Sudipta Rakshit

Mechanisms of ciprofloxacin removal by nano-sized magnetite.

An understanding of the interaction mechanisms of antibiotics with environmentally relevant sorbents is important to determine the environmental fate of antibiotics and to develop wastewater treatment strategies. Magnetite (Fe(3)O(4)(s)) is ubiquitous in the environment and occurs as a secondary corrosion product of iron nanoparticles that are commonly used as a remediation material. In this study, we aimed to assess the sorption mechanisms of ciprofloxacin (CIP), an important class of fluoroquinolone antibiotics, with magnetite nanoparticles using a combination of wet chemical and in situ ATR-FTIR spectroscopic measurements. Ciprofloxacin sorption was characterized as a function of pH (3.4-8.0), CIP concentration (1-500 μM), ionic strength (0.5, 0.1, and 0.01 M NaCl), and competing anion such as phosphate (0.1mM) to cover a broad range of environmentally relevant geochemical conditions. Results indicated a bell-shaped sorption envelop where sorption of CIP on nano-Fe(3)O(4)(s) increased from 45% to 80% at pH 3.44-5.97; beyond that sorption gradually decreased to a value of 25% at pH 8.39. Phosphate had negligible effect on CIP sorption. In situ ATR-FTIR results indicated inner-sphere coordination of CIP at the magnetite surface mediated by carboxylic acid groups. Results suggest that nano-Fe(3)O(4)(s) has the potential to remove CIP from wastewater effectively.

Antimony sorption at gibbsite-water interface.

Antimony (Sb) is extensively used in flame retardants, lead-acid batteries, solder, cable coverings, ammunition, fireworks, ceramic and porcelain glazes and semiconductors. However, the geochemical fate of antimony (Sb) remained largely unexplored. Among the different Sb species, Sb (V) is the dominant form in the soil environment in a very wide redox range. Although earlier studies have examined the fate of Sb in the presence of iron oxides such as goethite and hematite, few studies till date reported the interaction of Sb (V) with gibbsite, a common soil Al-oxide mineral. The objective of this study was to understand the sorption behavior of Sb (V) on gibbsite as a function of various solution properties such as pH, ionic strength (I), and initial Sb concentrations, and to interpret the sorption-edge data using a surface complexation model. A batch sorption study with 20 g L(-1) gibbsite was conducted using initial Sb concentrations range of 2.03-16.43 μM, pH values between 2 and 10, and ionic strengths (I) between 0.001 and 0.1M. The results suggest that Sb (V) sorbs strongly to the gibbsite surface, possibly via inner-sphere type mechanism with the formation of a binuclear monodentate surface complex. Weak I effect was noticed in sorption-edge data or in the isotherm data at a low surface coverage. Sorption of Sb (V) on gibbsite was highest in the pH range of 2-4, and negligible at pH 10. Our results suggest that gibbsite will likely play an important role in immobilizing Sb (V) in the soil environment.

Surface complexation of antimony on kaolinite.

Geochemical fate of antimony (Sb) - a similar oxyanion as arsenic (As) - in a variety of environment is largely unexplored. Kaolinite is an important, naturally occurring clay mineral in soils and aquifers and is known to control the fate of several contaminants via a multitude of geochemical processes, primarily adsorption. Here we report adsorption of antimony on kaolinite as a function of solution chemistry: initial antimony concentration, pH, ionic strength, and a competing anion. A surface complexation modeling (SCM) approach was undertaken to understand the potential mechanistic implications of sorption envelope data. In the SCM, a multicomponent additive approach, in which kaolinite is assumed to be a (1:1) mixture of quartz (≡SiOH) and gibbsite (≡AlOH), was tested. Results indicated that ionic strength has a minimal effect on antimony adsorption. For the lower initial antimony concentration (4.11 μM), the additive model with binuclear surface complexes on quartz and gibbsite showed a better fit at pH<6, but somewhat under predicted the experimental data above pH 6. At the higher initial antimony concentration (41.1 μM), the sorption envelope was of different shape than the lower load. The additive model, which considered binuclear surface complexes for quartz and gibbsite, resulted in over prediction of the adsorption data at pH>3.5. However, the additive model with binuclear surface complex on quartz and mononuclear surface complex on gibbsite showed an excellent fit of the data. Phosphate greatly influenced antimony adsorption on kaolinite at both low and high antimony loadings, indicating competition for available surface sites.

In situ attenuated total reflectance fourier-transform infrared study of oxytetracycline sorption on magnetite.

Adsorption of antibiotics on the surfaces of common mineral sorbents plays a major role in determining their fate in soils and sediments. The mechanisms of these reactions are, therefore, important for understanding and predicting the environmental fate of antibiotics. We used in situ attenuated total reflectance Fourier-transform infrared spectroscopy to elucidate the binding mechanisms of oxytetracycline (OTC) onto the surface of magnetite [FeO], a common Fe oxide mineral in soils and sediments, as a function of pH (3-9) and aqueous OTC concentration (5-150 μmol L). Comparison of dissolved OTC spectra to those of OTC-magnetite surface complexes indicated strong interactions of OTC molecules with the FeO surface via carbonyl (C=O) and amine (-NH) moieties of the amide group (-CONH) and the N atom of the dimethyl amino group [-N(CH)]. Increasing the aqueous OTC concentration led to increased OTC adsorption but did not notably alter the OTC binding mode at the magnetite surface. The results of this study would help to assess the importance of Fe oxide minerals in determining the environmental fate of OTC in soils and sediments.

Effect of solution properties, competing ligands, and complexing metal on sorption of tetracyclines on Al-based drinking water treatment residuals

In the current batch study, we investigated the effect of solution properties, competing ligands (phosphate (P(V)) and sulfate), and complexing metal (calcium (Ca2+)) on tetracycline (TTC) and oxytetracycline (OTC) sorption by Al-based drinking water treatment residuals (Al-WTR). The sorption behavior for both TTC and OTC on Al-WTR was pH dependent. The sorption in absence of competing ligands and complexing metal increased with increasing pH up to circum-neutral pH and then decreased at higher pH. The presence of P(V) when added simultaneously had a significant negative effect (p < 0.001) on the sorption of TTC and OTC adsorbed by Al-WTR at higher TTC/OTC:P ratios. However, when P(V) was added after the equilibration of TTC and OTC by Al-WTR, the effect was minimal and insignificant (p > 0.1). The presence of sulfate had a minimal/negligible effect on the sorption of TCs by Al-WTR. A significant negative effect (p < 0.001) on the adsorption of TCs by Al-WTR was observed in the pH range below 5 and at higher TCs:Ca2+ ratios, probably due to TCs-Ca2+ complex formation. Fourier transform infrared (FTIR) analysis indicated the possibility of inner-sphere-type bonding by the functional groups of OTC/TTC on Al-WTR surface. Results from the batch sorption study indicate high affinity of Al-WTR for TCs in the pH range 4–8 (majorly encountered pH in the environment) in the presence of competing ligands and complexing metal.

Water Treatment Residuals and Scrap Tire Rubber as Green Sorbents for Removal of Stormwater Metals.

Ozonation of real dye wastewater for removal of color and COD reduction covering a wide range in operating parameters forms the scope of the present work. The influence of parameters such as influent pH, ozone flow rate and initial effluent concentration on ozonation efficiency has been critically examined. It has been observed from the present investigation that a maximum of COD removal efficiency of 92.5% has been achieved under optimum operating conditions (pH=11; ozone flow rate: 6×10(-3) m(3)/minute). Further the biodegradability index of the dye effluent has increased from an initial value of 0.18 to 0.49 during ozonation indicating favorable adaptation of ozonation as a primer to the biochemical technique to enhance the efficiency of biochemical treatment.Bench scale tests were performed to evaluate two recycled wastes, water treatment residuals (WTR) and scrap tire rubber (STR), for adsorption of selected metals from urban stormwater, and assess their release from used sorbents. Aluminum-WTR alone could rapidly and effectively remove Cu, Pb, and Zn, while STR alone continuously released Zn accompanied with Cu and Pb adsorption. Zn leaching from STR was significantly reduced in the presence of WTR. Very little metals released from used combined adsorbents in NaNO3 solution, and only part of them were extracted with EDTA (a strong chelating agent), suggesting that metal release is not a concern in a typical stormwater condition. A combination of WTR and STR is a new, effective method for mitigation of urban stormwater metals-WTR can inhibit the STR leaching, and STR improves the hydraulic permeability of WTR powders, a limiting factor for stormwater flow when WTR is used alone.

Immobilization of tetracyclines in manure and manure-amended soils using aluminum-based drinking water treatment residuals

Veterinary antibiotics (VAs) are emerging contaminants of concern in the environment, mainly due to the potential for development of antibiotic-resistant bacteria and effect on microbiota that could interfere with crucial ecosystem functions such as nutrient cycling and decomposition. High levels of VAs such as tetracyclines (TCs) have been reported in agricultural soils amended with manure, which also has the potential to cause surface and groundwater contamination. Several recent studies have focused on developing methods to immobilize VAs such as composting with straw, hardwood chips, commercial biochar, aeration, mixing, heat treatment, etc. The major shortcomings of these methods include high cost and limited effectiveness. In the current study, we assessed the effectiveness of aluminum-based drinking water treatment residuals (Al-WTR) as a “green” sorbent to immobilize TCs in manure and manure-applied soils with varying physicochemical properties by laboratory incubation study. Results show that Al-WTR is very effective in immobilizing tetracycline (TTC) and oxytetracycline (OTC). The presence of phosphate resulted in significant (p < 0.01) decrease in TTC/OTC sorption by Al-WTR, but the presence of sulfate did not. attenuated total reflection (ATR)-FTIR spectroscopy indicate that TTC and OTC likely forming surface complexes via inner-sphere-type bonds in soils, manure, and manure-applied soils amended with Al-WTR.

Assessing redox properties of standard humic substances

Redox properties of humic substances (HS) control important biogeochemical processes. Thus, accurate estimation of redox properties of HS is essential. However, there is no general consensus regarding the best available measurement method of HS redox properties. In this study, we compared several common HS redox property measurement methods using anthraquinone-2,6-disulfonate (AQDS) as model compound, and standard Elliot soil humic acid (1S102H, ESHA), reference Pahokee peat (1R103H, PPHA), and Suwannee River natural organic matter (1R101N, SRNOM) as representative HS. We found that the H2/Pd reduction method followed by incubation with ferric citrate (FeCit) reagent was incomplete, and the H2/Pd reduction method followed by incubation with potassium ferricyanide (K3Fe(CN)6) was insensitive. Stannous chloride (SnCl2) reduction followed by titration of excess stannous (Sn2+) by potassium dichromate (K2Cr2O7) was found to be most accurate. These findings will help in future investigations on detailed characterizations of functional groups of HS responsible for oxidation/reduction reactions.

Influence of phosphate on tungstate sorption on hematite: A macroscopic and spectroscopic evaluation of the mechanism

The environmental fate of the tungstate (VI) oxyanion [ e.g. mono tungstate and several polytungstate, generally expressed by W (VI)] is largely controlled by sorption on soil minerals, especially on iron oxide minerals. Molecular scale evaluation of W (VI) retention on iron oxides in the presence of competing oxyanions is scarce in literature. Here we report surface interaction mechanisms of W (VI) on hematite in the presence of phosphate (P) using macroscopic and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic experiments. Batch sorption experiments were conducted using 2 g L-1 hematite and 100 μM W (VI) and P, in single ion system and in binary mixtures as a function of pH (4-11). In situ ATR-FTIR spectroscopic evaluation of P and W (VI) sorption on hematite was also carried out. The results from macroscopic experiments indicated that W (VI) sorption on hematite was not affected by P when W (VI) was added first. The influence of P on W (VI) sorption was noticed when W (VI) & P were added simultaneously or P was added first. The in situ ATR-FTIR spectroscopic data corroborated these findings. In addition, the spectroscopic data revealed that in the presence of P, the surface complexation mode of W (VI) differed as noted from either the absence of WO antisymmetric infrared (IR) band or the WOW stretching band. This study provides useful information on molecular level understanding of W (VI) surface complexation on hematite in the presence of competing ions such as P.