Filter cake is one of the main waste products of zinc processing industries and it contains a high amount of toxic heavy metals. In this research in order to reduce heavy metals leachability in filter cake, Portland cement, natural pozzolan, diammonium phosphate (DAP), triple superphosphate (TSP), lime, zinc oxide and ground granulated blast furnace slag (GGBFS) have been used. This research’s results suggest that even though leachability of lead in the filter cake is quite low, it should be noted that limestone doubles the solubility level of Pb due to the mineralogical nature of the waste, by converting its sulphate form (PbSO4) to its carbonate form (PbCO3), which is more soluble. TSP was the only additive capable of reducing the leachability of metal elements in the filter cake as well as stabilizing the amounts of Pb and Cd in TCLP extracts within the limit. Although all additives were able to reduce Cd leachability, TSP and ZnO had the most remarkable efficiency. According to XRD results, the main factor in lead stabilization using TSP was a reduction in the amount of PbSO4 (with high Ksp) and an increase in PbS (with low Ksp), which ultimately reduced lead leachability. The Pozzolan which was used for the first time as a stabilizer demonstrated a good performance in reducing the leachability of Cd, Ni and Cu. In addition, due to the pseudo-cementitious properties of Pozzolan, it can reduce leachability of metal elements, along with other additives such as cement, by increasing the solidification efficiency.

The reaction of Pd(II) with 1,8-dihydroxy-2-(pyrazol-5-ylazo)-naphthalen-3,6-disulphonic acid immobilized by physical sorption onto Dowex 1-X8 ion-exchange resin was investigated with the aim to develop the sorption-spectroscopic test method for the detection of low Pd(II) concentrations in water. The resin phase absorption spectra of the reagent and its Pd(II) complex were followed. The immobilized reagent has the spectral characteristics similar to those in the water and forms with Pd(II) 1:1 complex with the absorption maximum at 650 nm. Parameters, such as pH, wavelength and contact time have been optimized for a given amount of the sorbed reagent. The experimental conditions for the linear dependence of absorbance vs. Pd(II) concentration have been determined.

As one of the most common ways to get rid of municipal waste, landfill leachate, waste with complicated compositions and high levels of contaminants, has become a significant threat to the world's environment. Here, the impact of sewage sludge (SS) and derived-biochar (SSB) amendments on the immobilization and potential mobility of heavy metals in a contaminated soil-plant system was investigated. The sequential fractionation findings showed that using SS-2%, SSB-2%, and SSBC-1% reduced the potential mobility of heavy metals while increasing the residual fraction in polluted soils. The translocation and bioconcentration factors showed that heavy metals were slightly transferred into shoots from roots and lowered accumulation in roots from contaminated soils. Fourier transform infrared (FTIR) and X-ray photoelectron spectrum (XPS) comprehensive characterization results indicated the significant role of applied amendments for heavy metals transformation from the exchangeable-soluble fractions to the least available form by lowering their mobility to confirm the adsorption-based complexes, which results in the surface adsorption of heavy metals with functional groups. The electron paramagnetic resonance (EPR) results indicated the dominance of reactive oxygen species (ROS) in the intracellular formation of hydroxyl radicals (•OH) in maize plant roots and shoots. ROS (•OH) generation plays a critical influence in the interaction between the physiological processes of plants and heavy metals. Moreover, all the amendments increased maize growth and biomass production. Our study suggests that alone and combined application of SS and SSB have great potential to remediate heavy metals contaminated soil for environmental sustainability.

Aging is an important natural process affecting the physiochemical properties of biochar, while mechanistic understanding of its effect on the adsorbed heavy metals is still lacking. After adsorption of Cd²⁺ and Pb²⁺, biochars produced from wheat straw (WS) and maize straw (MS) at 300 and 500 °C (denoted as WS300, WS500, MS300, and MS500, respectively) were subjected to 60 cycles of wet–dry or freeze–thaw aging. The results showed that simulated aging treatment transformed the Cd²⁺ and Pb²⁺ adsorbed on the low-temperature biochars from the readily and potentially bioavailable fractions into the non-bioavailable one, while the fractionation of Cd²⁺ and Pb²⁺ adsorbed on WS500 and Pb²⁺ on MS500 barely changed. Spectroscopic characterization revealed that simulated aging enhanced the complexation of Cd²⁺ and precipitation of Pb²⁺ on the biochars. These findings suggest that heavy metals could be effectively immobilized on low-temperature biochars amended to contaminated soils in the long term.

Mixed pollution due to heavy metals (HMs), especially cadmium (Cd), lead (Pb), and arsenic (As), seriously endangers the safety of food produced in paddy soil. In the field experiments, foliar application of 2,3-dimercaptosuccinic acid (DMSA) at the flowering stage was found to significantly reduce the levels of Cd, Pb, total As, and inorganic As (iAs) in rice grains by 47.95%, 61.76%, 36.37%, and 51.24%, respectively, without affecting the concentration of metallonutrients, including Mn, K, Mg, Ca, Fe, and Zn. DMSA treatment significantly reduced the concentrations of Cd, Pb, and As in the panicle node, panicle neck, and rachis, while those in the flag leaves were significantly increased by up to 20.87%, 49.40%, and 32.67%, respectively. DMSA application promoted the transport of HM from roots and lower stalks to flag leaves with a maximum increase of 34.55%, 52.65%, and 46.94%, respectively, whereas inhibited the transport of HM from flag leaves to panicle, rachis, and grains. Therefore, foliar application of DMSA reduced Cd, Pb, and As accumulation in rice grains by immobilizing HMs in flag leaves. Thus, this strategy could act as a promising agronomic measure for the remediation of mixed HM contamination in paddy fields.

The traditional assessment of agricultural soil remediation technologies pay limited attention to sustainability and only considers the decrease in contaminant concentrations and cost, even though the sustainability of these technologies has been prioritized. This 3-year field study comprehensively assessed the sustainability of four commonly used agricultural soil remediation technologies in terms of metal(loid) removal efficiency, environmental merit, and cost. The farmland was contaminated by previous sewage irrigation with excessive amounts of As, Cd, and Pb. The four selected remediation technologies used were phytoextraction, intercropping of hyperaccumulators and cash crops, chemical immobilization, and turnover and attenuation (T&A). A risk reduction–environmental merit–cost model was utilized to compare these four technologies. Results showed that T&A reduced the health risks posed by excess metal(loid)s by ∼47% and yielded the highest risk reduction and lowest cost. Phytoextraction achieved the highest environmental merit because it produced the least interruption to the environment. A simplified assessment frame for soil remediation technology was established from a retrospective aspect using data from a real soil remediation project. Environmental merit is a less considered factor and more difficult to quantify than risk reduction or cost, thus requiring increased attention.

Goethite is a common iron hydroxide, which can be substituted by manganese (Mn) in the goethite structure. It is important to investigate the immobilization of uranium(VI) on Mn-substituted goethite (Mn-Goe) to understand the fate and migration of uranium in soils and sediments. In this study, the sorption of uranium(VI) by Mn-Goe was investigated as a function of pH, adsorbent dosage, contact time, and initial uranium concentration in batch experiments. Several material analysis techniques were used to characterize manganese substituted materials. Results indicated that Mn was successfully introduced into the goethite structure, the length of particles increased gradually, the surface clearly exhibited higher roughness with increasing Mn content, and that uranium(VI) sorption of synthetic Mn-Goe appeared to be higher than that of goethite. The sorption kinetics supported the results presented by the pseudo-second-order model. The sorption capacity of uranium on Mn-Goe was circa 77 mg g⁻¹ at pH = 4.0 and 25 °C. Fourier transform-infrared spectroscopy (FT-IR) analyses revealed that uranium ions were adsorbed through functional groups containing oxygen on the Mn-Goe structure. The enhancement of Mn-substitution for the uranium(VI) sorption capacity of goethite was revealed. This study suggests that goethite and Mn-Goe can both play a significant role in controlling the mobility and transport of uranium(VI) in the subsurface environment, which is helpful for material development in environmental remediation.

For further understanding leaching characteristics of heavy metals in tailings and better immobilization on heavy metals against acid rain, batch experiments were conducted. The leaching results of Cu(II), Zn(II), Cd(II) and Mn(II) can be well fit by second-order kinetics equation, and Pb(II) can be well fit by two-constant equation. The leaching intensity of heavy metals in tailings was ranged as: Mn(II)> Cu(II)> Cd(II)> Zn(II)> Pb(II). Triethylenetetramine functioned montmorillonite (TETA-Mt) was successfully synthesized and can obtain simultaneous immobilization effect compared with Mt and TETA, and immobilization rates on Cu(II), Cd(II), Mn(II) and Zn(II) can reach above 90%, the immobilization rate on Pb(II) can reach more than 75%. The mechanisms for efficient immobilization of heavy metals on TETA-Mt included buffering and adsorption abilities. The mechanism for TETA-Mt adsorption of heavy metals included physical absorption, chelation and chemical sedimentation. The results showed that TETA-Mt can be applied to effective immobilization of heavy metals in tailings and efficient remediation of acid mine drainage (AMD) in acid rain area.

There are global concerns about heavy metal (HM) contamination in soils, which in turn has produced an increased demand for soil remediation. Biochar has been widely documented to effectively immobilize metals in contaminated soils and has received increasing attention for use in soil remediation. Here, we review recent progresses in understanding metal-biochar interactions in soils, potential risks associated with biochar amendment, and application of biochar in soil remediation in China. These recent studies indicate that: (1) the remediation effect depends on the characteristics of both biochar and soil and their interactions; (2) biochar applications could decrease the mobility/bioavailability of HMs in soils and HM accumulation in plants; and (3) despite its advantages, biochar applications could pose ecological and health risks, e.g., by releasing toxic substances into soils or by inhalation of biochar dust. Research gaps still exist in the development of practical methods for preparing and applying different biochars that target specific HMs. In the future, the long term effects and security of biochar applications on soil remediation, soil organisms and plant growth need to be considered.

In this study, the feasibility of using zero-valent iron (ZVI) and Fe₃O₄-loaded biochar for Pb immobilization in contaminated sandy soil was investigated. A 180-day incubation study, combined with dry magnetic separation, chemical extraction, mineralogical characterization, and model plant (ryegrass, namely the Lilium perenne L.) growth experiment was conducted to verify the performance of these two materials. The results showed that both amendments significantly transferred the available Pb (the exchangeable and carbonates fraction) into more stable fractions (mainly Fe/Mn oxides-bound Pb), and ZVI alone showed a better performance than the magnetic biochar alone. The magnetic separation and extended X-ray absorption fine structure (EXAFS) analysis proved that Fe (oxyhydr)oxides on aged ZVI particles were the major scavengers of Pb in ZVI-amended soils. In comparison, the reduced Pb availability in magnetic biochar-amended soil could be explained by the association of Pb with Fe/Mn (oxyhydr)oxides in aged magnetic biochar, also the possible precipitation of soil Pb with soluble anions (e.g. OH⁻, PO₄³⁻, and SO₄²⁻) released from magnetic biochar. ZVI increased ryegrass production while Fe₃O₄-loaded biochar had a negative effect on the ryegrass growth. Moreover, both markedly decreased the Pb accumulation in aboveground and root tissues. The simple dry magnetic separation presents opportunities for the removal of Pb from soils, even though the efficiencies were not high (17.5% and 12.9% of total Pb from ZVI and biochar-treated soils, respectively). However, it should be noted that the ageing process easily result in the loss of magnetism of ZVI while the magnetic biochar tends to be more stable and has high retrievability during the dry magnetic separation application.