Study On Calcium Removal In Seawater By Electrosorption

In the process of seawater desalination, calcium ions are easy to sacle when the temperature of seawater or concentration of calcium ions are high. Sacling reduces the efficiency of desalination and results in wastes of energy. Electrosorption is a new technology which has a good performance for deionization from seawater. The advantages of electrosorption get widely attention in water treatment, electrosorption with some theoretical and practical basis has the broad application prospects.This paper applied the technology in decalcification, explored the the feasibility and regularity of electrosorption in decalcification.First of all, the LSX (low-silica X) molecular sieves was synthesized using NaAlO₂ and solbule glass (27.9% SiO₂) as raw materials, KOH and NaOH as the alkali source. The exchange capacities of those molecular sieves in pure calcium solution and seawater are 66.83mg/g and 31.06 mg/g respectively, and its selectivity coefficients to Ca²⁺ in the solution of Ca²⁺/Na⁺, Ca²⁺/K⁺ and Ca²⁺/Mg²⁺ are 744.45, 733.36 and 22.24 respectively. The LSX molecular sieves synthesized has a high selectivity to Ca²⁺, So it can be used to adsorb Ca²⁺ from seawater . In order to avoid molecular sieve coacervate in the electrode materials, the LSX molecular sieve was modified using silicane coupling agent, effects of the factors including amount of coupling agent and reaction time were studied on the surface modification.This paper attempted to add a certain proportion modified molecular sieve to the activated carbon to make composite electrodes, tried to use the synergy between activated carbon and modified molecular sieve to enhance the electrosorption capacity and selectivity to Ca²⁺ for electrodes. Effects of the factors including amount of molecular sieve, voltage, space between the electrodes and electrode mass on the decalcification were investigated. At the same time, the electrosorption of composite electrodes and the common activated carbon electrodes to Ca²⁺ in different mixed solution was compared. The results show that, in the solution of Ca²⁺/Na⁺, Ca²⁺/Mg²⁺ and Ca²⁺/K⁺, comparing the composite electrodes with common activated carbon electrodes, the electrosorption capacity and selectivity to calcium ions are increased; In the artificial or natural seawater solution, the electrosorption capacity of composite electrodes to Ca²⁺ is about 2 times that of the activated carbon electrodes .The electrosorption experiments were conducted at different voltages and different concentrations of CaCl2 solution. The results show that electrosorption to Ca²⁺ on composite electrodes follow Langmuir adsorption isotherm, indicating monolayer adsorption; n values of Freundlich adsorption isotherm are between 0.35 and 0.5, indicating ions adsorption on electrode is easy.Our study suggests that: After adding molecular sieves, the electrosorption of the electrode to Ca²⁺ in seawater has doubled. the electrosorption capacity of the composite electrodes is about one-sixth of LSX, molecular sieves remove calcium ions by ion exchange or adsorption property of the sieves, their regeneration need chemical reagents. The regeneration of electrodes do not need to consume additional chemical reagents, only consuming electricity, which is an important advantage. Overall, decalcification in seawater by electrosorption technology has good prospects.