In agriculture, wastewater is used as an alternative source to meet the water and nutritional requirements of plants. However, long-term application of wastewater may degrade soil attributes. This study aimed to evaluate the soil physical quality of Oxisol fertigated with treated sewage effluent (TSE). The experiment was conducted in an area under TSE application for 4 years in Oxisol (625 g kg⁻¹ clay) cultivated with Urochloa brizantha. The treatments consisted of six levels of TSE in irrigation depth, 0%, 11%, 31%, 60%, 87%, and 100%, with four repetitions. Undisturbed and disturbed soils samples were collected in three layers (0.00–0.10 m; 0.10–0.20 m, and 0.20–0.30 m). Aggregation, porosity and water infiltration attributes were evaluated. This work concludes a long-term study on the effects of TSE application on soil properties and on the Urochloa brizantha crop. In other works, carried out in the experimental area of the present study, it was found that TSE fertigation increases the yield and quality of Urochloa brizantha, increases soil fertility and does not lead to soil heavy metal contamination. We note the TSE fertigation does not change the aggregation, porosity, water infiltration rate and organic carbon content in the soil. Irrigation with TSE is recommended in areas with clayey soil and those cultivated with perennial grasses as it does not cause any damage to the physical quality of the soil. Thus, the TSE fertigation can be used on many crops as a source of water and nutrients, reducing the environmental contamination potential.

Ion sorption on soil and sediment has been reported to be potentially affected by bacteria which may interact both physically and chemically with solid surfaces. However, whether and how bacteria affect the sorption of inorganic phosphate (P) on soil colloids remains poorly known. Here, we comparably investigated the P sorption on four soil colloids (three highly weathered soils including two Oxisols and one Ultisol and one weakly weathered soil Alfisol) and their complexes with Bacillus subtilis and Pseudomonas fluorescens. Batch experiments showed a notable reduction in P sorption on the colloids of highly weathered soils by the two bacteria at varying P concentrations and pHs; whereas that on the colloids of Alfisol appeared to be unaffected by the bacteria. The inhibitory effect was confirmed by both greater decline in P sorption at higher bacteria dosages and the ability of the bacteria to desorb P pre-adsorbed on the colloids. Further evidence was given by isothermal titration calorimetric experiments which revealed an alteration in enthalpy change caused by the bacteria for P sorption on Oxisol but not for that on Alfisol. The B. subtilis was more efficient in suppressing P sorption than the P. fluorescens, indicating a dependence of the inhibition on bacterium type. After association with bacteria, zeta potentials of the soil colloids decreased considerably. The decrease positively correlated with the decline in P sorption, regardless of soil and bacterium types, demonstrating that the increment in negative charges of soil colloids by bacteria probably contributed to the inhibition. In addition, scanning electron microscopic observation and the Derjaguin–Landau–Verwey–Overbeek theory prediction suggested appreciable physical and chemical interactions between the bacteria and the highly weathered soil colloids, which might be another contributor to the inhibition. These findings expand our understandings on how bacteria mobilize legacy P in soils and sediments.

Extracellular polymeric substances (EPS) found in soils can reduce the mobility of heavy metals through the use of both electrostatic and non-electrostatic mechanisms. Their effects vary from one soil type to another. The influence of EPS from Escherichia coli on the adsorption behaviors of Cu(II) and Cd(II) by two bulk variable charge soils, Oxisol and Ultisol, was studied at constant and varied pH, and the results were compared to a constant charge Alfisol. The maximum adsorption capacities of the soils were significantly (P < 0.05) enhanced in the presence of EPS, with Cu(II) adsorption being greater. Interaction of EPS with soils made the soil surface charge more negative by neutralizing positive charges and shifting the zeta potentials in a negative direction: from −18.6 to −26.4 mV for Alfisol, +5.1 to −22.2 mV for Oxisol, and +0.3 to −28.0 mV for Ultisol at pH 5.0. The adsorption data fitted both the Freundlich and Langmuir isotherms well. Preadsorbed Cd(II) was more easily desorbed by KNO₃ than preadsorbed Cu(II) from both the control and EPS treated soils. The adsorption of both metals was governed by electrostatic and non-electrostatic mechanisms, although more Cu(II) was adsorbed through the non-electrostatic mechanism. The information obtained in this study will improve our understanding of the mechanisms involved in reducing heavy metals mobility in variable charge soils and hence, their bioavailability.

The aim of this study was to assess the effect of temperature on the toxicity of fipronil toward earthworms (Eisenia andrei) in two Brazilian soils (Entisol and Oxisol) with contrasting textures. In the case of Entisol, the influence of soil moisture content on toxicity was also investigated. Earthworms were exposed for 56 days to soils spiked with increasing concentrations of fipronil (8.95, 19.48, 38.22, 155.61, and 237.81 mg kg⁻¹ for Entisol; 12.99, 27.94, 48.42, 204.67, and 374.29 mg kg⁻¹ for Oxisol) under scenarios with different combinations of temperature (20, 25 and 27 °C) and soil moisture content (60 and 30% of water holding capacity (WHC) for Entisol and 60% WHC for Oxisol). The number of juveniles produced was taken as the endpoint, and a risk assessment was performed based on the hazard quotient (HQ). In Entisol, at 60% WHC the fipronil toxicity decreased at 27 °C compared with the other temperatures tested (EC₅₀ = 52.58, 48.48, and 110 mg kg⁻¹ for 20, 25, and 27 °C, respectively). In the case of Oxisol at 60% WHC, the fipronil toxicity increased at 27 °C compared with other temperatures (EC₅₀ = 277.57, 312.87, and 39.89 mg kg⁻¹ at 20, 25, and 27 °C, respectively). An increase in fipronil toxicity was also observed with a decrease in soil moisture content in Entisol at 27 °C (EC₅₀ = 27.95 and 110 mg kg⁻¹ for 30% and 60% WHC, respectively). The risk of fipronil was only significant at 27 °C in Entisol and Oxisol with water contents of 30% and 60% WHC, respectively, revealing that higher temperatures are able to increase the risk of fipronil toxicity toward earthworms depending on soil type and soil moisture content. The results reported herein show that soil properties associated with climatic shifts could enhance the ecotoxicological effects and risk of fipronil for earthworms, depending on the type of soil.

We evaluated the toxicity and risk (via toxicity exposure ratio approach — TER) of the insecticide fipronil to collembolan’s growth and reproduction in three tropical soils, under increasing atmospheric temperatures. Chronic toxicity tests were performed with Folsomia candida in tropical artificial soil (TAS), oxisol, and entisol spiked with increasing concentrations of fipronil, at three room temperature scenarios: a standard (20 ± 2 °C), a tropical condition (25 ± 2 °C) and a global warming simulation (27 ± 2 °C). Temperatures influenced the fipronil effects on the species reproduction differently between soil types. In TAS and oxisol the highest toxicities (EC₅₀-based) were found at 27 °C (EC₅₀ TAS = 0.81, 0.70, 0.31 mg kg⁻¹; EC₅₀ OXISOL = 0.52, 0.54, 0.40 mg kg⁻¹; at 20, 25, and 27 °C, respectively). In entisol, the toxicity at 27 °C was lower compared to 25 and 20 °C (EC₅₀ ENTISOL = 0.33, 0.24, 0.12 mg kg⁻¹, respectively). Fipronil concentrations also increased the proportion of small juveniles (growth reduction) in all tested soils. However, this effect was greater (EC₁₀-based) at higher temperatures (25 and/or 27 °C), regardless of the soil type. TER approach revealed a significant risk of fipronil in entisol, regardless of the tested temperature, while in other soils the risk was found significant only at the higher temperatures (25 and 27 °C for TAS, and 27 °C for oxisol). These results indicate that exposures to fipronil at high temperatures (e.g., those resulting from climate change) can threaten F. candida populations, depending on the soil type.

The tolerance of Mentha crispa L. (garden mint) cultivated in cadmium-contaminated oxisol for 120 days was analyzed using plant growth variables such as height, the number of leaves and shoots in different Cd exposure periods, as well as assessing the metal concentration absorbed and accumulated in the plant parts (root, stem, and leaves). The maximum adsorption capacity was estimated at 9220 mg kg⁻¹ and used as a reference to establish the different Cd concentrations to be applied in the soil. M. crispa showed tolerance and revealed a reduction of height, the number of leaves and shoots, root development, and secondary toxicity signs such as chlorosis and leaf wilting. Comparing to the stems and leaves, Cd was retained mainly in the roots. PERMANOVA showed that plant growth variables and Cd concentrations in the plant’s part were affected by the Cd exposure time. The canonical discriminant analysis demonstrated height as the most affected variable until 45 growing days, and different responses were observed after 75 days. However, the number of shoots was the variable most affected by higher Cd concentrations. The bioaccumulation and translocation factors for all treatments were lower than one, indicating that M. crispa can be considered as an excluder plant and applied for a phytostabilization strategy.

Due to its unique properties, the potential application of graphene oxide (GO) in treating environmental pollution has attracted wide attention. In this study, the UV-light catalyzed photoreduction of Cr(VI) by GO was assessed as well as its adsorption toward Cr(VI), and FTIR and XPS techniques were adopted to reveal the underlying mechanisms. The surfaces of GO were negatively charged across the pH range examined. Therefore, the increase in pH resulted in the decrease in Cr(VI) adsorption due to the enhancement in repulsion between Cr(VI) and GO surfaces. The kinetic studies showed that the Cr(VI) adsorption proceeded quickly during the 0–24 h stage, followed by a slow process until to the end of reaction (96 h). Additionally, the kinetic data could be properly described with the pseudo-first-order rate equation (R² = 0.9754). With the UV-light irradiation, Cr(VI) reduction in the presence of 0.5 g L⁻¹ GO was observed with the concentration of Cr(VI) decreased from 0.1 mM to zero within 12 h at pH 3.0, while which would be suppressed as the pH increased. The addition of EDTA could enhance the photocatalytic Cr(VI) reduction due to the consumption of the photogenerated holes (h⁺), leaving more Cr(III) species present in solution. The generation of h⁺ was further confirmed by the complete photodegradation of 4-CP during 48 h. Moreover, the changes in FTIR and XPS spectrum of GO before and after reaction indicated the oxidization of epoxy and hydroxyl groups by holes or reduction by electrons was involved in the photoreaction. The photoreduction of Cr(VI) could was also observed in an oxisol with the existence of GO, with the disappearance of 0.1 mM of aqueous Cr(VI) at pH 4.40 after 36 h. The results above could enhance our understanding on the essence of photoreactivity of GO, and indicated that the potential release of GO into soil environments would be helpful to eliminate the risk posed by Cr(VI) through the UV-light irradiated photocatalytic reduction.

Herbicides with long residual period may increase the risk of environmental contamination. Adequate management of forage can reduce the half-life of the picloram, one of the most herbicides used in weed control. This study aims to determine the half-life of picloram, using high-performance liquid chromatography in a cultivated soil with Brachiaria brizantha trimmed or not. Brachiaria brizantha was cultivated in 60 pots filled with samples of oxisol, and 30 others were kept uncultivated with this forage. This plant was cut off close to the ground, after 60 days of emergency on 30 vessels. Picloram was applied in all of the plots. Soil samples were collected at 2, 16, 30, 44, 58, 72, 86, 120, 150, and 180 days after the application of this herbicide. These samples were air-dried and stored at − 20 °C. Picloram was extracted by HPLC/UV-Vis detector. Half-life of this herbicide was calculated using kinetics models. The mere presence of roots in treatment with signalgrass cutoff did not reduce the concentrations of this herbicide, except when the emergence of new leaves occurred. The absence of B. brizantha cultivation in areas with application of picloram increases the risk of environmental contamination and successive crops due to the half-life of this herbicide. Brachiaria brizantha reduced half-life picloram and environmental risk in pastures. The validation method is suitable for determining picloram in low concentrations in soil.

The potential application of carbon nanotubes (CNTs) in waste water treatment and their effect on the fate of heavy metals in the environments have attracted wide attention. However, the influence of CNTs on the reduction of Cr(VI) to Cr(III) in soils remains unknown. In this study, Cr(VI) adsorption by carboxylated or hydroxylated multi-walled carbon nanotubes (MWCNT-COOH or MWCNT-OH) was investigated together with their catalytic effect on Cr(VI) reduction by citric acid. Across the initial concentration range examined (5–60 mg/L), the adsorption capacity of Cr(VI) by MWCNT-COOH and MWCNT-OH (pH 5.0) could reach to 8.09 and 7.85 mg/g, respectively. With the decrease in pH, the Cr(VI) adsorption by both MWCNTs increased, while their difference in adsorption capacity became more pronounced, evidenced by that the percentage of Cr(VI) adsorbed by MWCNT-COOH can be 1.3-fold higher than that of MWCNT-OH at a pH of 3.2. The Cr(VI) adsorption kinetics could be well described by pseudo-second-order (R² > 0.95) and intra-particle diffusion models (R² > 0.98). MWCNT-OH or MWCNT-COOH could accelerate the reduction of 0.1 mM Cr(VI) by 1.0 mM citric acid, with the first-order rate constant of 0.0325 and 0.0147 h⁻¹, respectively. This finding was explained as that the reactivity of citric acid might be enhanced with its adsorption on the MWCNT surfaces. The catalysis of the functionalized CNTs on the Cr(VI) reduction was inhibited as the pH increased. The addition of MWCNTs to an oxisol can enhance the Cr(VI) reduction because the final concentration of aqueous Cr(III), compared with that without addition of MWCNTs, increased from 20.7 to 32.6 μM. Meanwhile, re-adsorption of aqueous Cr(III) onto the solid surfaces was also observed. The results above are important for understanding on the effect of CNTs on the fate of Cr(VI) and how they can be used to remediate Cr(VI)-polluted soils.

Although several grasses have been evaluated for cadmium (Cd) phytoextraction, there are few studies assessing how Cd is accumulated and distributed in the tissues of Panicum maximum grown in mildly spiked soils. The evaluation of tillering, nutritional status and biomass yield of this grass, mainly along successive shoot regrowths, is not well studied so far. Thus, P. maximum Jacq. cv. Massai was grown for two periods in an Oxisol presenting bioavailable Cd concentrations varying from 0.04 (control) to 10.91 mg kg⁻¹ soil. Biomass yield of leaves and stems’ growth has decreased under the highest Cd exposure, but it did not occur in the regrowth period, indicating that Cd-induced toxicity is stronger in the early stages of development of P. maximum. The tillering was not compromised even the basal node presenting Cd concentrations higher than 100 mg kg⁻¹ DW. We identified a restriction on Cd transport upwards from basal node, which was the main localization of Cd accumulation. Apparently, P, K, Mg, S and Cu are involved in processes that restrict Cd translocation and confer high tolerance to Cd in P. maximum. The Cd-induced nutritional disorders did not negatively correlate with factors used to calculate phytoextraction efficiency. However, the nutritional adjustments of P. maximum to cope with Cd stress restricted the upward Cd transport, which decreased the phytoextraction efficiency from the available Cd concentration of 5.93 mg kg⁻¹ soil.