The concurrent presence of arsenic (As) and cadmium (Cd) contamination in soil is widespread and severe, highlighting the need for remediation. However, remediating As and Cd co-contaminated soils is more complex than remediating soils contaminated with a single heavy metal due to the opposite properties of As and Cd in soil. Thus, the different forms of As and Cd in co-contaminated soils and their transformation rules have been systematically reviewed in this paper. Simultaneously, hyperaccumulators and immobilization amendments used in the remediation of As–Cd co-contaminated soil were reviewed. Moreover, the mechanisms of phytoremediation and chemical immobilization techniques in the treatment of As and Cd co-contaminated soil and the remediation effects were expounded in detail. To promote the development of ecological civilization, this paper proposes further remediation strategies and guidance for the remediation of As–Cd co-contaminated soil.

Revegetation is urgently needed for the ecological restoration of lead-zinc tailings ponds. However, the harsh properties of lead-zinc tailings-including poor aggregation, nutrient deficiency, and low water retention capacity-severely hinder plant colonization and growth. This study effectively improved the physical structure and nutrient status of lead-zinc tailings using electrolytic manganese residue (EMR), a solid waste characterized by strong cohesiveness and high ammonium nitrogen content. Moreover, lead-zinc tailings amended with 1 % EMR had low leaching of toxic elements, indicating low environmental risk. X-ray computed tomography and nuclear magnetic resonance analyses revealed that EMR amendment significantly reduced total porosity of lead-zinc tailings while enhancing their water retention capacity (p < 0.05). The improvements likely resulted from the filling of interparticle voids by EMR clay particles and the formation of Ca₂(AlFe)(Si₂O₇)(SiO₄)O(OH), a hydrated gel similar to the hydrated calcium silicate gel. The hydrated gel formed by the reaction between Ca²⁺ from EMR and silicates on tailing surfaces may increase interparticle cohesion. A pot experiment showed that 1 % EMR amendment significantly promoted ryegrass growth in tailings, increasing fresh biomass by 13.7 % (p < 0.05). Overall, these findings indicate that EMR amendment is a promising strategy to enhance plant colonization and growth in the revegetation of lead-zinc tailings ponds.

Revegetation is recognized as one of the most effective strategies for the ecological restoration of tailings ponds. However, a systematic understanding of how both plant colonization time and plant type shape the microbial functional potential for coupled biogeochemical cycles remains insufficient. Here, we collected 24 samples comprising bare tailings and rhizosphere tailings from four dominant plant species (Miscanthus sinensis, Pinus massoniana, Lespedeza bicolor, Patrinia villosa) colonizing a lead–zinc mine tailings pond to investigate the effects of revegetation on the contents of carbon (C), nitrogen (N) and phosphorus (P) and microbial functional genes related to their cycles. The results showed that revegetation significantly increased the C, N and P contents in the rhizosphere tailings (p < 0.05), and these increased with plant colonization time. Compared with the bare tailings, the contents of C, N and P increased by 1.10 to 4.12 times, 1.06 to 4.84 times and 0.63 to 7.30 times, respectively. Furthermore, revegetation significantly enriches microbial C-, N- and P-cycling genes. The abundance of C fixation, organic degradation, nitrate reduction and organic P mineralization genes in tailings significantly increased after revegetation. Additionally, revegetation substantially enhanced the density, links and average degree of the network of microbial C-, N- and P-cycling genes. Pathway analysis using partial least squares path modeling indicated that revegetation positively affected microbial C-, N- and P-cycling genes, which were regulated by plant type and colonization time. Collectively, these findings suggest that revegetation can substantially enhance the biogeochemical cycling functions of microorganisms in tailings while also promoting their coupling.

The efficiency of phytoextraction highly depends on the growth of accumulating plants. To enhance cadmium (Cd) phytoextraction, 2,4-dichlorophenoxyacetic acid (2,4-D), a synthetic auxin, was applied foliarly to Celosia argentea Linn. The results showed that foliar application of 2,4-D induced hormesis effect which enhanced the growth of C. argentea. At an application rate of 1.0 mg/L, 2,4-D treatment most significantly increased the growth of C. argentea (p < 0.05). The increase in growth may be linked to changes in endogenous hormone balance induced by 2,4-D. After treatment with 1.0 mg/L 2,4-D, a significant decrease in abscisic acid (ABA), Jasmonic acid (JA) and trans-Zeatin-riboside (tZR) levels and a significant increase in indole-3-butyric acid (IBA), 3-Indolepropionic acid (IPA) and 6-Benzylaminopurine (BAP) levels were observed in C. argentea. Specifically, the reductions in ABA and JA may be correlated with a decrease in stomatal closure on the leaf surface and increases in IPA and BAP may support root and plant growth development. Furthermore, a low concentration of 2,4-D significantly increased the uptake and translocation of Cd in C. argentea. At 1.0 mg/L treatment, Cd phytoextraction efficiency increased by 108 %. Residues of 2,4-D in soil were below 0.01 mg/kg after 7 days when applied foliarly at 1.0 mg/L. These findings suggest that foliar application of low-dose 2,4-D is a simple, efficient, and environmentally safe method to improve Cd phytoextraction efficiency in contaminated soils.