Adsorption Of Sodium Lignosulfonate From Aqueous Solution By Using In-situ Formed Magnesium Hydroxide Particles

Sodium lignosulfonate (SLSN) is a conventional anionic surfactant, which is usually used as oil-displacing agent for oilfield production to enhance oil recovery. If there are a large number of surfactants in the water body, it will cause short-term or long-term changes for the ecological system and do great harm to the aquatic animals and plants and humans. In this paper, in-situ formed magnesium hydroxide (Mg(OH)2) nanoparticles by soluble magnesium salts under alkaline conditions as adsorbents,which were used to remove simulated wastewater containing SLSN. The in-situ formed magnesium hydroxide particles had a small size, large adsorptive surface area and positive surface charge, which enables them effectively to attract negatively charged SLSN by the strong electrostatic attraction. The effects of adsorption parameters such as pH, magnesium chloride concentration, contact time, initial SLSN concentration, temperature and coexisting anions on adsorption properties were investigated in this paper. The Zeta potential of magnesium hydroxide and sodium lignosulfonate as a function of pH were measured in order to ensure the Isoelectric Point (IEP). The Mg(OH)2 particles before and after the adsorption of SLSN were characterized in terms of morphology, structure, and surface property by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier infrared spectroscopy (FT-IR), and Zeta potential measurements. Furthermore, the adsorption mechanism was discussed combining Visual MINTEQ, which is a chemical equilibrium simulation software. The experiment results showed that:(1) The in-situ formed Mg(OH)2 particles can be obtained by Ca(OH)2, besides NaOH, which can achieve equal good removal efficiency. Besides, Ca(OH)2 is cheap and available compared with NaOH, leading to the decrease of the cost in the treatment of actual industrial wastewater.(2) X-ray diffraction analysis indicated the in-situ formed Mg(OH)2 particles with high purity, low baseline and sharp peak. Furthermore, because SLSN molecules were adsorbed on Mg(OH)2 particles, the density of diffraction peaks became slightly weaker after adsorption.(3) Compared with in-situ formed Mg(OH)2, the FT-IR spectrum of Mg(OH)2 after adsorption of SLSN occurred bending vibration of the characteristic peak S=O and -SO32- groups, which further confirmed that SLSN molecules were adsorbed on Mg(OH)2 particles.(4) The results showed that the in-situ formed Mg(OH)2 can significantly remove SLSN molecules. MgCl2 concentration?solution pH?contact time and temperature had great effects on the removal of SLSN. The optimum pH was 10.5. The appropriate MgCl2 concentration was 2.0 g/L. Adsorption equilibrium reached at 90 s and removal efficiency can be above 90%.(5) Visual MINTEQ chemical equilibrium simulation software analyzed that the change of pH value will result in different morphologies of the soluble magnesium salts such as Mg2+, MgOH+, and Mg(OH)2. Due to the change of surface charge, the different morphologies of the soluble magnesium salts play an important role to the adsorption of SLSN.(6) Fitting the experimental data with Langmuir and Freundlich isotherm models, the results indicated that the Freundlich isotherm was the best choice to describe the adsorption behavior. The adsorption of SLSN on Mg(OH)2 nanoparticles was multilayer adsorption. The adsorption thermodynamic studies showed that the adsorption reaction of SLSN on Mg(OH)2 nanoparticles was exothermic in nature and the adsorption was less favorable at higher temperatures.(7) Desorption experiment showed that the structure of SLSN molecules after desorption had no change and thus SLSN can be recycled.(8) Wastewater generally contains common anions such as NO3-, HCO3-, H2PO4-, and SO42-, and the effects of the common anions on the adsorption efficiency of SLSN were investigated. Results showed that there was no significant change in SLSN adsorption on Mg(OH)2 after adding competing anions of NO3-, HCO3-, and H2PO4-, while SO42- produced a slight change in SLSN adsorption on Mg(OH)2. The results indicated that the in-situ formed Mg(OH)2 nanoparticles had high adsorption selectivity towards SLSN and coexisting anions had no significant effects on the removal of SLSN.