Liping Weng

Adsorption of levofloxacin onto goethite: Effects of pH, calcium and phosphate

Adsorption of levofloxacin (LEV), one of the extensively used antibiotics, onto goethite was investigated using batch experiments. The adsorption of LEV on goethite was pH-dependent. A maximum adsorption was reached at pH 6. Above or below pH 6, the adsorption decreased. In the presence of calcium (Ca(2+)), a decrease in adsorption was observed, due to probably formation of Ca(2+)-LEV complexes in solutions. Phosphate also showed a significant inhibition on LEV adsorption over a pH range of 3-10. Phosphate competed with LEV for binding sites on the surface of goethite, and the electrostatic competition between LEV and phosphate on goethite surface might be another reason for the decrease in adsorption. These results indicated that Ca(2+) and phosphate have a great influence on the distribution of LEV in soils and waters, which will eventually affect its antibacterial activity in the environment.

Modeling of levofloxacin adsorption to goethite and the competition with phosphate

Interaction between various compounds in natural systems may influence the adsorption of these species and their environmental fate. In this work, we studied the interactions between a widely used antibiotic levofloxacin (LEV) and phosphate at the surface of goethite (α-FeOOH), which was important to better understand the competitive adsorption of antibiotics and oxyanions in natural systems. The presence of phosphate decreased LEV adsorption to goethite significantly over the whole pH range. The other way around, LEV had a little influence on phosphate adsorption. Eight types of LEV-goethite complexes were proposed and modeled in our study. Electrostatic competition was the main reason for the competition of binary components (LEV and phosphate) to goethite surface. Adsorption of single component (LEV or phosphate) to goethite was well predicted using the CD-MUSIC (Charge Distribution Multi-Site Complexation) model. In competition experiments, phosphate adsorption was still predicted well, but LEV adsorption was overestimated in model calculations. This is because less negative charge of LEV is located at outer electrostatic plane in our study, which decreases their electrostatic competition to goethite surface.

Enhanced transport of ferrihydrite colloid by chain-shaped humic acid colloid in saturated porous media

Both humic acid and colloid particle size effectively regulate colloid transport. However, little is known about effect of particle size and configuration of humic acid colloid (HAcolloid) on enhanced-transport of ferrihydrite colloid (FHcolloid) in porous media. Co-transport of HAcolloid and FHcolloid at different pH was systematically investigated by monitoring breakthrough curves (BTCs) in saturated sand columns. The colloid transport model and the (X)DLVO theory were used to reveal the mechanism of HAcolloid-enhanced FHcolloid transport in the columns. Results showed that HAcolloid enhanced FHcolloid transport in neutral and alkaline conditions. In neutral conditions, small HAcolloid (F-HAcolloid) with chain-shaped structure enhanced FHcolloid transport more prominently than pristine granular HAcolloid. The chain-shaped F-HAcolloid caused osmotic repulsion and elastic-steric repulsion between colloids and sand, leading to enhanced transport. However, the granular HAcolloid readily occurred as deposition due to attachment and straining, which decreased the enhanced transport of FHcolloid. In alkaline conditions, both HAcolloid and F-HAcolloid were chain-shaped, with longer chains of HAcolloid than F-HAcolloid. Ferrihydrite colloid transport was enhanced by HAcolloid more significantly than F-HAcolloid due to stronger repulsion between mixed HAcolloid-FHcolloid and sand. It suggested that regulation of particle size and morphology of HAcolloid would enhance FHcolloid transport and further help in understanding FHcolloid-facilitated contaminants transport in porous media.

Simultaneous analysis of small organic acids and humic acids using high performance size exclusion chromatography

An accurate and fast method for simultaneous determination of small organic acids and much larger humic acids was developed using high performance size exclusion chromatography. Two small organic acids, i.e. salicylic acid and 2,3-dihydroxybenzoic acid, and one purified humic acid material were used in this study. Under the experimental conditions, the UV peaks of salicylic acid and 2,3-dihydroxybenzoic acid were well separated from the peaks of humic acid in the chromatogram. Concentrations of the two small organic acids could be accurately determined from their peak areas. The concentration of humic acid in the mixture could then be derived from mass balance calculations. The measured results agreed well with the nominal concentrations. The detection limits are 0.05 mg/L and 0.01 mg/L for salicylic acid and 2,3-dihydroxybenzoic acid, respectively. Applicability of the method to natural samples was tested using groundwater, glacier, and river water samples (both original and spiked with salicylic acid and 2,3-dihydroxybenzoic acid) with a total organic carbon concentration ranging from 2.1 to 179.5 mg C/L. The results obtained are promising, especially for groundwater samples and river water samples with a total organic carbon concentration below 9 mg C/L.

Enhanced cadmium immobilization in saturated media by gradual stabilization of goethite in the presence of humic acid with increasing pH

Goethite (Gt) and humic acid (HA) are important components of soil that significantly affect Cd mobility. In this study, the co-transport of Cd2+ and Gt with/without HA in saturated sand columns was investigated by monitoring the breakthrough curves at different pH values. A solute transport model was used to study Cd2+ transport and retention in the saturated sand in the presence of Gt and HA, and a colloid transport model was used to describe the Gt colloid (GtC) transport in the columns. Our results showed that the transport behaviors of Cd2+ and Gt colloids/aggregates were regulated by pH. Cadmium transport was significantly inhibited at high pH due to its adsorption on the sand and Gt. Moreover, Gt retention was gradually stabilized with increasing pH regardless of its forms, i.e., individual colloids (GtC) or larger assemblages of particles due to aggregation (GtA). This retention was obviously enhanced in the presence of HA. Thus, the superposition of increased Cd2+ adsorption on Gt and Gt retention (stabilization) enhanced the immobilization of Cd2+ at high pH. In addition to stabilizing Gt, HA further enhance Cd2+ adsorption on Gt, thus promoting Cd2+ immobilization. However, only a small amount of organic-matter-bound Cd2+ was observed in the columns with injected HA. The major fractions of retained Cd2+ were exchangeable Cd2+ and Fe-oxide-bound Cd2+. Our results provide new insights into the roles of Gt and HA in the transport and mobilization of Cd2+ in soil-groundwater systems.

Cation accumulation leads to the electrode aging in soil microbial fuel cells

PurposeThe electrode aging in soil microbial fuel cells (MFCs) disturbed the removal of pollutants and sensitivity of electrophysiological signal. Therefore, surveying the causes of aging electrodes could assist to take the prevention measures for remediation and biosensor application of soil MFCs.Materials and methodsThe surface morphology, element accumulation on the surface of electrodes, and element migration in soils between electrodes were investigated by scanning electronic microscopy, energy-dispersive spectrometry, and X-ray photoelectron spectroscopy in a constructed soil MFC.Results and discussionThe rust was observed on the anode, and the soil gypsification was noted on the cathode after a long-term (300-day) contribution of soil MFCs. The major elements (At%) Na and Ca in soils on two electrodes increased by 338–562 and 100–119%, respectively. Beside, Al and Fe of increment (24 and 21%) in the anode and Mg and Fe of augmenter (84 and 155%) in the cathode were detected.ConclusionsThe stacking of Ca and Fe besides Na in soils adjacent to electrodes probably led to the electrode corrosion and soil gypsification on the surface of electrodes. Thus, the electrode aging of soil MFCs should be paid more attention in further applications.

Influence of calcium and phosphate on pH dependency of arsenite and arsenate adsorption to goethite

In the environment, simultaneous presence of arsenic (As) of different oxidation states is common, which hampers our understanding of As behavior. In the current study, the pH dependency of arsenite (As(III)) and arsenate (As(V)) adsorption to goethite under the influence of calcium (Ca2+) (as a major cation) and phosphate (PO43-) (as a major anion) was studied, and the reliability of the CD-MUSIC model prediction was tested. The results show that the presence of the major ions led in general to a weaker and more complicated pH dependency of As adsorption. Calcium promoted As(V) adsorption especially at high pH, which can reverse the direction of the pH dependency. The presence of Ca2+ can even decrease As(III) adsorption when As(V) and/or PO43- are present. Phosphate competed strongly with both As(III) and As(V) in their adsorption, especially at intermediate and low pH. In the multi-component system, As(III) adsorbs weaker than As(V) over the environmental relevant pH range, therefore it is often the dominant As species in solution and soluble As(III) concentration generally decreases with increasing pH. In the same pH range, As(V) adsorption shows a complicated pH dependency. Soluble As(V) reaches a minimum around pH 6u202fat high concentration of major bivalent cations (e.g. Ca2+), whereas soluble As(V) will decrease with pH at low bivalent cation concentrations. The experimental results can be reliably predicted and explained with the CD-MUSIC model. The outcome of this study can provide understanding needed in the risk assessment and remediation of As contaminated soils and water.

Long-term effect of biochar amendment on the biodegradation of petroleum hydrocarbons in soil microbial fuel cells

Biochar is extensively applied in amendment of contaminated soils. However, the effect of biochar on the biodegradation of petroleum hydrocarbons and electricity generation in soil microbial fuel cells (MFCs) remains unclear. Here, three biochars respectively derived from poultry (chicken manure, CB), agriculture (wheat straw, SB) and forestry industries (wood sawdust, WB) were investigated after 223u202fdays of amendment. Consequently, high removal for alkanes was in CB with the mineral nutrition and phosphorus while aromatics were in SB with the most N content and the highest molecular polarity. The lowest removal efficiency of total petroleum hydrocarbons was observed in WB with the highest surface area, whereas the most charge was obtained. The different performance of soil MFCs was due to physicochemical properties of biochar and colonized microbial communities of bacteria and archaea. The abundance of Actinotalea increased by 144-263% in SB and CB while that of Desulfatitalea distinctly increased in WB. Meanwhile, species from Methanosarcina, Methanoculleus, Halovivax and Natronorubrum exerted probably a methanogenic degrading role. This study revealed that the degrader, azotobacter and electricigens exhibited a close relationship in order to degrade hydrocarbons and generate electricity in soil bioelectrochemical remediation systems.

Enhancement effect of earthworm (Eisenia fetida) on acetochlor biodegradation in soil and possible mechanisms

Acetochlor is a widely used chloroacetanilide herbicide and has posed environmental risks in soil and water due to its toxicity and high leaching capacity. Earthworm represents the dominant invertebrate in soil and can promote the decomposition of organic pollutants. The effect of earthworm on acetochlor degradation in soil was studied by soil column experiment with or without acetochlor and earthworm in sterile and natural soils. The degradation capacities of drilosphere components to acetochlor were investigated by microcosm experiments. Bacterial and fungal acetochlor degraders stimulated by earthworm were identified by high-throughput sequencing. The degradation kinetics of acetochlor suggested that both indigenous microorganisms and earthworm played important roles in acetochlor degradation. Acetochlor degradation was quicker in soil with earthworms than without earthworms, with the degradation rates increased by 62.3u202f±u202f15.2% and 9.7u202f±u202f1.7% in sterile and natural treatments respectively. The result was related to the neutralized pH, higher enzyme activities and enhanced soil microbial community diversity and richness in the presence of earthworms. Earthworm cast was the degradation hotpot in drilosphere and exhibited better anaerobic degradation capacity in microcosm experiments. The acetochlor degradation rate of cast in anaerobic environment was 12.0u202f±u202f0.1% quicker than that in aerobic environment. Residual acetochlor in soil conferred a long-term impairment on fungal community, and this inhibition could be repaired by earthworm. Earthworm stimulated indigenous degraders like Sphingomonas and Microascales and carried suspected intestinal degraders like Mortierella and Escherichia_coli to degradation process. Cometabolism between nutrition cycle species and degraders in casts also contributed to its faster degradation rates. The study also presented some possible anaerobic degradation species like Rhodococcus, Pseudomonas_fulva and Methylobacillus.

Distinct effect of humic acid on ferrihydrite colloid-facilitated transport of arsenic in saturated media at different pH

Both humic acid and arsenic (As) have a strong affinity to ferrihydrite colloids (FHC). However, little is known about effects of humic acid colloid (HAC) and FHC interaction on transport and deposition of As in porous media. Co-transport of HAC-FHC and As at different pH was systematically investigated by monitoring their breakthrough curves (BTCs) in saturated sand columns. Colloid and solute transport models were used to reveal mechanisms of As transport with HAC-FHC in porous media. Results showed that HAC-FHC regulated As transport. Under neutral pH conditions, As transport was facilitated by FHC loading small chain-shaped HAC, while it was slightly retarded by FHC loading granular HAC. Under alkaline conditions, HAC was chain-shaped and coupled with FHC to facilitate As transport. However, mobile colloid-adsorbed As was less evident due to relatively low adsorption on FHC under alkaline pH in comparison with the transport under neutral pH. The presence of HAC occupied adsorption sites on FHC and thus decreased As adsorption. Arsenic adsorption on mobile FHC decreased with increasing HAC concentration. Therefore, HAC had an antagonistic effect on mixed colloid-facilitated As transport at neutral and alkaline conditions. Arsenic was readily co-deposited with FHC, especially at low pH. This study suggested that morphology and concentration of HAC, mobility of FHC, and solution pH would control As transport in porous media. The fate of As changed from co-transport with HAC-FHC to co-deposition when environmental pH decreased.