Fluoride removal from natural volcanic underground water by an electrocoagulation process: Parametric and cost evaluations.

Excessive fluoride content in groundwater can cause serious risks to human health, and sources of groundwater intended for human consumption should be treated to reduce fluoride concentrations down to acceptable levels. In the particular case of the island of Tenerife (Canary Islands, Spain), the water supply comes mainly from aquifers of volcanic origin with a high content of fluorides that make them unacceptable for human consumption without prior conditioning treatment. The treatments that generate a high rejection of water are not acceptable because water is a scarce natural resource of high value. An electrocoagulation process was investigated as a method to treat natural groundwater from volcanic soils containing a hazardously high fluoride content. The operating parameters of an electrocoagulation reactor model with parallel plate aluminum electrodes were optimized for batch and continuous flow operations. In the case of the batch operation, acidification of the water improved the removal efficiency of fluoride, which was the highest at pH 3. However, operation at the natural pH of the water achieved elimination efficiencies between 82 and 92%, depending on the applied current density. An optimum current density of 5 mA/cm2 was found in terms of maximum removal efficiency, and the kinetics of fluoride removal conformed to pseudo-second-order kinetics. In the continuous-flow operation, with the optimal residence time of 10 min and a separation of 0.5 cm between the electrodes, it was observed that the current density that would be applied would depend on the initial concentration of fluoride in the raw water. Thus, an initial fluoride concentration of 6.02 mg/L required a current density >7.5 mA/cm2 to comply with the legal guidelines in the product water, while for an initial concentration of 8.98 mg/L, the optimal current density was 10 mA/cm2. Under these operating conditions, the electrocoagulation process was able to reduce the fluoride concentration of natural groundwater to below 1.5 mg/L according to WHO guidelines with an operating cost between 0.20 and 0.26 €/m3 of treated water.

Electrochemical characteristics of bioresorbable binary MgCa alloys in Ringer's solution: Revealing the impact of local pH distributions during in-vitro dissolution.

Biodegradable magnesium-calcium (MgCa) alloy is a very attractive biomaterial. Two MgCa alloys below the solid solubility of Ca were considered, as to solely investigate the effect of Ca content on the behavior of magnesium and the pH changes associated to metal dissolution. X-ray diffraction analysis and optical microscopy showed that both Mg-0.63Ca and Mg-0.89Ca alloys were solely composed of α(Mg) phase. Degradation characteristics and electrochemical characterization of MgCa alloys were investigated during exposure to Ringer's solution at 37 °C by electrochemical impedance spectroscopy and scanning electrochemical microscopy. The impedance behavior showed both capacitive and inductive features that are related to the alloy charge transfer reaction and the relaxation of the absorbed corrosion compounds, and can be described in terms of an equivalent circuit. Scanning electron microscopy (SEM) was employed to view the surface morphology of the MgCa samples after 1 week immersion in Ringer's solution showing extensive precipitation of corrosion products, whereas the substrate shows evidence of a non-uniform corrosion process. Energy dispersive analysis showed that the precipitates contained oxygen, calcium, magnesium and chlorine, and the Mg:Ca ratios were smaller than in the alloys. Scanning electrochemical microscopy (SECM) was used to visualize local pH changes associated to these physicochemical processes with high spatial resolution. The occurrence of pH variations in excess of 3 units between anodic and cathodic half-cell reactions was monitored in situ.

Ex vivo antimicrobial efficacy of the EndoVac system plus photodynamic therapy associated with calcium hydroxide against intracanal Enterococcus faecalis.

AIM: To evaluate the ex vivo efficacy of the EndoVac system and photodynamic treatment (PDT) as adjuncts to chemomechanical debridement associated with calcium hydroxide (CaOH2 ) in reducing the levels of intracanal Enterococcus faecalis. METHODOLOGY: One hundred and twenty-five sterile premolar teeth were conventionally accessed, prepared and then contaminated with E. faecalis (ATCC 29212) for 30 days. Teeth were randomly divided into 4 groups: Control (chemomechanical debridement with conventional irrigation); Endovac (chemomechanical debridement with EndoVac system); PDT (chemomechanical debridement with conventional irrigation and PDT) and Endovac+PDT (chemomechanical debridement with EndoVac and PDT). The irrigants used in all groups were 5.25% sodium hypochlorite and 17% EDTA. After treatment, an intracanal dressing (CaOH2 ) was applied in all canals for 7 days. Samples were obtained before (T1) and after the therapeutic procedures (T2) and, after intracanal medication (T3), plated onto BHI media and incubated (37 °C, 48 h) to determine the colony-forming units (CFU mL(-1) ). RESULTS: The overall mean cell counts (CFU mL(-1) ) of E. faecalis were high at the initial contamination (T1). A significant reduction (P < 0.05) of E. faecalis mean counts was observed in all groups from baseline (T1) to both post-therapy samplings (T2 and T3); no differences amongst the groups were detected. No significant change in bacterial counts from T2 to T3 was detected. CONCLUSION: The adjunctive use of the EndoVac system and the photodynamic treatment, in combination or not, was as effective as the conventional chemomechanical debridement associated with CaOH2 on reducing the counts of intracanal E. faecalis.

The effect of temperature on the nucleation of corrosion pits on titanium in Ringer's physiological solution.

This paper describes the effect of temperature on the nucleation of corrosion pits on titanium microelectrodes in Ringer's physiological solution. The results are shown for potentials far below the pitting potential, and describe breakdown of passivity with no permanent propagation of pits. Nucleation events could be observed at all the temperatures used, although they were very rare events at 20 degrees C. The frequency of breakdown rises significantly with increase in temperature. Examples are shown of current transients due to both pit nucleation and to metastable pit propagation, the latter being rare events. Analysis shows that these events constitute a significant fraction of the passive corrosion rate of titanium.