Adsorption/desorption of arsenite and arsenate on chitosan and nanochitosan.

Equilibrium sorption studies of anionic species of arsenite, As(III) ions and arsenate As(V) ions onto two biosorbents, namely, chitosan and nanochitosan, have been investigated and compared. The results and trends in the sorption behavior are novel, and we have observed during the sorption process of the As(III) and As(V) on chitosan, a slow process of desorption occurred after an initial maximum adsorption capacity was achieved, before reaching a final but lower equilibrium adsorption capacity. The same desorption trend, however, is not observed on nanochitosan. The gradual desorption of As(III) and As(V) in the equilibrium sorption on chitosan is attributed to the different fractions of the dissociated forms of arsenic on the adsorbent surface and in solution and the extent of protonation of chitosan with the changing of solution pH during sorption. The change of solution pH during the sorption of arsenite ions on chitosan was also influenced by the interaction between the buffering effect of the arsenite species in the aqueous medium and the physical properties of chitosan. The final equilibrium adsorption capacity of chitosan for As(III) and As(V) was found to be around 500 and 8000 µg/g, respectively, whereas the capacities on nanochitosan are 6100 and 13,000 µg/g, respectively.

Influence of livestock activities on residue antibiotic levels of rivers in Hong Kong.

Occurrence of 10 antibiotics in the Yuen Long (YLR), Kam Tin (KTR), and Shing Mun (SMR) rivers of Hong Kong and possible influence of livestock activities on the concentrations of antibiotics were investigated. Tetracycline (30-497 ng/L), sulfadiazine (2-80 ng/L), sulfamethoxazole (2-152 ng/L), ofloxacin (5-227 ng/L), and erythromycin (1-315 ng/L) were detected in all the three rivers; chlortetracycline (23-227 ng/L), oxytetracycline (7-104 ng/L), ciprofloxacin (12-68 ng/L), and roxithromycin (1-105 ng/L) were detected in YLR and KTR, whereas norfloxacin (3-34 ng/L) was detected in KTR only. Significant correlation between livestock population and antibiotic contamination was observed in YLR only, indicating the influences of other sources in KTR and SMR. Among the antibiotics, significant correlation was observed between tetracyclines and sulfonamides indicating the major influence of livestock farms, whereas tetracyclines/sulfonamides were negatively correlated with fluoroquinolones/macrolides implying the differential origin of the latter class of antibiotics. Water quality of KTR and YLR were highly influenced by the non-point source pollutions, while of SMR was relatively good. Particularly, Escherichia coli populations of the YLR and KTR were 3-4 logs higher than those of the SMR indicating the involvement of livestock farms and sewerages. Good correlation between tetracyclines (TCs)/sulfonamides (SAs) and number of livestock farms and a negative correlation between TCs/SAs and fluoroquinolones (FQs)/macrolides (MLs) could be used as an indicator to trace the possible source of pollution.

Mechanism of arsenic removal using chitosan and nanochitosan.

Chitosan, a natural polysaccharide copolymer of glucosamine and N-acetyl-glucosamine, possesses one free primary amine and two free hydroxyl groups on each glucosamine unit. It has a polycationic nature and an abundance of amine functional groups. The sorption equilibrium and kinetics of arsenate onto chitosan flakes have been studied. The effect of pH on the adsorption capacity and the uptake kinetics is an important parameter to investigate the adsorption mechanism of anionic species such as arsenate ions on the protonated amine groups of chitosan. The equilibrium sorption and batch kinetic studies of arsenate ions on chitosan were performed at initial As concentration of 250-11,000 µg L(-1) and initial pH ranging from pH=3.50-5.50. The experimental results showed that initially for approximately the first 30 min there is a rapid and high adsorption of arsenate ions onto the chitosan leading to a maximum uptake capacity after this short time. However, this stage is followed by a slow desorption of arsenate from the chitosan with a steady increase in solution pH. A novel reversible pseudo-first order kinetic model was developed and applied to correlate this newly reported adsorption-desorption phenomenon. The physical and chemical properties of chitosan were studied and presented in terms of its surface and structural properties such as the degree of deacetylation, crystallinity, surface charge and its swelling properties.