Buffer solution

Summary

A buffer solution (more precisely, pH buffer or hydrogen ion buffer) is an acid or a base aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications. In nature, there are many living systems that use buffering for pH regulation. For example, the bicarbonate buffering system is used to regulate the pH of blood, and bicarbonate also acts as a buffer in the ocean. Principles of buffering Buffer solutions resist pH change because of a chemical equilibrium between the weak acid HA and its conjugate base A−: When some strong acid is added to an equilibrium mixture of the weak acid and its conjugate base, hydrogen ions (H+) are added, and the equilibrium is shifted to the left, in accordance with Le Chatelier's principle. Because of this,

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An accuracy study of resistive methods for elliptic PDEs

William Van Rooij

We numerically study the resistive method for the numerical approximation of elliptic PDEs. In particular we focus on the resistive method for weakly setting solution values in specific subdomains or interfaces in the domain.

2016

Influence of pH on dehalorespiring consortia & first steps in the development of a methodology for acid neutralization in enhanced in situ bioremediation

Nicole Munz

Enhanced in situ bioremediation of chlorinated solvents is, especially in source zone remediation, often limited through the accompanied high acidification of groundwater. This acid built-up originating from the dechlorination itself and fermentation products, such as organic acids, presents a draw-back in the efficiency of dehalorespiring bacteria. It is therefore indispensable to develop and implement appropriate buffering strategies to ensure a constant transformation of toxic chloroethenes to the final non-toxic product ethene. In this study pH tolerance of different dehalorespiring bacteria was tested. The limiting acidic pH varies significantly among the consortia. The consortium SL23b4 (PCE to ethene) grows at all the examined pH (pH 5 to pH 7.5) with a pH optimum between 6 and 7 (depending on the dechlorination step), whereas Maroc 6b (PCE to ethene) was only found to grow at incubation pH of 7, and very slowly at pH 6.5. The consortium 2SO (cis-DCE to ethene) seems to be inhibited at acidic pH (no growth at pH 5 and 5.5) having a pH optimum above neutral pH. In most cases incomplete dechlorination was observed in cultures growing at acidic pH or below the pH optima, accumulating toxic intermediate dechlorination products as a consequence. Additionally, high rates of dechlorination were always accompanied by pH drops up to 1.2 pH units, giving evidence of high acid production. pH had also influence on the community composition. In the consortium SL23b4 a shift from a Dehalococcoides- (pH 6-7.5) to a Sulfurospirillum spp.- (pH5 -5.5) dominated community was observed. Consortium Maroc 6b showed significantly different community compositions due to a pH change of only 0.5 pH units. Dehalococcoides spp. and Dehalobacter spp. were dominating at pH 7 and pH 6.5, respectively. Consortium 2SO is dominated by Dehalococcoides spp. at all pH, although at pH 7 a much higher diversity seems to prevail, with the threefold number of OTUs. Tests on the tolerance of the dehalorespiring consortia for elevated PCE concentration showed similar patterns for the different dechlorination steps, as for pH cultures, and confirmed a generally higher sensitivity towards PCE and pH changes for the last steps of dechlorination. Conclusively, Dehalococcoides spp., the only known genera able to perform complete dechlorination, seem to be highly sensitive to pH and PCE changes. Further, abiotic experiments on silicate mineral dissolution, using forsteritic olivine, were performed to simulate the buffer capacity of the minerals for the acid production during microbial reductive dechlorination. In these experiments, the buffer capacity was insufficient to counterbalance the constant acid titration rate, resulting in a final pH between 3 and 4 (initial pH of 7). The formation of precipitates, such as iron oxide, and the re-precipitation of magnesium were observed in some cases, showing a decrease in magnesium concentrations.

2010

Chloroethenes are the most prevalent groundwater contaminants in developed countries. They accumulate in the environment as a dense non aqueous phase liquid (DNAPL) which acts as a long term source of contamination. Enhanced in situ anaerobic bioremediation of source zones by reductive dechlorination is a promising technology. Nevertheless, it can be hindered by two major problems: (1) toxicity of high concentrations of chloroethenes in source zones towards organohalide respiring bacteria, (2) groundwater acidification due to dechlorination and fermentation processes. In a first time, the present work aims to deepen the knowledge on a consortium which can dechlorinate at high concentrations of PCE. Then, pursue the investigation of the use of silicate minerals in biotic batch experiments with organohalide respiring bacteria to observe the effect of dissolution on dechlorination activity. Once these data collected, it will allow to do biotic experiments on continuous-flow columns, in order to get closer to field conditions. To this end, a first estimation of the ratio of mineral dissolution kinetics between batch and columns experiments was also performed during this master thesis. Microcosms and biomolecular experiments were conducted to demonstrate the ability of Point Mugu (PM) culture to be used in contaminated source zones. Community pattern analysis showed that it is mainly constituted of Dehalococcoides spp. and Desulfitobacterium spp. Influence of the electron donor choice and the PCE concentration and the community’s ability to grow on zwitterionic buffer was evaluated in microcosms experiments. Butanol and lactate can both be provided as soluble electron donor. As the literature suggests, its tolerance to saturated aqueous concentrations (0.9 mM) of PCE was attested. Moreover, this consortium was able to grow on zwitterionic buffer. These three advantages make PM consortium a good candidate for future planned column experiments. Acidic conditions are a significant issue as low pH inhibits bacterial activity. It is therefore essential to develop appropriate buffering strategies in contaminated zones to allow the transformation of toxic chloroethenes into ethene. The use of silicate minerals as a buffer agent was investigated in microcosms with several consortia. With SL23-6 consortium, a complete dechlorination of 0.25 mmol of PCE to cis-DCE was performed in 50 mL batches containing 4 g of olivine, diopside and fayalite (fraction 50-100 μm) whereas the same amount of andradite and nepheline inhibited dehalogenating activity. Five minerals were tested on SL23-b4 consortium (diopside, fayalite, grossular, andradite and forsterite). They all presented the similar pattern of dechlorination with an inhibition characterized by an accumulation of cis-DCE and no production of ethene. All silicate minerals tested proved their ability to keep pH in optimal ranges if they are suitably dosed. Models were implemented from these experiments with the software PHREEQC to allow future predictions on buffering behaviors of these minerals during dechlorination process. Furthermore, abiotic experiment on fayalite anaerobic dissolution was performed in porous medium with a constant inflow medium at pH 4.5 and a hydraulic residence time of 2.3 days. In those conditions, mineral dissolution was 20 to 100 times slower in column than in head-over-end stirred reactor.

2011

Influence of pH on dehalorespiring consortia & first steps in the development of a methodology for acid neutralization in enhanced in situ bioremediation

Nicole Munz

2010

2011