Daoyong Zhang

Complexation between Hg(II) and biofilm extracellular polymeric substances: An application of fluorescence spectroscopy

The three-dimensional excitation emission matrix (EEM) fluorescence spectroscopy was employed to investigate the interaction of extracellular polymeric substances (EPS) from natural biofilm with Hg(II). The EEM spectra demonstrated that EPS with molecular weight over 14 kDa had two protein-like fluorescence peaks. The fluorescence intensity at both peaks was strongly dependent on the solution pH in the absence and presence of Hg(II), with the maximal fluorescence intensity at neutral pH. Fluorescence of both protein-like peaks was significantly quenched by Hg(II). The values of conditional stability constants (log K(a)=3.28-4.48) derived from modified Stern-Volmer equation are approximate to those for humic substances and dissolved organic matter (DOM), indicating that fluorescent components in EPS have strong binding capacity for Hg(II). Our findings suggest that EPS from biofilm is a class of important organic ligands for complexation with Hg(II) and may significantly affect the chemical forms, mobility, bioavailability and ecotoxicity of heavy metals in the aquatic environment.

Biomineralization based remediation of As(III) contaminated soil by Sporosarcina ginsengisoli

Arsenic is a highly toxic metalloid and has posed high risk to the environment. As(III) is highly mobile in soil and leached easily into groundwater. The current remediation techniques are not sufficient to immobilize this toxic element. In the present study, an As(III) tolerant bacterium Sporosarcina ginsengisoli CR5 was isolated from As contaminated soil of Urumqi, China. We investigated the role of microbial calcite precipitated by this bacterium to remediate soil contaminated with As(III). The bacterium was able to grow at high As(III) concentration of 50mM. In order to obtain arsenic distribution pattern, five stage soil sequential extraction was carried out. Arsenic mobility was found to significantly decrease in the exchangeable fraction of soil and subsequently the arsenic concentration was markedly increased in carbonated fraction after bioremediation. Microbially induced calcite precipitation (MICP) process in bioremediation was further confirmed by ATR-FTIR and XRD analyses. XRD spectra showed presence of various biomineralization products such as calcite, gwihabaite, aragonite and vaterite in bioremediated soil samples. The results from this study have implications that MICP based bioremediation by S. ginsengisoli is a viable, environmental friendly technology for remediation of the arsenic contaminated sites.

Toxic effects of amoxicillin on the photosystem II of Synechocystis sp. characterized by a variety of in vivo chlorophyll fluorescence tests

Amoxicillin is one of the widely used antibiotics of environmental concern. This study shows that amoxicillin has toxic effects on the photosynthesis of Synechocystis sp. Its inhibitory effects on photosystem II (PSII) of Synechocystis sp. were investigated by using a variety of in vivo chlorophyll fluorescence tests. The inhibitory effects of amoxicillin on PSII activity of Synechocystis sp. are concentration-dependent. Amoxicillin exposure leads to slowing down of electron transport on both donor side and acceptor side and causes accumulation of P680(+). Q(A)(-) reoxidation test revealed that amoxicillin hinders electron transfer from Q(A)(-) to Q(B)/Q(B)(-) and more Q(A)(-) is oxidized through S(2)(Q(A)Q(B))(-) charge recombination. Analysis of PSII heterogeneity demonstrated that an exposure to amoxicillin increases the proportion of inactive PSII (PSII(X)) centers and the proportion of PSII centers with small antenna (PSIIbeta). These changes finally result in deterioration of full photosynthesis performance.

Bioremediation of strontium (Sr) contaminated aquifer quartz sand based on carbonate precipitation induced by Sr resistant Halomonas sp.

Contamination of aquifers or sediments by radioactive strontium ((90)Sr) is a significant environmental problem. In the present study, microbially induced calcite precipitation (MICP) was evaluated for its potential to remediate strontium from aquifer quartz sand. A Sr resistant urease producing Halomonas sp. was characterized for its potential role in bioremediation. The bacterial strain removed 80% of Sr from soluble-exchangeable fraction of aquifer quartz sand. X-ray diffraction detected calcite, vaterite and aragonite along with calcite-strontianite (SrCO(3)) solid solution in bioremediated sample with indications that Sr was incorporated into the calcite. Scanning electron micrography coupled with energy-dispersive X-ray further confirmed MICP process in remediation. The study showed that MICP sequesters soluble strontium as biominerals and could play an important role in strontium bioremediation from both ecological and greener point of view.

Binding of dicamba to soluble and bound extracellular polymeric substances (EPS) from aerobic activated sludge: A fluorescence quenching study

Binding of dicamba to soluble EPS (SEPS) and bound EPS (BEPS) from aerobic activated sludge was investigated using fluorescence spectroscopy. Two protein-like fluorescence peaks (peak A with Ex/Em=225 nm/342-344 nm and peak B with Ex/Em=275/340-344 nm) were identified in SEPS and BEPS. Humic-like fluorescence peak C (Ex/Em=270-275 nm/450-460 nm) was only found in BEPS. Fluorescence of the peaks A and B for SEPS and peak A for BEPS were markedly quenched by dicamba at all temperatures whereas fluorescence of peaks B and C for BEPS was quenched only at 298 K. A dynamic process dominated the fluorescence quenching of peak A of both SEPS and BEPS. Fluorescence quenching of peak B and C was governed a static process. The effective quenching constants (logK(a)) were 4.725-5.293 for protein-like fluorophores of SEPS and 4.23-5.190 for protein-like fluorophores of BEPS, respectively. LogK(a) for humic-like substances was 3.85. Generally, SEPS had greater binding capacity for dicamba than BEPS, and protein-like substances bound dicamba more strongly than humic-like substances. Binding of dicamba to SEPS and BEPS was spontaneous and exothermic. Electrostatic force and hydrophobic interaction forces play a crucial role in binding of dicamba to EPS.

Binding of phenanthrene to extracellular polymeric substances (EPS) from aerobic activated sludge: a fluorescence study.

Extracellular polymeric substances (EPS) are the essential components of activated sludge for removal of pollutants from wastewater. Limited information is available on the binding constants and binding mode of organic pollutants to EPS. In the present study, binding of phenanthrene (PHE) to extracellular polymeric substances (EPS) from aerobic activated sludge was investigated using fluorescence spectroscopy. Two protein-like fluorescence peaks (Ex/Em=225nm/340nm for peak A; Ex/Em=275-280nm/338nm for peak B) were identified in the three-dimensional excitation-emission matrix (3DEEM) fluorescence spectroscopy of the EPS. The fluorophores in EPS were clearly quenched by PHE and the quenching processes were static. The quenching constants (lnK(a)) were in the range of 11.27-13.82M(-1) and the binding constants (logK(b)) in the range of 6.11-8.98M(-1). The binding site number increased with increasing temperature. The corresponding thermodynamic parameters DeltaG, DeltaH and DeltaS were calculated. The interaction of the fluorophores in EPS and PHE is spontaneous and exothermic. The binding of EPS with PHE is dominated by the hydrophobic interactions. Fluorophore A has stronger hydrophobic interaction with PHE than fluorophore B.

Effects of arsenic on growth and photosystem II (PSII) activity of Microcystis aeruginosa

Effects of arsenic on growth, pigments content, oxygen evolution and photosystem II (PSII) activity of Microcystis aeruginosa were investigated in the present study. Various concentrations of As(III) did not show significant effects on growth and total carotenoids content within 24 h of treatment. After 48 h of treatment, 10 mg L(-1) As(III) significantly inhibited the growth and synthesis of carotenoids of M. aeruginosa, while As(III) at concentrations ranging from 0 to 1 mg L(-1) showed no significant inhibition. Chlorophyll a synthesis, oxygen evolution and chlorophyll fluorescence were more sensitive to As(III) exposure than carotenoid synthesis and growth. Chlorophyll a content, fast fluorescence rise transients and fluorescence decay kinetics appeared to be affected after the cells were exposed to 1 and 10 mg L(-1) As(III) for more than 24 h. Treatment with 10 mg L(-1) As(III) for 24 h or longer led to flattening of the fluorescence transient and drastic decrease of amplitude of fast phase of QA- reoxidation kinetics. Exposure to As(III) mainly inhibited the quantum yield for primary photochemistry, density of reaction centers and photosynthesis performance index, and increased the dissipated energy. The decrease in amplitude of the fast and middle phases further revealed that once electron transfer from QA- to QB was inhibited by As(III), more QA- was reoxidized via S2(QAQB)- charge recombination. As(III) stress may result in an increased stability of the S2QB- and S2QA- recombination.

Anaerobic Nitrate-Dependent Iron (II) Oxidation by a Novel Autotrophic Bacterium, Citrobacter freundii Strain PXL1

Anaerobic Fe(II) Oxidizing Denitrifiers (AFODN), a type of newly found Fe(II)-oxidizing bacteria, play an important role in iron and nitrogen cycling. In the present study, a novel AFODN strain PXL1 was isolated from anaerobic activated sludge. Phylogenetic analysis of 16S rRNA gene sequence revealed similarity between this strain and Citrobactor freundii. The strain reduced 30% of nitrate and oxidized 85% of Fe(II) over 72 h with an initial Fe(II) concentration of 3.4 mM and nitrate concentration of 9.5 mM. Oxidation of iron was dependent on the reduction of nitrate to nitrite in the absence of other electron donors or acceptors. Nitrate reduction and Fe(II) oxidation followed first-order reaction kinetics. Iron oxides accumulated in the culture were analyzed by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) spectroscopy and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS). The strain recovered deposited oxidized Fe in the form of amorphous Fe oxides.

Removal of antimony (Sb(V)) from Sb mine drainage: biological sulfate reduction and sulfide oxidation-precipitation.

Antimony (Sb(V)) in Sb mine drainage has adverse effects on the receiving water environments. This study for the first time demonstrated the feasibility of using sulfate-reducing bacteria (SRB) to convert sulfate ions in SMD into sulfides that reduce Sb(V) to Sb(III) and to form complex with Sb(III) as precipitate. The principal compound in the precipitate was stibnite (Sb2S3) at pH 7 and pH 9. The Sb(V) removal mechanism is sulfate-reduction and sulfide oxidization-precipitation, different from the conventional SRB-precipitation processes for heavy metals. The Sb(V)/sulfate ratio is noted an essential parameter affecting the Sb removal efficiency from SMD.

Remediation of Cr(VI) from chromium slag by biocementation

Here we demonstrate a calcifying ureolytic bacterium Bacillus sp. CS8 for the bioremediation of chromate (Cr(VI)) from chromium slag based on microbially induced calcite precipitation (MICP). A consolidated structure like bricks was prepared from chromium slags using bacterial cells, and five stage Cr(VI) sequential extraction was carried out to know their distribution pattern. Cr(VI) mobility was found to significantly be decreased in the exchangeable fraction of Cr slag and subsequently, the Cr(VI) concentration was markedly increased in carbonated fraction after bioremediation. It was found that such Cr slag bricks developed high compressive strength with low permeability. Further, leaching behavior of Cr(VI) in the Cr slag was studied by column tests and remarkable decrease in Cr(VI) concentration was noticed after bioremediation. Cr slags from columns were characterized by SEM-EDS confirming MICP process in bioremediation. The incorporation of Cr(VI) into the calcite surface forms a strong complex that leads to obstruction in Cr(VI) release into the environment. As China is facing chromium slag accidents at the regular time intervals, the technology discussed in the present study promises to provide effective and economical treatment of such sites across the country, however, it can be used globally.