Adsorption Of Heavy Metals By Carbonate Hydroxylapatite

Several industrial wastewater streams may contain heavy metals such as Cr, Cu, Pb, Zn, Ni, etc. including the waste liquids generated by metal finishing or the mineral processing industries. The toxic metals, probably existing in high concentrations must be effectively treated/removed from the wastewaters. If the wastewater were discharge into the sewerage system may effect negatively the subsequent biological wastewater treatment.Previous works also mentioned that the synthetic CHAP had higher efficiency in the removal of heavy metals (Pb²⁺, Cd²⁺, Cu²⁺, Hg²⁺) than HAP. In this study present a new process for CHAP synthesis by using eggshell waste, historically a waste byproduct from the food products industry, for decrease the cost of sorbent. Carbonate hydroxylapatite samples were prepared as follows: the pretreated (washed and dried) eggshell wsate was ground and sieved into powder (30 mesh) at first. Then, the eggshell was added to H3PO4 aqueous in molar ratio 1:1, kept at 30⁴0℃for 2 to 3 hours, under pH 1³. Insoluble matter was separated and removed by filtration. Subsequently, Ca(OH)2 was added into the solution, kept at 50⁶0℃for 24 h, adjusting pH 9¹2 by NaOH, The adjusting pH to 7 by NH4Cl. At last, the suspension was dried and ground into powder manually (30 mesh) to get the sample. The sample was studied by Scan Electron Microscope and Energy Disperse Spectroscopy.The adsorption of heavy metals with CHAP was divided into two parts, including the adsorption to single heavy metal and the multi- metals at the same time. The effect of various parameters on adsorption process such as initial concentration of metal ions, solution pH, contact time, amount of CHAP and temperature were studied to optimize the conditions for maximum adsorption. The sorption isotherm, the dynamic and thermodynamic of process were study. Then the sample after the adsorption of heavy metals was studied by Scan Electron Microscope and Energy Disperse Spectroscopy.At last the desorption was studied with NaCl(0.2mol/L), NaNO3(0.2mol/L), HAC(pH=3.93), HAC(pH=4.93), CaCl2(0.05mol/L), CaCl2(0.1mol/L) and ultrasonic to the sample of CHAP after adsorption the Zn²⁺.The present study showed that the CHAP was effective in removing Cd²⁺, Zn²⁺, and Ni²⁺ from aqueous solutions. The results showed that the removal efficiency of Cd²⁺ and Zn²⁺ by CHAP could reach 93% and 98.67% respectively, when the initial Cd²⁺ concentration 80mg/L and Zn²⁺ concentrarion 100mg/L and the solid/liquid ratio was 2.5g/L, and when solution pH and contact time was 6 and 6⁷ and 1h and 45 min, respectively. The equilibrium sorption data for single metal systems at 40℃could be described by the Langmuir and Freundlich isotherm models. At low pH the main mechanism between CHAP and metal ions are ion-exchange and surface adsorption. Possibly some precipitation of metal could occur at higher pH. The results showed that zinc adsorption on CHAP would be an endothermic process, and might occur more effectively at relatively high temperatures. The results of multi- heavy metals adsorption showed that the capacity adsorption less than single metal, which competitive for the active point. Desorption experiments showed that various extractant reagents were not efficient enough to desorb substantial amount of metal ions from the CHAP. The results obtained show that CHAP has a high affinity to heavy metals. Neddle-shaped minerals were found after adsorption heavy metals.