Removal of fluoride and hydrated silica from underground water by electrocoagulation in a flow channel reactor.

This paper concerns simultaneous removal of fluoride and hydrated silica from groundwater (4.08 mg L-1 fluoride, 90 mg L-1 hydrated silica, 50 mg L-1 sulfate, 0.23 mg L-1 phosphate, pH 7.38 and 450 μS cm-1 conductivity) by electrocoagulation (EC), using an up-flow EC reactor, with a six-cell stack in a serpentine array, opened at the top of the cell to favor gas release. Aluminum plates were used as sacrificial electrodes. The effect of current density (4 ≤ j ≤ 7 mA cm-2) and mean linear flow rate (1.2 ≤ u ≤ 4.8 cm s-1), applied to the EC reactor, on the elimination of fluoride and hydrated silica was analyzed. The removal of fluoride followed the WHO guideline (<1.5 mg L-1), while the hydrated silica was abated at 7 mA cm-2 and 1.2 cm s-1, with energy consumption of 2.48 kWh m-3 and an overall operational cost of 0.441 USD m-3. Spectroscopic analyses of the flocs by XRD, XRF-EDS, SEM-EDS, and FTIR indicated that hydrated silica reacted with the coagulant forming aluminosilicates, and fluoride replaced a hydroxide from aluminum aggregates, while sulfates and phosphates were removed by adsorption process onto the flocs. The well-engineered EC reactor allowed the simultaneous removal of fluoride and hydrated silica.

Castañeda LF ，Coreño O ，Nava JL ，Carreño G ... -  《-》

Removal of hydrated silica, fluoride and arsenic from groundwater by electrocoagulation using a continuous reactor with a twelve-cell stack.

The simultaneous removal of hydrated silica, fluoride and arsenic from deep well water (hydrated silica 72 mgL-1, fluoride 4.4 mgL-1, arsenic 106.2 μgL-1, sulfate 50 mgL-1, phosphate 0.99 mgL-1, pH = 8.2 and conductivity 659 μScm-1) by electrocoagulation (EC) was investigated. The EC was performed in a continuous electrochemical reactor using aluminum plates as sacrificial anodes coupled directly to a jar test device. The effect of current density (4 ≤ j ≤ 8 mA cm-2) and mean linear flow rates in the EC reactor (0.057 ≤ u ≤ 0.57 cm s-1) on the hydrated silica, fluoride, and arsenic removal efficiencies was analyzed. The abatement of hydrated silica was obtained at 8 mA cm-2 and 0.057 cm s-1, while the residual concentrations of F- and As after the same electrolysis were 0.19 mg L-1 and 9.8 μg L-1, satisfying the WHO guidelines for F- (≤1.5 mg L-1) and As (≤10 μg L-1). Spectroscopic analyses on aluminum flocs revealed that they are predominantly composed of aluminum silicates. Arsenates adsorb on aluminum flocs and fluoride replaces a hydroxyl group from aluminum aggregates.

Rosales M ，Coreño O ，Nava JL 《-》

Arsenic and fluoride removal from groundwater by electrocoagulation using a continuous filter-press reactor.

We investigated simultaneous arsenic and fluoride removal from ground water by electrocoagulation (EC) using aluminum as the sacrificial anode in a continuous filter-press reactor. The groundwater was collected at a depth of 320 m in the Bajío region in Guanajuato Mexico (arsenic 43 µg L(-1), fluoride 2.5 mg L(-1), sulfate 89.6 mg L(-1), phosphate 1.8 mg L(-1), hydrated silica 112.4 mg L(-1), hardness 9.8 mg L(-1), alkalinity 31.3 mg L(-1), pH 7.6 and conductivity 993 µS cm(-1)). EC was performed after arsenite was oxidized to arsenate by addition of 1 mg L(-1) hypochlorite. The EC tests revealed that at current densities of 4, 5 and 6 mA cm(-2) and flow velocities of 0.91 and 1.82 cm s(-1), arsenate was abated and residual fluoride concentration satisfies the WHO standard (CF < 1.5 mg L(-1)). Spectrometric analyses performed on aluminum flocs indicated that these are mainly composed of aluminum-silicates of calcium and magnesium. Arsenate removal by EC involves adsorption on aluminum flocs, while fluoride replaces a hydroxyl group from aluminum aggregates. The best EC was obtained at 4 mA cm(-2) and 1.82 cm s(-1) with electrolytic energy consumption of 0.34 KWh m(-3).

Guzmán A ，Nava JL ，Coreño O ，Rodríguez I ，Gutiérrez S ... -  《-》

Abatement of hydrated silica, arsenic, and coexisting ions from groundwater by electrocoagulation using iron electrodes.

The paper deals with the removal of arsenic (As), hydrated silica (HS), and coexisting ions from groundwater by electrocoagulation (EC) using a laboratory-scale up-flow reactor with sacrificial iron anodes (1018 steel, >99% wt. Fe). Natural groundwater, taken in the northern region of Mexico, contained 25.7 μg L-1 As, 237.8 mg L-1 HS, 1.43 mg L-1 F-, 45.0 mg L-1 SO42-, 0.61 mg L-1 PO43-, pH 8.62, and 577 μS cm-1 conductivity. The effect of current densities (4≤j≤8 mA cm-2) and mean linear flow velocities (1.1≤u≤4.6 cm s-1) on the pollutant's removal was systematically addressed. The best EC trial that showed the lowest overall cost and complied with the WHO guideline (<10 μg L-1 As) was obtained at j = 6 mA cm-2 and u = 2.3 cm s-1, reaching residual concentrations of As and HS of 4.6 μg L-1 and 150.0 mg L-1, respectively. A large amount of HS was found after electrolysis; therefore, a second EC was applied to reduce the HS concentration further. This time, residual concentrations of HS and As of 37.0 mg L-1 and 1.2 μg L-1 were obtained, with electrolytic energy consumption and overall cost of EC of 0.872 kWh m-3 and 0.178 USD m-3, respectively. XRF, EDS, XRD, and FTIR analyzes on flocs indicate that hydrated silica reacts with iron, forming iron silicates with divalent cations as flocs. Arsenic and PO43- are abated by adsorption on flocs. The modest removal of F- and SO42- (44% and 12%, respectively) is due to its weak adsorption on flocs.

Valentín-Reyes J ，Trejo DB ，Coreño O ，Nava JL ... -  《-》

Concurrent arsenic, fluoride, and hydrated silica removal from deep well water by electrocoagulation: Comparison of sacrificial anodes (Al, Fe, and Al-Fe).

Some studies have reported the removal of As (As) and fluoride (F-) using different sacrificial anodes; however, they have been tested with a synthetic solution in a batch system without hydrated silica (SiO2) interaction. Due to the above, concurrent removal of As, F-, and SiO2 from natural deep well water was evaluated (initial concentration: 35.5 μg L-1 As, 1.1 mg L-1F-, 147 mg L-1 SiO2, pH 8.6, and conductivity 1024 μS cm-1), by electrocoagulation (EC) process in continuous mode comparing three different configurations of sacrificial anodes (Al, Fe, and Al-Fe). EC was performed in a new reactor equipped with a small flow distributor and turbulence promoter at the entrance of the first channel to homogenize the flow. The best removal was found at j = 5 mA cm-2 and u = 1.3 cm s-1, obtaining arsenic residual concentrations (CAs) of 1.33, 0.45, and 0.77 μg L-1, fluoride residual concentration ( [Formula: see text] ) of 0.221, 0.495, and 0.622 mg L-1, and hydrated silica residual concentration ( [Formula: see text] ) of 21, 34, and 56 mg L-1, with costs of approximately 0.304, 0.198, and 0.228 USD m-3 for the Al, Fe and Al-Fe anodes, respectively. Al anode outperforms Fe and Al-Fe anodes in concurrently removing As, F- and SiO2. The residual concentrations of As and F- complied with the recommendations of the World Health Organization (WHO) (As < 10 μg L-1 and F- < 1 mg L-1). The spectroscopic analyses of the Al, Fe, and Al-Fe aggregates showed the formation of aluminosilicates, iron oxyhydroxides and oxides, and calcium and sodium silicates involved in removing As, F-, and SiO2. It is concluded that Al would serve as the most suitable sacrificial anode.

Castañeda LF ，García I ，Nava JL ，Coreño O ... -  《-》