This study presents an electrocoagulation-flocculation process to remove the sulphate ions contained in water from abandoned mines in the state of Guanajuato by electrocoagulation (EC) using aluminum as the sacrificial anode in a continuous multi-stage electro-coagulation reactor (Figure 1) with a flocculator-clarifier. The concentration of sulfates was determined by an acid-base equilibrium method and a turbidimetric method at a wavelength of 420 nm, respectively. The best sulfate removal was approximately 58%.
Introduction
Electrocoagulation is an efficient process to remove sulfates contained in water. This process generates aluminum hydroxides [1,2,3]. The principal reaction on the anode, the electro dissolution of aluminum generates aluminum ions (Al3+); therefore, the aluminum ions are transformed to aluminum hydroxides (Al(OH)3(s)).
The sulfate removal mechanism by EC is carried through by adsorption in which aluminum hydroxides flocs entrap an SO42- ion [1,2].
Methodology
The electrocoagulation was achieved in a continuous multi-stage electrochemical reactor without re-circulation, where the coagulant is produced in a inner way of the reactor. Each resulting solution, after passing EC reactor, was immediately passed to the flocculation and clarification unit (jar test), this solution was mixed at slow speed (30 rpm) for 15 min to form flocs. Then the aggregates were allowed to precipitate in static solution for 1 hr. Subsequently the concentration of sulfate ions contained in the resulting solution was determined by a turbidimetric method. The EC process was performed at current densities of 4, 5 and 6 mA cm–2 at each mean volumetric flow rates.
For he characterization of the multi-stage reactor, the theoretical aluminum dose in the EC reactor is that given for CAl3+ = (N) (j*L*MW/z*F*S* ur)*106, [1,2,3].
where j is the current density (A cm–2), L is the length of one channel (cm), MW is the molecular weight of aluminum (26.98 g mol–1), Z is the number of electrons exchanged (z=3), F is the Faraday constant (96485 C mol–1), S is the channel width (cm), ur is the mean linear flow rate (cm s–1), N is the number of channels (N=6), and 106 is a conversion factor use to obtain the aluminum concentration in mg L–1.
the energy consumption by electrolysis (Es,vol). Es,vol which were evaluated by means of Es,vol = (Ecell*I)/(3.6*S*B* ur), [1,2,3].
Where Es,vol is the energy consumption, I is the current intensity during electrolysis (C s-1), Ecell is the cell potential (J C-1), B is the channel height (cm), and 3.6 is a conversion factor used to obtain Es,vol in units of kWh m-3.
Results
The water used to achieved this study was collected from the Nopal Mine located in the City of Guanajuato, the characteristics of the water are the following: pH = 8.1, conductivity = 3.72 mS cm-1, sulfate concentration: 4292 mg L-1
The best removal of sulfate ion in terms of energy consumption was obtained at a linear speed of 7.41 cm s-1 with 0.049 kWh m-3 the concentration decreased from 4292 to 1813.2 mg L-1 with a sulfate removal % of 57.7, the energy consumption at a linear speed of 3.7 cm s-1 was of 0.1064 kWh m-3 with a sulfate removal % of 31.2, and finally the energy consumption at a linear speed of 5.55 cm s-1 was of 0.0668 kWh m-3 with a sulfate removal % of 38.2. The concentration of aluminum for each linear speed (3.7, 5.55, 7.42 cm s-1) was of 25.32, 13.39 and 8.92 mg L-1 respectively.
Conclusions
The best removal of sulfate ion in terms of energy consumption was obtained at a linear speed of 7.41 cm s-1 with 0.049 kWh m-3, where the sulfate ion removal was of 57.7%, having an energy consumption inversely proportional to the linear speed. Using this type reactor is posible to remove a singnificant quatity of sulfates, but not sufficient to meet the standard concentration for a the sulfate NOM-127-SSA1-1994, 400 mg L-1 regulation compliance.
References
[1] Del Angel P., Carreño G., Nava J.; “Removal of Arsenic and Sulfates from an Abandoned Mine Drainage by Electrocoagulation. Influence of Hydrodynamic and Current Density”; Int. J. Electrochem. Sci., 9 (2014) 710 - 719
[2] Sandoval M. Nava J. Carreño G., Sulfate Ions Removal from an Aqueous Solution Modeled on an Abandoned Mine by Electrocoagulation Process with Recirculation, Int. J. Electrochem. Sci., 12 (2017) 1318 – 1330.
[3] Flores O.J., Nava J.L., Carreño G., Elorza E., Martinez F., 2013. Arsenic removal from groundwater by electrocoagulation in a pre-pilot continuous filter press reactor. Chemical Engineering Science, 97, 1-6.