Current research focuses more on redox of toxic Cr(VI), with less attention to Cr(III) changes in flooded soil. First, the structure of acid birnessite and cryptomelane was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and other test methods. This study investigated farmland soil in Yuxi, Yunnan Province, under flooding stress induced by the addition of two distinct concentrations of manganese oxides. Throughout the experiment, key physicochemical properties of the soil—including pH, redox potential (Eh), Cr(VI) concentration, and chromium speciation—were systematically measured and analyzed. Structural characterization demonstrated distinct morphological and surface area properties. Specifically, acid birnessite, with petal-like stacked spheres, has a specific surface area of 103.76 m²/g, while cryptomelane, strip-shaped, has an area of 95.92 m²/g. The submergence experiment yielded the following phenomena: (1) During the 60-day flooding experiment, soil amended with 0.5% or 1% acid birnessite or cryptomelane exhibited an increase in Eh compared to the control group. (2) At the end of the 60-day submergence period, the Cr(VI) concentration in the soil treated with 1% acid birnessite increased by 2.4 times compared to the control group. In addition, after 60 days, Cr(VI) concentrations in the soil exceeded 5 mg/L in soils with manganese oxide added to them. This study evaluates how manganese oxides oxidize Cr(III), aiding in assessing their environmental risks and long-term impacts on metal transformation. The findings help predict chromium behavior in farm soils and guide remediation strategies.

Effective phytoremediation of multi-metal contaminated soils requires understanding species-specific strategies that integrate rhizospheric engineering with physiological defense mechanisms. This study compared the phytoremediation responses of Medicago sativa, Bidens pilosa, and Celosia argentea under cadmium (Cd), lead (Pb), and zinc (Zn) co-contamination. B. pilosa and C. argentea exhibited higher shoot metal accumulation and translocation factors (TF > 1.5), attributed to efficient root-to-shoot transport and favorable rhizospheric conditions, including moderate pH and elevated β-glucosidase, acid phosphatase, and alkaline phosphatase activities. In contrast, M. sativa showed limited shoot accumulation due to sustained O₂ release and rhizospheric alkalization, which reduced metal solubility and availability. At the physiological level, all species experienced metal-induced oxidative stress, evidenced by elevated root malondialdehyde (MDA) concentrations. However, shoot responses diverged markedly: B. pilosa maintained oxidative homeostasis with stable shoot MDA levels, while C. argentea and M. sativa showed progressive oxidative burden. Metallothionein-based detoxification strategies also varied tissue and species specifically. Root tissues generally upregulated glutathione (GSH) and phytochelatin (PC) biosynthesis, yet foliar responses differed: GSH predominated in B. pilosa leaves, whereas PCs dominated in C. argentea and M. sativa. Antioxidant enzyme profiles further distinguished species strategies: B. pilosa and C. argentea showed coordinated catalase (CAT) and peroxidase (POD) upregulation in shoots, while M. sativa exhibited enzymatic imbalances indicative of compromised oxidative defense. These findings establish a mechanistic framework linking rhizosphere modification, metal translocation efficiency, and antioxidative capacity, providing critical insights for rational plant selection in tailored phytoremediation programs targeting multi-metal contaminated environments.

This study investigated the potential of microbial-assisted phytoremediation using Eichhornia crassipes (water hyacinth) to reduce heavy metal and salinity pollution in produced water collected from Aadi Oil Field in Gujar Khan, Pakistan. Produced water was analyzed for physicochemical parameters and heavy metal content using Inductively Coupled Plasma–Optical Emission Spectrometry (ICP-OES) to establish baseline data. E. crassipes plants augmented with indigenous, contaminant-tolerant microbial isolates were employed in a 15-day laboratory experiment. The results showed a resilient growth response, with plant height increasing to approximately 11–15 cm and root length extending up to 10–13 cm across treatments. Biomass also improved, with wet weights reaching 21–24 g from an initial 20 g. The treatment effectively reduced key physicochemical parameters: pH was stabilized from an initial alkaline value of 9.14 to near-neutral values (7.0–7.5), and total dissolved solids (TDSs) were reduced by approximately 50%. Heavy metal removal rates varied, with the highest efficiency of 79.2% for Silver (Ag) and the lowest (18.5%) for Mercury (Hg) This study demonstrates that E. crassipes actively participated in phytoremediation by absorbing and accumulating heavy metals and reducing salinity. The association with contaminant-tolerant microbes appeared to enhance the plant’s tolerance and overall treatment efficacy, indicating that plant–microbe interactions offer a sustainable strategy for the treatment of produced water.