Hydrophobic molecules (and their degradation products) are the most studied compounds due to their persistence in the environment. They also play a role in biogeochemical reactions in the context of radioactive waste disposal. In contrast, hydrophilic compounds are less documented while they may complex metals (including radionuclides due to their functional groups), and contribute to their transfer in the natural environment. Such compounds can be produced by the degradation of cellulosic material present in the waste (Glaus et al., 1999). Abiotic and biotic degradation of these compounds may occur. Microbes will be present in the near field of a geological disposal facility, and, depending on the geology, in the host rock itself. Microbial metabolism has the potential to play a significant role on the evolution of biogeochemical processes in such conditions (Rizoulis et al., 2012). Recently, biodegradation of isosaccharinic acid under alkaline conditions has been studied in view to its capacity to complex metallic ions and radionuclides (Kuippers et al., 2015 and references therein). Gluconic acid may also be present in radioactive waste. It may present the same complexing properties than isosaccharinic acid, but little is known about its fate under alkaline conditions. This study focuses on the (bio)degradation of gluconic acid under different alkaline conditions in order to evaluate the role of electron acceptor in the presence and absence of cement. Microorganisms were isolated from sediment extracts and cultured at pH~10 under anaerobic conditions. Three inocula were prepared, (i) without electron acceptor, (ii) with sulfate or (iii) iron (III) as electron acceptors, and then transferred into batch experiments. Four conditions were studied: fermentation, with SO4 or with Fe(III) in the absence of cement, and a SO4/Fe(III) mixture in the presence of cement. Initial gluconic acid concentration in batch was 2mM. Abiotic experiments were also performed in the same conditions. Experiments were done in triplicate. Samples were analysed after 1 day, 9 days, 19 days, 71 days and 135 days of incubation. Major elements (Ca, Na, Mg, K, Cl, NO3 and SO4), inorganic and organic carbon in solution and gas (e.g., CO2, CH4, H2S, O2) in headspace were analysed. pH and Eh were also measured in solution. Gluconic acid and its transformation products (e.g., lactate, pyruvate, …) were analyzed by HPLC coupled to a pulsed amperometric detector and by gas chromatography coupled to mass spectrometry (GC/MS). During biotic experiments, a decrease in pH was observed after 1 and 9 days of incubation, whereas it was stable in abiotic conditions suggesting a role of microbial activity in this trend. The decrease was most pronounced in SO4 condition (pH decreased from 10 to 6) while it was low in batch containing cement, due to the buffer effect of the cement. Note that pH was 10.5 in the presence of cement after 71 and 135 days of incubation. Concentration of dissolved organic carbon (DOC) decreased and dissolved inorganic carbon (DIC) increased in all biotic experiments, suggesting biodegradation of gluconic acid. This degradation seems to be faster in experiments with sulfate reducing bacteria and with cement. In abiotic conditions, a decrease of DOC was only noticed in the experiments with cement. This decrease combined with the low concentration of DIC could indicate sorption phenomena on the cement matrix. Molecular analyses show the same trends as DOC concentrations with a rapid decrease in the concentration of gluconic acid in experiments with Fe(III), SO4 and cement. Some transformation products (TPs), such as pyruvic and lactic acids, appear in all biotic experiments, with a production of TPs more important in the batch with SO4. CO2 and CH4 were also measured in headspace of all experiments attesting the degradation of gluconic acid. In abiotic cement experiment, no TPs are detected, confirming the sorption of gluconic acid on cement. Microbial degradation of gluconic acid is possible under alkaline conditions. The transformation is favoured in the presence of electron acceptor such as Fe(III) or SO4. Bacteria using SO4 seem to be more efficient in these conditions. Cement takes part in the decrease of gluconic acid concentration by sorbing this compound and can induce competition between sorption and biotransformation of gluconic acid. References Glaus et al., 1999. Degradation of cellulosic materials under the alkaline conditions of a cementitious repository for low and intermediate level radioactive waste part I: Identification of degradation products. Analytica Chimica Acta, 398, pp. 111–122. Kuippers et al.., 2015. Microbial degradation of isosaccharinic acid under conditions representative for the far field of radioactive waste disposal facilities. Mineralogical Magazine 79, pp. 1443-1454. Rizoulis et al., 2012. The potential impact of anaerobic microbial metabolism during the geological disposal of intermediate-level waste. Mineralogical Magazine 76, pp. 3261-3270