scale immobilization and precise application rate for the tested soil type. DTPA and CaCl₂ extractable Cd (46 and 51%) and Pb (68 and 70%) in field soil were decreased with applied treatments. Speciation of Cd and Pb also promoted conversion of metal exchangeable contents to less-available forms. Activated functional groups on amendments’ surface (_OH bonding, C_O and CO, -O-H, Si–O–Si, carboxylic and ester groups) sequestered metals by precipitation, adsorption, ion exchange or electro static attributes. Application of lime at 2400 kg/acre (T4) and CA2 at 1200 kg/acre was more effective in reducing rice shoot and grains metal contents. Moreover, obtained results in terms of pH, extractable content, speciation and yield, and microanalysis of amendments highlights the remarkable efficiency of lime and composite amendment to sorb Cd and Pb providing the key evidence of these amendments for metals immobilization and environmental remediation. Considering these results, lime and CA2 are potential amendments for co-contaminated rice field especially in context of alluvial soil.

Long-term effectiveness and persistence are two important criterias to evaluate alternative remediation technology of heavy metal polluted soils. Pot and field studies showed addition of sepiolite was effective in immobilizing Cd in polluted soils, with significant reduction in TCLP extracts (0.6%–49.6% and 4.0%–32.5% reduction in pot and field experiments, respectively) and plant uptake (14.4%–84.1% and 22.8%–61.4% declines in pot and field studies, correspondingly). However, the applications of sepiolite offered a limited guarantee for the safety of edible vegetables in Cd-polluted soils, depending on the soil type, the Cd pollution type and level, and the dose and application frequency of chemical amendments. Bioassays, such as plant growth, soil enzymatic activities and microbial community diversity, indicated a certain degree of recovery of soil metabolic function. Therefore, sepiolite-assisted in situ remediation is cost-effective, environmentally friendly, and technically applicable, and can be successfully used to reduce Cd enter into the food chain on field scale.

Organic and inorganic mixtures can be developed as immobilizing agents that could reduce heavy metal accumulation in crops and contribute to food safety. Here, inorganic materials (lime, L; zeolite, Z; and sepiolite, S) and organic materials (biochar, B, and compost, C) were selectively mixed to produce six composite soil amendments (LZBC, LSBC, LZC, LZB, LSC, and LSB). Given the fact that LZBC showed the best performance in decreasing soil Cd availability in the incubation experiment, it was further applied in the field condition with 14 vegetables as the test crops to investigate its effects on crop safety production in polluted greenhouse. The results showed that LZBC addition elevated rhizosphere soil pH by 0.1–2.0 units and reduced soil Cd availability by 1.85–37.99%. Both the biomass and the yields of edible parts of all vegetables were improved by LZBC addition. However, LZBC addition differently affected Cd accumulation in edible parts of the experimental vegetables, with the observation that Cd contents were significantly reduced in Allium fistulosum L., Amaranthus tricolor L., and Coriandrum sativum Linn., but increased in the three species of Lactuca sativa. Further health risk assessment showed that LZBC application significantly decreased daily intake of metal (DIM), health risk index (HRI), and target hazard quotient (THQ) for Cd in Allium fistulosum L., Amaranthus tricolor L., and Coriandrum sativum Linn., whereas increased all the indexes in Lactuca sativa. Our results showed that the effect of a composite amendment on Cd accumulation in different vegetables could be divergent and species-dependent, which suggested that it is essential to conduct a pre-experiment to verify applicable species for a specific soil amendment designed for heavy metal immobilization.

With the improvement of environmental protection and standards, the recovery and recycling of copper ions released from industrial wastewater discharge has aroused sufficient research interest. A new adsorbent (ABsep) derived from natural sepiolite (Sep) by modification technology of hexadecyltrimethoxysilane (HDTMS) or tetraethylorthosilicate (TEOS) and flocs separation method for adsorption of Cu²⁺ in wastewater have been investigated in this paper. The changes of crystal structure and physicochemical properties of Sep during modification process showed that HDTMS and TEOS were loaded onto the Sep surface without inserting into the Sep interlayer structure. The adsorption experimental results presented a smaller amount of ABsep (1 g/L) achieved 97.5% removal of 50 mg/L Cu²⁺ at pH 6 and temperature of 298 K within a shorter contact time (50 min). It is interesting the adsorption process of Cu²⁺ on ABsep was spontaneous and exothermic, with physical adsorption dominating, as result of combination of physical adsorption, electrostatic gravitational force, and chemical reaction. Because of good physical–chemical adsorption performance of ABsep surface to Cu²⁺, high removal rates of Cu²⁺ from aqueous phase could be achieved after three adsorption regeneration cycles, this indicated the ABsep was expected to be a promising adsorbent of Cu²⁺ removal for wastewater treatment.

Various geochemical studies have yielded conflicting data on whether humic coatings decrease or increase adsorption of heavy metals by soil minerals. The objective of the present study is to determine how humate pre-adsorption affects subsequent retention of Cd and Pb by palygorskite and sepiolite, as special silicate clay minerals of soil in many arid regions. For this purpose, a series of equilibrium batch experiments were conducted on the interactions of Pb and Cd with Ca–palygorskite and Ca–sepiolite before and after humate adsorption. The results showed that the Langmuir (L), Freundlich (F), Langmuir–Freundlich (LF), and Toth (T) equations satisfactorily described metal sorption data on the minerals. In the presence of humate as the pre-adsorbate, the values for sorption capacities of palygorskite and sepiolite for Cd and Pb slightly decreased. This can be attributed to the competition between humates and metal ions for mineral active sites and steric hindrance of the adsorbed humates, which reduces the access of metal ions to the mineral surface and internal channels. Humate coatings decreased the adsorption equilibrium constants of Cd, suggesting that the affinity of the organo-clays for Cd sorption is lower than those of Ca–clays. The values for the heterogeneity factor (β) generally showed an increasing trend with increasing humate coverage on palygorskite and sepiolite, which can be explained by the increased diversity of adsorption centers on humate–clay complexes. It may be concluded that the presence of humate bound on fibrous clay surfaces can influence the sorption, and hence the bioavailability and mobility of heavy metals in fibrous clay-containing arid and semiarid soils.

Farmland soils that surround a historical zinc smelting area in northwestern Guizhou, China, are characterized by high levels of heavy metal accumulation. Previous studies have mainly focused on the potential risk evaluations of heavy metals in soil and crops. However, at present, the effects of amendment applications on the bioavailability of heavy metals and on microbial community in the heavily contaminated soils of the mining region are still unclear. A pot experiment was conducted to determine the effect of applying a combined amendment (e.g. lime, sepiolite, and vermicompost) on the diversity and composition of microbial community in the contaminated soil. The results showed that the contents of DTPA- and TCLP-extractable heavy metals (e.g. Cd, Pb, and Zn) decreased and that the pH, SWC, EC, and soil available nutrient (e.g. AN, AP, and AK) contents increased after the application of the combined amendment. Furthermore, application of the combined amendment decreased the diversity of soil bacterial and fungal communities and increased the relative abundances of the dominant bacterial and fungal communities such as Proteobacteria, Bacteroidetes, and Ascomycota; however, the relative abundances of Acidobacteria and Actinobacteria decreased. Redundancy analysis (RDA) and structural equation model (SEM) analysis showed that the bioavailability of heavy metals decreased and that soil physicochemical characteristics improved and had positive or negative effects on the diversity and composition of soil microbial community.

Raw and modified fibrous clay minerals palygorskite (Pal) and sepiolite (Sep) were tested for their ability to remove ammonium from ammonium polluted water. Palygorskite and sepiolite underwent thermal treatment at 400°C (T-Pal and T-Sep respectively). Raw and thermally treated samples were characterized using XRD, SEM, BET, FTIR, TGA, zeta potential, and XRF. The techniques verified the effect of thermal treatment on sample structures and the enhancement of negative charge. Both raw and thermally activated materials were applied in batch kinetic experiments, and found to be efficient adsorbents in their raw forms, since Pal and Sep achieved 60 and 80% NH₄⁺–N removal respectively within 20 min of contact for initial NH₄⁺–N concentration of 4 mg/L. Similar removal rates were gained for other concentrations representative of contaminated aquifers that were examined, ranging from 1 to 8 mg/L. Results for the modified T-Pal and T-Sep minerals showed up to 20% higher removal rate. Saturation tests indicated the positive effect of thermal treatment on the minerals since T-Pal and T-Sep removal efficiency reached 85% and remained stable for 24 h. However, competitive ions in real water samples can influence the NH₄⁺–N removal efficiency of the examined samples. At almost all the examined samples, the nonlinear Freundlich isotherm and linear pseudo-second kinetic models showed better fitted all examined samples thus indicating heterogeneous chemisorption.

Sodium pentachlorophenol (NaPCP) is a highly toxic and persistent organic pollutant. With sepiolite as the support, a series of TiO₂-Sep nanocomposites (NCs) with different Ti/Sep ratios were developed. The objective was to understand the effect of Ti/Sep ratio on the structure and activity of the NCs in aqueous and soil systems and to evaluate the feasibility of the NCs for in situ soil remediation. The prepared NCs were characterized with XRD, SEM, TEM, and N₂ adsorption–desorption, respectively. The results showed that high surface area and good dispersion of TiO₂ on sepiolite surface were obtained. The photocatalytic activities in aqueous and soil of the as-developed NCs were examined using NaPCP as a model pollutant. Compared with bare sepiolite and TiO₂, the heterogeneous NCs showed significantly higher photocatalytic performance in decomposing NaPCP, and the photocatalytic activities varied with the content of TiO₂ in the NCs. In aqueous media, treatment with TiO₂-S-30 showed excellent degradation efficiency with about 90% NaPCP decomposed in 140 min. Nevertheless, the sample TiO₂-S-20 promotes maximum rate reduction of NaPCP with above 90% within 20-h irradiation in soil. The results indicate that an appropriate Ti/Sep ratio could significantly enhance the activities of NCs on NaPCP remediation and the role of carrier sepiolite is more important in soil media than that in aqueous phase. The excellent performance of the TiO₂-Sep in wastewater degradation and soil remediation can be attributed to the synergistic effects between the high photocatalytic activity of TiO₂ nanoparticles and the strong adsorption capacity of sepiolite nanofibers. This work revealed that sepiolite adsorption coupled with TiO₂ photocatalysis can be one promising technique for in situ remediation of NaPCP-contaminated soil.

This work is devoted to the development of Ag-ZnO/sepiolite photocatalysts as novel nanostructured materials by the immobilization of Ag-doped ZnO on the surface of fibrous clay. Herein, innovative Ag-ZnO/sepiolite photocatalysts were successfully prepared through a simple hydrothermal route using diverse Ag dopant concentrations (2 and 5%). Structural, morphological, and optical properties of the obtained photocatalysts were characterized by XRD, TEM, MEB, and DRS-UV–Vis spectroscopy. The results confirmed that Ag-doped ZnO nanoparticles with a diameter of 10–30 nm are homogeneously distributed on the sepiolite fibers’ surface. The silver dopant was effectively incorporated into the zinc oxide, leading to a slight distortion of the hexagonal wurtzite structure and a reduction of the bandgap energy with increased silver doping. The photocatalytic activity towards the degradation of methylene blue (MB) dye was analyzed for all the samples under UV–Vis light. Compared to ZnO alone and undoped ZnO/SEP, the Ag-ZnO/SEP5% nanostructured materials exhibited a significantly improved photocatalytic activity, with full decolorization after 4 h of UV–Vis irradiation (60 W). The photocatalysis of organic pollutants matched well with a pseudo-first-order kinetic. The enhanced photocatalytic activity was ascribed to the low bandgap energy (3 eV), the reduction of the recombination of electron hole, and the sepiolite support.