The study of the removal conditions of free cyanide from wastewaters by layered double hydroxide through experimental design technique, kinetic and equilibrium studies

The wastewaters of the mining and metallurgical activities must be treated before discharging into the environment due to the highly toxicity of cyanide. One of the most efficient technologies for the removal of cyanide from wastewaters is adsorption. In this research, layered double hydroxides (LDHs) known as anionic clays was synthesized by co-precipitation process using a 3:1 ratio of magnesium nitrate and aluminum nitrate and used for removal of cyanide from aqueous solutions. The chemical and mineralogical compositions of LDH were investigated using XRD, XRF and Scanning Electron Microscopy (SEM) and Wavelength Dispersive X-ray (WDX) analysis methods. The largest particle size of LDH was determined about 4 nm using Scherrer equation which indicates that the synthesized LDH is in the range of nano-sized materials. The effect of temperature, absorbent dosage, stirring speed and pH on the adsorption capacity of LDH for cyanide was investigated using experimental design method by DX7 software. The optimum conditions were determined to be temperature=60oC, adsorbent weight=1.5 g, rotation speed=500 rpm and pH=9.52. In this condition, the maximum adsorption capacity of synthesized LDH for cyanide was 73.80 mg/g. The equilibrium data and the kinetic data were best modeled by Langmuir isotherm model and pseudo-second-order kinetic model, respectively. The rate limiting step was determined to be intraparticle by adopting the adsorption data with Weber and Morris model and the adsorption of cyanide onto LDH is occurred through ion-exchange mechanism due to chemical composition and mineralogical structure of LDH. LDH synthesized using salts of industrial purity also showed high potential for removal of cyanide ions from the solution. Also, the treatment of a real wastewater by LDH showed that with an absorbent dose of 20 g /L, more than 84% of the cyanide can be removed from the wastewater.