peer reviewed | Contamination of land and aquatic ecosystems with heavy metals (HMs) is a global issue having the persistent potential to damage the quality of food and water. In the present study, Tagetes erecta L. plants were used to assess their potential to uptake HMs from wastewater. Plants were grown in soil for 20 days and then transplanted in hydroponic system containing Hoagland nutrient solution. After more than 15 days of growth, plants were then subjected to wastewater from tannery and surgical industries in different concentrations ranging from 25 to 100% in combination of citric acid (5 and 10 mM). After 6 weeks of treatment, plants were collected and segmented into roots, stem, and leaves for characterizing the morphological properties including plant height, roots length, fresh and dry mass of roots, stem, and leaves. For evaluation of the effect of wastewater on the plants, photosynthetic pigments; soluble proteins; reactive oxygen species (ROS); antioxidant enzymes SOD, POD, CAT, and APX; and metal accumulation were analyzed. Application of industrial wastewater revealed a significant effect on plant morphology under wastewater treatments. Overall growth and physiological attributes of plant decreased, and metal accumulation enhanced with increasing concentration of wastewater. Similarly, the production of ROS and antioxidant enzymes were also increased. Chlorophyll, protein content, and enzyme production enhanced with CA (5 and 10 mM) mediation; however, ROS production and EL were reduced. Metals analysis showed that the maximum accumulation of Pb was in roots, while Cr and Ni in the stem which further increased under CA mediation. Overall, the metal accumulation ability was in the order of Pb > Ni > Cr under CA.

Many organic materials have been used to decrease heavy-metal bioavailability in soil via in-situ remediation due to its high efficiency and easy operation; meanwhile, cheap materials have also been pursued to decrease the cost of remediation. Agricultural wastes exhibit their potential in remediation materials due to their low cost; however, raw agricultural wastes have a low ability to immobilize heavy metals in soil. Attempts have been made to modify agricultural wastes to improve the efficiency of heavy-metal passivation. In this study, novel agricultural waste-based materials, raw sugarcane bagasse (SB), citric acid modified (SSB) and citric-acid/Fe₃O₄ modified (MSB) sugarcane bagasse at 0.5% and 1% addition rates, were compared for their effectiveness in soil Cd passivation and Cd accumulations in pakchoi plants in a 30-day pot experiment. The addition of SB did not decrease soil bioavailable Cd effectively and slightly decreased Cd accumulation in plant roots and leaves. In comparison, SSB and MSB exhibited a great potential to decrease the transformation, translocation and accumulation of Cd with the decrease being greater at 1% than 0.5% rate in the soil-pakchoi system. For example, the addition of SSB and MSB at 0.5% decreased the concentration of Cd in leaves by 10%, and 16%, and at 1% decreased the concentration by 25% and 30%, respectively. High pH and abundant functional groups of three amendments played important roles in Cd immobilization. The enhanced microbial activities might also contribute to Cd passivation. However, plant growth was decreased in the amended treatments except SSB at 0.5% rate. The results suggest that citric-acid-modified sugarcane bagasse at addition rate of 0.5% has a potential to immobilize Cd in soil and decrease Cd accumulation in edible part of pakchoi effectively without decreasing vegetable growth.

Odontarrhena muralis is one of the most promissing plant species for Ni phytomining, and soil amendments can further increase its Ni phytoextraction ability. Here we investigated whether Ni phytomining/phytoremediation using this Ni hyperaccumulator can benefit from applying citric acid to a serpentine soil that is naturally enriched in Ni (>1000 mg kg⁻¹). Synchrotron micro X-ray fluorescence (μ-SXRF) was used to image Ni and other metal distributions in whole fresh leaves of O. muralis. Leaf Ni accumulation in plants grown on citric acid-amended soil increased up to 55% while Co, Cr, Fe, Mn, and Zn concentrations were 4-, 14-, 6-, 7- and 1.3-fold higher than the control treatment. O. muralis presented high bioconcentration factors (leaf to soil concentration ratio) to Ni and Zn whereas Cr was seemingly excluded from uptake. The μ-SXRF images showed a uniform distribution of Ni, preferential localization of Co in the leaf tip, and clear concentration of Mn in the base of trichomes. The citric acid treatments strongly increased the Co fluoerescence intensity in the leaf tip and altered the spatial distribution of Mn across the leaf, but there was no difference in Ni fluorescence counts between the trichome-base region and the bulk leaf. Our data from a serpentine soil suggests that citrate treatment enhances Ni uptake, but Co is excreted from leaves even in low leaf concentrations, which can make Co phytoming using O. muralis unfeasible in natural serpentine soils.

Dibenzofuran (DBF) derivatives have caused serious environmental problems, especially those produced by paper pulp bleaching and incineration processes. Prominent for its resilient mutagenicity and toxicity, DBF poses a major challenge to human health. In the present study, a new strain of Pseudomonas aeruginosa, FA-HZ1, with high DBF-degrading activity was isolated and identified. The determined optimum conditions for cell growth of strain FA-HZ1 were a temperature of 30 °C, pH 5.0, rotation rate of 200 rpm and 0.1 mM DBF as a carbon source. The biochemical and physiological features as well as usage of different carbon sources by FA-HZ1 were studied. The new strain was positive for arginine double hydrolase, gelatinase and citric acid, while it was negative for urease and lysine decarboxylase. It could utilize citric acid as its sole carbon source, but was negative for indole and H2S production. Intermediates of DBF 1,2-dihydroxy-1,2-dihydrodibenzofuran, 1,2-dihydroxydibenzofuran, 2-hydroxy-4-(3′-oxo-3′H-benzofuran-2′-yliden)but-2-enoic acid, 2,3-dihydroxybenzofuran, 2-oxo-2-(2′-hydrophenyl)lactic acid, and 2-hydroxy-2-(2′-hydroxyphenyl)acetic acid were detected and identified through liquid chromatography-mass analyses. FA-HZ1 metabolizes DBF by both the angular and lateral dioxygenation pathways. The genomic study identified 158 genes that were involved in the catabolism of aromatic compounds. To identify the key genes responsible for DBF degradation, a proteomic study was performed. A total of 1459 proteins were identified in strain FA-HZ1, of which 100 were up-regulated and 104 were down-regulated. A novel enzyme “HZ6359 dioxygenase”, was amplified and expressed in pET-28a in E. coli BL21(DE3). The recombinant plasmid was successfully constructed, and was used for further experiments to verify its function. In addition, the strain FA-HZ1 can also degrade halogenated analogues such as 2, 8-dibromo dibenzofuran and 4-(4-bromophenyl) dibenzofuran. Undoubtedly, the isolation and characterization of new strain and the designed pathways is significant, as it could lead to the development of cost-effective and alternative remediation strategies. The degradation pathway of DBF by P. aeruginosa FA-HZ1 is a promising tool of biotechnological and environmental significance.

During their lifecycle, many engineered nanoparticles (ENPs) undergo significant transformations that may modify their toxicity, behaviour, and fate in the environment. Therefore, understanding the possible environmentally relevant transformations that ENPs may undergo as a result of their surroundings is becoming increasingly important. This work considers industrially produced ceria (CeO2) and focuses on a particle library consisting of seven zirconium-doped variants (Ce1-xZrxO2) where the Zr doping range is x = 0–1. The study assesses their potential transformation in the presence of environmentally relevant concentrations of phosphate. These ENPs have an important role in the operation of automotive catalysts and therefore may end up in the environment where transformations can take place. Samples were exposed to pH adjusted (c. 5.5) solutions made up of either 1 mM or 5 mM each of KH2PO4, citric acid and ascorbic acid and the transformed particles were characterised by means of DLS – size and zeta potential, UV/VIS, TEM, FT-IR, EDX and XRD. Exposure to the phosphate solutions resulted in chemical and physical changes in all ceria-containing samples to cerium phosphate (with the monazite structure). The transformations were dependent on time, ceria concentration in the particles (Ce:Zr ratio) and phosphate to ceria ratio. The presence of Zr within the doped samples did not inhibit these transformations, yet the pure end member ZrO2 ENPs showed no conversion to phosphate. The quite dramatic changes in size, structure and composition observed raise important questions regarding the relevant form of the materials to investigate in ecotoxicity tests, and for regulations based on one or more dimensions in the nanoscale.

Aggregation, an important environmental behavior of silver nanoparticles (AgNPs) influences their bioavailability and cytotoxicity. The work studied the influence of dissolved oxygen (DO) or the redox potential on the stability of AgNPs in aqueous environments. This study employed time-resolved dynamic light scattering (TR-DLS) to investigate the aggregation kinetics of citrate-coated AgNPs. Our results demonstrated that when DO was present, the aggregation rates became much faster (e.g., 3–8 times) than those without DO. The hydrodynamic sizes of AgNPs had a linear growth within the initial 4–6 h and after the linear growth, the hydrodynamic sizes became random for AgNPs in the presence of DO, whereas in the absence of DO the hydrodynamic sizes grew smoothly and steadily. Furthermore, the effects of primary particles sizes (20, 40, and 80 nm) and initial concentrations (300 and 600 μg/L) of AgNPs on aggregation kinetics were also investigated.

Raw biochar with high pH possibly stimulated ammonia (NH₃) volatilization in the agricultural soil. We hypothesized that the modified biochar (MBC) with low pH can synchronically decrease the NH₃ and nitrous oxide (N₂O) losses. We performed a two-year experiment to clarify how citric acid MBC influence the NH₃ volatilization and N₂O emission as well as the underlying mechanisms. Two typical paddy soils, i.e., Hydragric Anthrosol and Haplic Acrisol, receiving equal urea N with 240 kg ha⁻¹ but varied rates of MBC with 0, 5, 10, and 20 t ha⁻¹ (named Urea, Urea + MBC5, Urea + MBC10, and Urea + MBC20, respectively) were studied. The results showed that MBC-amended treatments effectively mitigated the NH₃ volatilization from Hydragric Anthrosol and Haplic Acrisol by 29.6%–57.9% and 30.5%–62.4% in 2017, and by 16.5%–21.0% and 24.5%–35.0% in 2018, respectively, compared to Urea treatment. In addition, significantly lower N₂O emissions with averaged 38.3% and 43.1% in 2017, and 51.7% and 26.7% were recorded under Hydragric Anthrosol and Haplic Acrisol, respectively, following the MBC application (P < 0.05). Increased MBC addition performed higher efficacy on mitigating NH₃ volatilization, particularly in the first rice season, while this “dosage effect” was not found for N₂O reduction. Lowered pH in overlying water, enhanced adsorption of NH₄⁺-N and its nitrification rate likely contributed to the lower NH₃ volatilization as result of MBC addition. The nirS and nosZ gene copies were not changed by MBC, while the nirK gene copies were decreased as result of MBC amendment by 8.3%–25.2% under Hydragric Anthrosol and by 21.8%–24.9% under Haplic Acrisol. Consequent lower ratio of nirK/(nirS + nosZ) explained the mitigation effect of MBC on N₂O emission. In conclusion, the present two-year study recommends that MBC applied at a low dosage can perform positive effect on controlling the nitrogenous gas pollutants from paddy soil.

In the current investigation, we presented the success of the modified hydrothermal method for synthesizing the iron-oxide nanoparticles (Fe₂O₃-NPs) efficiently. These NPs were further characterized by using different techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM) micrographs, energy-dispersive X-ray spectroscopy (EDAX)/Mapping pattern, Raman Spectroscopy Pattern, ultra violet (UV) and Photoluminescence (PL). All these analyses revealed highly pure nature of Fe₂O₃-NPs with no internal defects, and suggested its application for plant growth improvement. Therefore, we further investigated the separate as well as combined effects of the Fe₂O₃-NPs and citric acid (CA) in the alleviation of arsenic (As) toxicity in the soybean (Glycine max L.), by evaluating the different plant growth and metabolic attributes. Results of our study revealed that As-induced growth inhibition, reduction of photosynthesis, water use efficiency (WUE), and reactive oxygen species (ROS) accumulation whereas application of the Fe₂O₃-NPs and CA significantly reversed all these adverse effects in soybean plants. Moreover, the As-stress induced malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) production were partially reversed by the Fe₂O₃-NPs and CA in the As-stressed plants by 16% and 10% (MDA) and 29% and 12% (H₂O₂). This might have resulted due to the Fe₂O₃-NPs and CA induced activities of the antioxidant defense in plants. Overall, the Fe₂O₃-NPs and CA supplementation separately and in combination positively regulated the As tolerance in soybean; however, the effect of the combined application on the As tolerance was more profound relative to the individual application. These results suggested the synergetic effect of the Fe₂O₃-NPs and CA on the As-tolerance in soybean. However, in-depth mechanism underlying the defense crosstalk between the Fe₂O₃-NPs and CA needs to be further explored.

Selenium (Se) and arsenic (As) are toxic contaminants in surface water and drinking water. The human body needs little quantity of Se, but too high dose is not allowed. Metal oxides such as iron oxides were used for adsorption or co-precipitation removal of As from water. However, the regeneration and stability problems of metals oxides sorbents are unsatisfactory , and there is not enough adsorbent for Se removal from water also. We developed the acrylic amine fiber (AAF) for adsorption reomval of Se and As from water and systematically studied the influenced factors. Batch experiments were conducted for investigating the adsorption edges, while column filtration tests were employed for dynamic application edges. At neutral pH, the Langmuir isotherm fittings gave the maximum adsorption capacities of As(V), As(III), Se(VI) and Se(IV) are 270.3, 40.5, 256.4, and 158.7 mg/g, respectively. Effects of co-existing inorganic anions on As(V) and Se(VI) adsorption using AAF gave the order of PO₄³⁻ > SO₄²⁻ > NO₃⁻ > SiO₃²⁻, while different organic acids obey the order of citric acid > oxalic acid > formic acid. Fourier transform infrared analysis showed the PO₄³⁻ and SO₄²⁻ competition mechanisms are electrostatic repulsions, while the competition of organic acids derived from acid-base reaction between the carboxyl group and the amino group. Column filtration and regeneration results showed that the spent AAF can be regenerated using 0.5 mol/L HCl solution and reused with no much decrease of adsorption capacity.

Adding exogenous low-molecular weight organic acids is an effective technique to improve phytoremediation of Cd-contaminated soil and has been well documented, but how acid application rate affects remediation efficiency and its underlying limiting factors remains elusive. We investigated this using pot experiments with rapeseed (Brassica napus L.) as the model plant. Plastic pots packed with a sandy loam contaminated by Cd at 4.838 mg/kg were amended with acetic acid, oxalic acid, citric acid, malic acid and tartaric acid, respectively, at an application rate gradient varying from 0.0 to 12.0 mmol/kg. Plants in each pot were harvested after growing for five months, and we then measured the exchangeable, carbonate, Fe–Mn oxide, organic and residual Cd in the rhizosphere, as well as Cd in both roots and shoots. The results showed that all organic acids improved plant uptake of Cd and, compared with the control without acid addition, they could improve Cd uptake by more than 100%. The enhanced Cd extraction was due to the increase in exchangeable Cd in the rhizosphere. Plant Cd was weakly correlated to the amount of Cd lost from a unit volume of the rhizosphere due to root extraction (R² = 0.06), but a good negative correlation was found between them after normalizing the lost Cd by root biomass (R² = 0.36). Mass balance analysis revealed that the average Cd content in soil (rhizosphere and bulk soils combined) was much higher than the Cd content in the rhizosphere, and the improved Cd mobility after acid addition was thus due to the increased chelation. As diffusion of ligands in water is one order in magnitude smaller than diffusion of Cd ions, our results suggested that Cd migration from the bulk soil into the rhizosphere was a major factor limiting Cd phytoextraction by rapeseed after adding the exogenous organic acids.