Xueyuan Gu

Adsorption of tetracycline onto goethite in the presence of metal cations and humic substances

Adsorption of tetracycline, one of the most widely used antibiotics, onto goethite was studied as a function of pH, metal cations, and humic acid (HA) over a pH range 3-10. Five background electrolyte cations (Li(+), Na(+), K(+), Ca(2+), and Mg(2+)) with a concentration of 0.01 M showed little effect on the tetracycline adsorption at the studied pH range. While the divalent heavy metal cation, Cu(2+), could significantly enhance the adsorption and higher concentration of Cu(2+), stronger adsorption was found. The results indicated that different adsorption mechanisms might be involved for the two types of cations. Background electrolyte cations hardly interfere with the interaction between tetracycline and goethite surfaces because they only form weak outer-sphere surface complexes. On the contrary, Cu(2+) could enhance the adsorption via acting as a bridge ion to form goethite-Cu(2+)-tetracycline surface complex because Cu(2+) could form strong and specific inner-sphere surface complexes. HA showed different effect on the tetracycline sorption under different pH condition. The presence of HA increased tetracycline sorption dramatically under acidic condition. Results indicated that heavy metal cations and soil organic matters have great effects on the tetracycline mobility in the soil environment and eventually affect its exposure concentration and toxicity to organisms.

Insights into tetracycline adsorption onto goethite: Experiments and modeling

The surface adsorption behavior of tetracycline (TC), a zwitterionic antibiotic, to goethite was investigated as a function of pH, ionic strength and TC concentration using batch adsorption experiments and structural information was derived from attenuated total reflectance Fourier transform infrared spectrum observations. The spectroscopic results suggested that the tricarbonylamide group and the phenolic diketone group of the TC molecule were involved in interacting with the goethite surface depending on the pH level. A charge distribution surface complexation model was developed to describe the macroscopic adsorption trends. Two inner-sphere surface complexation species could successfully describe observed adsorption trends: under acidic condition TC may interact with the surface of goethite, forming a monodentate complex through the tricarbonylamide group, while under alkaline condition forming a more stable bidentate complex via the tricarbonylamide and phenolic diketone groups. The model could well predict the adsorption behavior of TC under a relatively wide range of pH, ionic strength and surface coverage. However, since the model did not fully consider the molecular size of TC, the model might overestimate the adsorption when TC surface coverage is higher than 1.42 μmol m(-2).

Surface complexation modeling of Cr(VI) adsorption at the goethite-water interface.

In this study, a charge distribution multisite surface complexation model (CD-MUSIC) for adsorption of chromate onto goethite was carefully developed. The adsorption of Cr(VI) on goethite was firstly investigated as a function of pH, ionic strength and Cr(VI) concentration. Results showed that an inner-sphere complexation mechanism was involved because the retention of Cr(VI) was little influenced by ionic strength. Then two surface species: a bidentate complex (≡Fe2O2CrOOH) and a monodentate complex (≡FeOCrO3(-3/2)), which is constrained by prior spectroscopic evidence were proposed to fit the macroscopic adsorption data. Modeling results showed that the bidentate complex was found to be the dominant species at low pH, whereas, with increasing pH, monodentate species became more pronounced. The model was then verified by prediction of competitive adsorption of chromate and phosphate at various ratios and ionic strengths. The model successfully predicted the inhibition of chromate with the presence of phosphate, suggesting phosphate has higher affinity to goethite surface than Cr(VI). Results showed that the model developed in this study for Cr(VI) onto goethite was applicable for various conditions. It is a useful supplement for the surface complexation model database for oxyanions onto goethite surfaces.

Effects of metal cations and fulvic acid on the adsorption of ciprofloxacin onto goethite

Ciprofloxacin (CIP) can be strongly adsorbed by ferric oxides, but some influencing factors, such as multivalent cations and soil organic matter, have not been evaluated extensively. In this study, the interaction between CIP and four divalent metals (Ca, Cd, Cu, and Pb) was investigated using potentiometric titration and the results indicated that CIP can bind to the divalent metals in the following affinity order: Cu(II) > Pb(II) > Cd(II) > Ca(II). The effects of metals and fulvic acid (FA) on the adsorption behavior of CIP onto goethite surfaces were also examined using batch experiments. It was found that metal cations enhanced the CIP retention on goethite surfaces in the same order as the affinity order with CIP, indicating that metals likely increased CIP retention through cation bridging. FA was found to promote CIP sorption rather than compete with it, and the coexistence of FA and Cu(II) in the system exhibited an addictive effect with CIP sorption, indicating that they might influence the sorption separately under the studied loading condition. Taken together, our results suggested that the coexistence of divalent cations or soil organic matter will enhance CIP sorption on goethite surfaces, hence reducing its mobility and bioavailability in the environment.

Free atmospheric phosphine concentrations and fluxes in different wetland ecosystems, China

Atmospheric phosphine (PH(3)) fluxes from typical types of wetlands and PH(3) concentrations in adjacent atmospheric air were measured. The seasonal distribution of PH(3) in marsh and paddy fields were observed. Positive PH(3) fluxes are significantly related to high air temperature (summer season) and increased vegetation. It is concluded that vegetation speeds up the liberation of PH(3) from soils, while water coverage might function as a diffusion barrier from soils or sediments to the atmosphere. The concentrations of atmospheric PH(3) (ng m(-3)) above different wetlands decrease in the order of paddy fields (51.8 ± 3.1) > marsh (46.5 ± 20.5) > lake (37.0 ± 22.7) > coastal wetland (1.71 ± 0.73). Highest atmospheric PH(3) levels in marsh are found in summer. In paddy fields, atmospheric PH(3) concentrations in flourishing stages are higher than those in slowly growing stages.

Oxidative stress responses and insights into the sensitivity of the earthworms Metaphire guillelmi and Eisenia fetida to soil cadmium.

Soil toxicological tests are commonly performed using Eisenia fetida as the standard earthworm species, but it is tolerant to a wide range of pollutants. Therefore, the inclusion of susceptible species is crucial for the accurate estimation of soil contamination. In this study, we examined the sensitivity to soil cadmium (Cd) of anecic Metaphire guillelmi and epigeic E. fetida by measuring multiple indexes of oxidative stress. Using subcellular partitioning analysis, we further elucidated the inherent mechanism underlying the species-specific sensitivity of the two earthworm species. Among the battery of biochemical indexes, reactive oxygen species and protein carbonyl groups served as sensitive biomarkers. According to their respective response thresholds, M. guillelmi was more sensitive than E. fetida and they differed in their dose-response relationships. In E. fetida, the activities of three antioxidant enzymes, superoxide dismutase (SOD), catalase (CAT) and glutathione S-transferase (GST), exhibited a hormesis-like U-shaped dose-response relationship, while in M. guillelmi SOD, glutathione peroxidase (an analogue of CAT) and GST showed an inverted U-shaped relationship. The concentrations of Cd in the subcellular fractions and whole body of the earthworms well fit (R2>0.9) a saturation model versus bioavailable Cd concentrations determined by the diffusive gradients in thin films technique. Despite the lower accumulation capacity of M. guillelmi, the Cd-binding capacity (Cmax) of its subcellular heat-stable protein fraction, the so-called biologically detoxified metal pool, was 2.7 times lower than that of E. fetida, whereas the Cd binding affinity (logK) of its heat-denatured protein fraction, i.e. the metal-sensitive fraction, was 3.0 times higher, which accounted for the high susceptibility of M. guillelmi to soil Cd. Our results suggest that because of their sensitivity, as exemplified by M. guillelmi, native earthworm species should be taken into account in soil risk assessments to avoid underestimation of the toxicity of various pollutants.

Insights into tetrabromobisphenol A adsorption onto soils: Effects of soil components and environmental factors

Concerns regarding tetrabromobisphenol A (TBBPA), the most widely utilized brominated flame retardant in the world, are growing because of the wide application and endocrine-disrupting potential of this compound. To properly assess its environmental impacts, it is important to understand the mobility and fate of TBBPA in soil environments. In this study, the effects of soil components, dissolved organic carbon (DOC) and heavy metal cations on TBBPA adsorption onto two Chinese soils (red soil and black soil) were investigated using batch sorption experiments. The desorption behavior of TBBPA when the two soils are irrigated with eutrophicated river water was also investigated. The results showed that pH greatly affects the adsorptive behavior of TBBPA in soils. Iron oxide minerals and phyllosilicate minerals are both active surfaces for TBBPA sorption, in addition to soil organic matter (SOM). DOC (50 mg OC L(-1)) exhibited a limited effect on TBBPA sorption only under neutral conditions. TBBPA sorption was only minimally affected by the heavy metals (Cu2+, Pb2+ and Cd2+) in the studied pH range. Eutrophicated river water significantly enhanced the desorption of TBBPA from red soil due to the change in soil solution pH. These findings indicate that mobility of TBBPA in soils is mainly associated with soil pH, organic matter and clay fractions: it will be retained by soils or sediments with high organic matter and clay fractions under acidic conditions but becomes mobile under alkaline conditions.

Insights into tetracycline adsorption onto kaolinite and montmorillonite: experiments and modeling

Adsorption of tetracycline (TC) on kaolinite and montmorillonite was investigated using batch adsorption experiments with different pH, ionic strength, and surface coverage. As a result, pH and ionic strength-dependent adsorption of TC was observed for the two clay minerals. The adsorption of TC decreased with the increase of pH and ionic strength, and high initial TC concentration had high adsorption. In addition, a triple-layer model was used to predict the adsorption and surface speciation of TC on the two minerals. As a result, four complex species on kaolinite (≡X−∙H3TC+, ≡X−∙H2TC±, ≡SOH0∙H2TC±, and ≡SOH0∙HTC−) and three species on montmorillonite (≡X−∙H3TC+, ≡X−∙H2TC±, and ≡SOH0∙HTC−) were structurally constrained by spectroscopy, and these species were also successfully fitted to the adsorption edges of TC. Three functional groups of TC were involved in these adsorption reactions, including the positively charged dimethylamino group, the C=O amide I group, and the C=O group at the C ring. Combining adsorption experiments and model in this study, the adsorption of TC on kaolinite and montmorillonite was mainly attributed to cation exchange on the surface sites (≡X−) compared to surface complexation on the edge sites (≡SOH) at natural soil pH condition. Moreover, the surface adsorption species, the corresponding adsorption modes, and the binding constants for the surface reactions were also estimated.

Joint toxicity of tetracycline with copper(II) and cadmium(II) to Vibrio fischeri: effect of complexation reaction

Abstract Co-contamination of antibiotic and heavy metals commonly occurs in the environment. Tetracycline (TC), a common antibiotic, can behave as an efficient organic ligand to complex with cations. In this paper, the joint toxicity of TC with two commonly existing metals, copper(II) and cadmium(II), towards a luminescent bacteria, Vibrio fischeri, are investigated. Results showed that coexistence of TC and Cu(II) showed a significant antagonistic effect, while TC and Cd(II) showed a synergistic effect. The aqueous speciation of TC with two metal cations was calculated using a chemical equilibrium software Visual MINTEQ and results indicated that a strong complexation exist between TC and Cu(II), while much weaker interaction between TC and Cd(II). Traditional joint toxicity prediction model based on independent action failed to predict the combined toxicity of TC with metals. A new method based on speciation calculation was used to evaluate the joint toxicity of ligands and cations. It is assumed that the metal–ligand complexes are non-toxic to V. fischeri and the joint toxicity is determined by the sum of toxic unit of free metal-ions and free organic ligands. It explained the joint toxicity of the mixed systems reasonably well. Meanwhile, citric acid (CA) and fulvic acid (FA) were also introduced in this study to provide a benchmark comparison with TC. Results showed it is also valid for mixed systems of CA and FA with metals except for the Cd–CA mixture.

Stress response and potential biomarkers in spinach (Spinacia oleracea L.) seedlings exposed to soil lead.

Oxidative stress and biochemical responses of spinach seedlings to soil Pb stress were investigated by pot experiments. The seedlings were exposed to 0-500 mg kg(-1) extraneous Pb. After 30 days of germination, production of O(2)(-), HSP 70, HSP 60, superoxide dismutase (SOD) activities, carbonyl groups and lipid peroxidation was significantly induced by soil Pb. After 50 days, HSP 70 and HSP 60 decreased, and HSP 60 was significantly inhibited at 500 mg kg(-1). The results indicated that Pb probably induced oxidative stress and proteotoxicity to the seedlings through O(2)(-) accumulation, and that SOD, HSP 70 and HSP 60 were important defense mechanisms to alleviate the oxidative stress. It is found that O(2)(-), HSP 70 and HSP 60 were the most sensitive parameters and had the potential to act as biomarkers for early warning of soil Pb contamination. Concentrations of soil Pb, exposing time and combination of multiple parameters should be also taken into consideration when assessing soil Pb pollution by these biomarkers.