AsIII oxidation by Thiomonas arsenivorans in up-flow fixed-bed reactors coupled to As sequestration onto zero-valent iron-coated sand.

The combined processes of biological As(III) oxidation and removal of As(III) and As(V) by zero-valent iron were investigated with synthetic water containing high As(III) concentration (10 mg L(-1)). Two up-flow fixed-bed reactors (R1 and R2) were filled with 2 L of sieved sand (d = 3 ± 1 mm) while zero-valent iron powder (d = 76 µm; 1% (w/w) of sand) was mixed evenly with sand in R2. Thiomonas arsenivorans was inoculated in the two reactors. The pilot unit was studied for 33 days, with HRT of 4 and 1 h. The maximal As(III) oxidation rate was 8.36 mg h(-1) L(-1) in R1 and about 45% of total As was removed in R2 for an HRT of 1 h. A first order model fitted well with the As(III) concentration evolution at the different levels in R1. At the end of the pilot monitoring, batch tests were conducted with support collected at different levels in R1. They showed that bacterial As(III) oxidation rate was correlated with the axial length of reactor, which could be explained by biomass distribution in reactor or by bacterial activity. In opposition, As(III) oxidation rate was not stable in R2 due to the simultaneous bacterial As(III) oxidation and chemical removal by zero-valent iron and its oxidant products. However, a durable removal of total As was realized and zero-valent iron was not saturated by As over 33 days in R2. Furthermore, the influence of zero-valent iron and its oxidant corrosion products on the evolution of As(III)-oxidizing bacteria diversity was highlighted by the molecular fingerprinting method of PCR-DGGE using aoxB gene as a functional marker of aerobic As(III) oxidizers.

Cadmium biosorption by ozonized activated sludge: the role of bacterial flocs surface properties and mixed liquor composition.

Cadmium uptake by activated sludge was studied following modifications of sludge composition and surface properties induced by ozone treatment. Ozone leads to the solubilization of sludge compounds as well as their mineralization, especially humic like substances. Small particles were formed following floc disintegration, leading to a decrease of average floc size. The study of surface properties underlined the mineralization as the number of surface binding sites decreased with the increase of ozone dose. Depending on ozone dose, cadmium uptake by activated sludge flocs was either increased or decreased. Different mechanisms were involved: below 10 mg O(3)/g TS, the increase of floc specific surface area following floc size decrease as well as the release of phosphate ions yielded an increase by 75% of cadmium uptake, due to the better availability of biosorption sites and the increase of precipitation. Inversely, at higher ozone doses, the number of biosorption sites decreased due to oxidation by ozone. Moreover, dissolved organic matter concentration increased and provided ligands for metal complexation. Cadmium uptake was therefore limited for ozone doses ranging from 10 to 16.8 mg O(3)/g TS.

Fate of cadmium in activated sludge after changing its physico-chemical properties by thermal treatment.

The effect of thermal treatment of activated sludge on cadmium uptake was investigated in respect with potential modifications of floc surface properties. Soluble fraction biochemical composition and floc size measurements evidenced floc destructuration and the release of (in)organic ligands in solution. Characterization of sludge samples by potentiometric titrations and IR spectra showed the transfer of functional groups from particulate to soluble fraction as well as the higher availability of phosphate groups originating from cell membrane phospholipids after thermal treatment. Batch biosorption tests demonstrated that cadmium uptake was highly affected by sludge modifications due to thermal treatment. For temperatures below 95 degrees C, floc size decrease allowed a better availability of binding sites, resulting in a higher sorption capacity. At temperatures above 95 degrees C, the effect of released soluble ligands and of the lower total number of surface functional groups limited cadmium uptake. Uptake mechanisms were also affected by sludge thermal treatment as surface complexation involving ion exchange tends to become predominant over precipitation.