Anne-Claire Texier

Rare earth elements removal by microbial biosorption: a review.

Abstract This paper reviews published work on the sorption of rare earth elements by microbial biomass. In a first part, the biosorption capacities and the various experimental conditions performed in batch reactor experiments are compared. Secondly, sorption modelling generally used in biosorption studies are described. Thirdly, the microbial cell wall characteristics of the metallic ion binding sites are considered. From these observations it seems that the important functional groups for metallic ion fixation are the carboxyl and the phosphate moieties. Moreover, the competing effect of various ions like aluminium, iron, glutamate, sulphate etc. is described. Finally, some adsorption results of the rare earth elements in dynamic reactors are presented.

p-Cresol biotransformation by a nitrifying consortium.

The oxidizing ability of a nitrifying consortium exposed to p-cresol (25 mg CL(-1)) was evaluated in batch cultures. Biotransformation of the phenolic compound was investigated by identifying the different intermediates formed. p-Cresol inhibited the ammonia-oxidizing process with a decrease of 83% in the specific rate of ammonium consumption. After 48 h, ammonium consumption efficiency was 96+/-9% while nitrate yield reached 0.95+/-0.06 g NO(3)(-)-Ng(-1)NH(4)(+)-N consumed. High value for nitrate production yield showed that the nitrifying metabolic pathway was only affected at the specific rate level being nitrate the main end product. The consortium was able to totally oxidize p-cresol at a specific rate of 0.17+/-0.06 mg p-cresol-Cmg(-1) microbial protein h(-1). p-Cresol was first transformed to p-hydroxybenzaldehyde and p-hydroxybenzoate, which were later completely mineralized. In the presence of allylthiourea, a specific inhibitor of ammonia monooxygenase (AMO), p-cresol was oxidized to the same intermediates and in a similar pattern as obtained without the AMO inhibitor. AMO seemed not to be involved in the p-cresol oxidation process. When p-hydroxybenzaldehyde was added (25 mg CL(-1)), the nitrifying process was inhibited in the same way as observed with p-cresol, indicating that p-hydroxybenzaldehyde could be the main compound responsible for nitrification inhibition. p-Hydroxybenzaldehyde was accumulated during 15 h before complete consumption at a specific rate value eight times lower than the p-cresol consumption rate. Results showed that p-hydroxybenzaldehyde oxidation was the limiting step in p-cresol mineralization by the nitrifying consortium.

Contribution of quinone-reducing microorganisms to the anaerobic biodegradation of organic compounds under different redox conditions

The capacity of two anaerobic consortia to oxidize different organic compounds, including acetate, propionate, lactate, phenol and p-cresol, in the presence of nitrate, sulfate and the humic model compound, anthraquinone-2,6-disulfonate (AQDS) as terminal electron acceptors, was evaluated. Denitrification showed the highest respiratory rates in both consortia studied and occurred exclusively during the first hours of incubation for most organic substrates degraded. Reduction of AQDS and sulfate generally started after complete denitrification, or even occurred at the same time during the biodegradation of p-cresol, in anaerobic sludge incubations; whereas methanogenesis did not significantly occur during the reduction of nitrate, sulfate, and AQDS. AQDS reduction was the preferred respiratory pathway over sulfate reduction and methanogenesis during the anaerobic oxidation of most organic substrates by the anaerobic sludge studied. In contrast, sulfate reduction out-competed AQDS reduction during incubations performed with anaerobic wetland sediment, which did not achieve any methanogenic activity. Propionate was a poor electron donor to achieve AQDS reduction; however, denitrifying and sulfate-reducing activities carried out by both consortia promoted the reduction of AQDS via acetate accumulated from propionate oxidation. Our results suggest that microbial reduction of humic substances (HS) may play an important role during the anaerobic oxidation of organic pollutants in anaerobic environments despite the presence of alternative electron acceptors, such as sulfate and nitrate. Methane inhibition, imposed by the inclusion of AQDS as terminal electron acceptor, suggests that microbial reduction of HS may also have important implications on the global climate preservation, considering the green-house effects of methane.

Benzene transformation in nitrifying batch cultures.

The effect of benzene on the nitrifying activity of a sludge produced in steady‐state nitrification was evaluated in batch cultures. Benzene at 10 mg/L inhibited nitrate formation by 53%, whereas at 5 mg/L there was no inhibition. For initial benzene concentrations of 0, 7, and 10 mg/L, the specific rates of NO3‐‐N production were 0.545 ± 0.101, 0.306 ± 0.024, and 0.141 ± 0.010 g NO3‐‐N/g microbial protein‐N·h, respectively. The specific rates of benzene consumption at 7, 12, and 20 mg/L were 0.034 ± 0.003, 0.050 ± 0.006, and 0.027 ± 0.002 g/g microbial protein‐N·h, respectively. Up to a concentration of 10 mg/L, benzene was first oxidized to phenol, which was later totally oxidized to acetate. Benzene at higher concentrations (20 and 30 mg/L) was converted to intermediates other than acetate, phenol, or catechol. These results suggest that this type of nitrifying consortium coupled with a denitrification system may have promising applications for complete removal of nitrogen and benzene from wastewaters.

2-Chlorophenol consumption and its effect on the nitrifying sludge.

The kinetic behavior of a nitrifying sludge exposed to 2-chlorophenol (2-CP) was evaluated in batch culture. The assays were performed using a stabilized nitrifying sludge. In control assays with (mg L(-1)): NH(4)(+)-N (100) and NaHCO(3)(-)-C (250), the substrates were consumed in 8h, the ammonium consumption efficiency was 99% and the NO(3)(-) yield higher than 0.9. When 5mg 2-CP-C L(-1) was added, it was transformed into an unidentified intermediate and the nitrifying efficiency decreased to 10%. Ammonium specific consumption rate diminished 95%, but the NO(3)(-) yield remained higher than 0.9. The biomass previously exposed to 2-CP was newly suspended with NH(4)(+)-N or NO(2)(-)-N in order to evaluate the ammonium and nitrite oxidizing processes. The consumption efficiencies and NO(3)(-) yields were similar to those obtained in control assays. However, the total time required for ammonium and nitrite consumption increased to 120 and 42 h, respectively. Specific consumption rates for NH(4)(+)-N and NO(2)(-)-N decreased by 95% and 83% respectively, compared to control assays. Thus, the previous contact to 2-CP had more influence on ammonium oxidizing process than the nitrite oxidizing process. These are the first evidences where a nitrifying sludge exposed to 2-CP are reported.

Effect of phenol and acetate addition on 2-chlorophenol consumption by a denitrifying sludge

Chlorophenols are widely distributed in the environment. Various strategies have been used to improve their biological elimination under anaerobic conditions; however, such information is still scarce. The aim of this study was to evaluate in batch assays the consumption of 2-chlorophenol (2-CP) by a denitrifying sludge and the influence of acetate or phenol as co-substrates in the 2-CP consumption. It was observed that phenol (69 and 92 mg phenol-C L−1) and acetate (60 and 108 mg acetate-C L−1) enhanced 2-CP consumption by the denitrifying sludge, increasing both the efficiency (up to 100%) and specific rate of 2-CP consumption. When phenol was added at 92 mg C L−1, the specific consumption rate of 2-CP increased 2.6 times with respect to the control lacking co-substrates, whereas with acetate (108 mg C L−1) the increase was 9.0 times. Acetate appeared to be a better co-substrate for 2-CP consumption, obtaining a specific consumption rate of 2.48±0.14 mg 2-CP-C g−1 VSS d−1 at 108 mg acetate-C L−1. The mass balance analysis indicated that the denitrifying sludge was able to simultaneously mineralize 2-CP, phenol or acetate (E2− CP, EPhenol, and EAcetate close to 100% [E=consumption efficiency], YHCO3− of 0.90±0.10 [Y=yield ]) and reduce nitrate to nitrogen gas (ENO3− of 100% and YN2 of 0.96±0.02). It was shown that the addition of co-substrates as phenol or acetate could be a good alternative for improving the elimination of chlorophenols from wastewaters by denitrifying sludges.

Qualitative and quantitative determination of a humic model compound in microbial cultures by cyclic voltammetry.

The humic model compound, anthraquinone–2,6–disulfonate (AQDS), was characterized and measured in microbial cultures by cyclic voltammetry (CV). Under the experimental conditions, the formal reduction potential (E°´) of the couple AQDS/AHQDS– was found to be of –0.520 V vs. SCE (standard calomel electrode) at pH value of 7.0. Control experiments showed that there were no interferences of the culture medium or the microbial consortium on the quantitative determination of the quinone. The linear equation E°´ = –0.294 – 0.032 pH was found, showing that the pH used (7.0–7.8) did not affect significantly the AQDS determination by CV and AHQDS– was the predominant hydroquinone formed. A calibration curve was obtained by plotting current response versus AQDS concentration with a linear correlation (r = 0.999) from 0.2 to 10 mM of AQDS. This technique was applied in a denitrifying culture to establish kinetic profiles for AHQDS– formation coupled to acetate and p–cresol oxidation. CV results showed that organic matter oxidation by the denitrifying sludge was stoichiometrically associated to AQDS reduction into AHQDS–. CV was shown to be a useful tool for monitoring oxidation/reduction processes of quinones occurring in complex microbial media.

Batch Nitrifying Cultures in Presence of Mixtures of Benzene, Toluene, and m-Xylene

Benzene, toluene, and m-xylene compounds in individual (5.0 ± 0.5 mg C l−1) and mixed solutions (2.5 ± 0.2 mg C l−1 for each one) in nitrifying batch cultures induced a decrease in the specific rates of NH4 + consumption (81 ± 6%) and NO3 − production (39-79%). However, after 24 h, ammonium consumption efficiency and conversion of consumed NH4 +-N into NO3 −-N were close to 100% and there was no significant accumulation of nitrite in the medium. After 24 h, no aromatic intermediate was detected in the cultures and 50% of the mixed compounds was converted to acetate. The following order of biotransformation was found: m-xylene > toluene > benzene. Transformation rate of m-xylene was 0.051 ± 0.005 g C (g protein-N h)−1 in individual and mixed solutions. When m-xylene was added, benzene was oxidized at a faster rate (0.051 ± 0.005 g C (g protein-N h)−1) whereas toluene at a slower rate (0.012 ± 0.002 g C (g protein-N h)−1).

p-Cresol mineralization and bacterial population dynamics in a nitrifying sequential batch reactor.

The ability of a nitrifying sludge to oxidize p-cresol was evaluated in a sequential batch reactor (SBR). p-Cresol was first transformed to p-hydroxybenzaldehyde and p-hydroxybenzoate, which were later mineralized. The specific rates of p-cresol consumption increased throughout the cycles. The bacterial population dynamics were monitored by using denaturing gradient gel electrophoresis (DGGE) and sequencing of DGGE fragments. The ability of the sludge to consume p-cresol and intermediates might be related to the presence of species such as Variovorax paradoxus and Thauera mechernichensis. p-Cresol (25 to 200mgC/L) did not affect the nitrifying SBR performance (ammonium consumption efficiency and nitrate production yield were close to 100% and 1, respectively). This may be related to the high stability observed in the nitrifying communities. It was shown that a nitrifying SBR may be a good alternative to eliminate simultaneously ammonium and p-cresol, maintaining stable the respiratory process as the bacterial community.

Simultaneous Elimination of Carbon and Nitrogen Compounds of Petrochemical Effluents by Nitrification and Denitrification

Human activities have resulted in the increase of nitrogen and carbon content in wastewater and groundwater affecting the environment (Bremmen, 2002). The average water consumption in Mexico is close to 0.25 m3/d, resulting in municipal and industrial wastewater generation between 168 and 232 m3/s, respectively. Only 12 and 20% of wastewater has received some treatment (Monroy et al., 2000). The increase of nitrogen compounds such as nitrate, nitrite and ammonium in superficial and groundwater has caused several environmental effects such as eutrophication, toxicity to aquatic organisms, loss of biodiversity (Galloway, 1998; Mateju et al., 1992; Schimel et al., 1996); and human health damages as methahemoglobinemia (Mateju et al., 1992; Morgan-Sagastumen et al., 1994); formation of nitrosoamines which are potentially carcinogenic compounds (Cerhan et al., 2001, as cited in Gonzalez-Blanco et al., 2011) and gastric cancer (Knobeloch et al., 2000; Ward et al., 2005). In the north of Gulf of Mexico, an important hypoxic zone has been detected where oxygen concentration is lower than 2 mg/l due to high nitrate discharges and eutrophication (Alexander et al., 2000; McIsaac et al., 2002). Nitrate concentrations between 7 and 156 mg/l (Anton & Diaz, 2000) and higher than 80 mg/l (Munoz et al., 2004; Pacheco et al., 2001) have been determined in aquifers of middle and south of Mexico, respectively. These nitrate concentrations are higher than the maximum levels established by Secretary of Environmental and Natural Resources (SEMARNAT), NOM-003-ECOL-1997 (15 and 40 mg total nitrogen/l) (Diario Oficial de la Federacion, 1998) and the United States Environmental Protection Agency (USEPA, 2007) (10 mg N-NO3-/l, 1 mg N-NO2-/l and 10 mg N-NH4+/l). Therefore, it is clear the need of applying effective wastewater treatments for reducing nitrogen contamination.