Remediation of cadmium and lead contaminated soils using Fe-OM based materials.

Iron oxide-lignin composites (GLS) were prepared based on the significant role of Fe-OM in the environmental behaviour of heavy metals and lignin binding with iron oxide preferentially in soil. GLS was applied in Cd/Pb immobilization and the stability under acid rain was investigated. The results show that the iron oxide appeared weakly crystalline or amorphous similar to 2-line ferrihydrite after the addition of lignin. Agglomerates of nanoparticles with higher adsorption capacity were observed for GLS. The mobility factor (MF) of Cd/Pb in the soil decreased rapidly after adding GLS. At the 3% dosage, the MF of Cd and Pb in the soil was decreased by 58.94% and 78.15% respectively, which was approximately 5 times that of goethite (GE). The mobile and exchangeable Cd/Pb were converted to organic, amorphous Fe oxide-bound and residue fractions. Under acid rain conditions, MF continues to decline for the GLS group, increasing the organic and amorphous Fe oxide-bound fractions, while for control group (CK) and GE, the trend was the opposite. Lignin could inhibit iron oxide dissolution and stabilize the combination of Cd/Pb and iron oxides in soil. The better stability performance of GLS for Cd/Pb may be related to the higher adsorption capacity and microstructural difference after iron oxide combined with lignin.

Cd and Pb immobilisation with iron oxide/lignin composite and the bacterial community response in soil.

Iron oxide is a natural mineral that generally exists in the form of iron oxide-organic complexes (Fe-OM) in soil. Lignin is a naturally occurring polymer that is considered to be an important part of soil carbon cycling. In this study we prepared a composite material (MGE) with iron oxide and lignin based on the Fe-OM present in the soil. MGE was then applied to remediate Cd and Pb in contaminated soil. The results show that DTPA-Cd and DTPA-Pb levels were reduced by 58.87% and 78.09%, respectively. The bacterial community diversity index decreased in the iron oxide (GE) group, but a slight increase was observed in the MGE group. In terms of species composition in the MGE group, the abundance of Proteobacteria, Gemmatimonadota and Acidobacteriota increased, while the abundance of Bacteroidota, Actinobacteriota and Firmicutes decreased. The outcome in the GE group was the opposite. In the MGE group, HCl-Fe2+, HCl-Fe3+, and pH were significantly higher than in the other groups, indicating that MGE stimulated the growth of iron-reducing bacteria (FeRB) and promoted iron redox reactions. Iron oxide could be reduced to Fe2+ due to the activity of FeRB, and then Fe2+ would be oxidised and hydrolysed, which led to an increase in soil pH. Secondary minerals were formed during this process. With the oxidation of Fe2+ and the formation of secondary minerals, Cd and Pb could be stabilised in the oxides and were not easily released through a co-precipitation mechanism.

Effect of lignosulfonate on the adsorption performance of hematite for Cd(II).

Lignin is a precursor of humus in soil and sediment. Lignin can be separated from vascular plants in the form of lignosulfonate via pulping processes. On the other hand, composites of iron oxide and organic matter can adsorb heavy metals, and thus influence the migration of these heavy metals in the environment. In this paper, a hematite/lignosulfonate composite (HLS) was prepared via coprecipitation to compare the adsorption performance of hematite (α-Fe2O3) toward Cd(II) before and after the incorporation of lignosulfonate (LS). The HLS is found to exhibit a weakly crystalline structure and possess a large number of nanoscale particles. Specific surface area of HLS (291.97 m2/g) is about 11 times that of α-Fe2O3, and the pore volume of HLS (0.22 cm3/g) is twice that of α-Fe2O3. The adsorption of Cd(II) is well illustrated by the pseudo-second-order adsorption kinetics and the initial adsorption rate (h) of HLS is 13.83 times that of α-Fe2O3. The maximum adsorption capacities are significantly improved from 4.89-6.35 mg/g (α-Fe2O3) to 39.03-53.65 mg/g (HLS). A greater affinity and more favorable association between Cd(II) and HLS is observed via fitting models. The incorporation of LS provides HLS with significantly better adsorption properties toward Cd(II) than α-Fe2O3, as is further confirmed by FT-IR and XPS characterization. Fe-O-O-H and Fe-O-H structures as well as more hydroxyl groups are observed, which promote the adsorption performance since the process are mainly influenced by complexation via coordination bonds.

[Immobilization Impact of Goethite-Fulvic Acid Composites on Pb-Cd Contaminated Soil].

The Pearl River Delta region is the major economic zone of the Greater Bay Area:it presents a large number of industrial-mining enterprises and is interested by severe heavy metal pollution (mainly caused by Pb and Cd). The research and development of safe and efficient heavy metal remediation materials and technologies is fundamental in order to guarantee regional environmental quality and habitat safety. Goethite-fulvic acid composites were prepared using goethite minerals present in the red soils of the Pearl River Delta region by passivation repair, and were applied to immobilize Pb and Cd in a co-contaminated soil. The results showed that a higher ratio of fulvic acid in the composites enhanced the immobilization effect on Pb and Cd:the immobilization efficiency (IE) of Pb and Cd increased with the passivation time, and then stabilized. Additionally, the exchangeable and carbonate-bound fractions of Pb and Cd in the soil decreased, while the Fe-Mn oxide-bound and residual fractions increased. The cation exchange capacity (CEC), as well as the available K and NH4+-N in the soil increased after the application of the composite materials. We conclude that goethite-fulvic acid composites can be effectively applied to the remediation of Pb-Cd contaminated soil.

Characterization of goethite-fulvic acid composites and their impact on the immobility of Pb/Cd in soil.

The coprecipitation of organic matter (OM) and minerals is a relatively common phenomenon in soil, and it has a significant influence on the surface properties and reactivity of minerals. In turn, the fate of pollutants in soil is greatly affected by the organic-mineral composites. In this study, goethite-fulvic acid (Ge-FA) composites with varying FA mass ratios in the range of 0-15% were synthesized by coprecipitation. The sample properties were studied using XRD, FTIR, SEM-EDS and N2 gas adsorption techniques. The influence of Ge-FA on the mobility of Pb/Cd in soil was investigated. The crystal forms of Ge-FA changed from goethite (FA≤4%) to hematite (FA≥5%), and the FA affected the FeO bond vibrations. These results demonstrated that FA was successfully introduced into the iron oxide. Ge-FA changed from a filamental morphology to an aggregate as the FA ratio increased. The coprecipitation resulted in blockages of iron oxides, thereby decreasing the specific surface area and pore volume. The adsorption amount of Pb(II) on Ge-FA increased as the FA ratio increased, but no significant change was observed for Cd(II). With the application of Ge-FA, the exchangeable concentrations of Pb and Cd in contaminated soil decreased by 42.4%-93.6% and 15.8%-43.7%, respectively. The exchangeable and carbonate bound fractions of Pb and Cd decreased and were transformed into the FeMn bound and residual fractions.

Potassium lignosulfonate as a washing agent for remediating lead and copper co-contaminated soils.

Lignin is a renewable aromatic polymer which is present in large quantities in the cell walls of terrestrial plants and is the main binding agent for fibrous plant components. Potassium lignosulfonate (KLS), as a by-product of pulping processes, can be applied to agricultural fields as a soil conditioner and chelate fertilizer. In this study, based on its solubility and complexing ability, batch washing and column leaching was explored to evaluate the potential application of KLS in the washing remediation of soil contaminated with lead and copper. Under optimum conditions KLS concentration of 8%, pH of 5.24 and 6 h duration, the removal ratios for the simulated and mining area soils in the batch experiment were 67.40% and 52.87% respectively, for Pb and 73.42% and 55.20% respectively, for Cu. In the column leaching experiment, the removal efficiencies of Pb and Cu increased with solution volume and that the removal ratios of the simulated and mining area soils were 36.46% and 20.31%, respectively, for Pb and 39.74% and 22.76%, respectively, for Cu. KLS can reduce the ratio of acid-soluble and reducible fractions, which may have the most potential hazardous and poisonous for plants. Cation exchange capacity (CEC) and pH of soil were all stable after washing. However, the organic matter and available K, N, and P in the treated soil, which are important factors for plant growth increased significantly. This study showed that KLS can remove Pb and Cu from soil, while improve nutrient (ammonium nitrogen, available phosphorus and available potassium) levels of soil.