Ultrasound Assisted Synthesis Of Ca-Al Hydrotalcite And Their Composites For U (Ⅵ) And Cr (Ⅵ) Adsorption

Layered double hydoxides (LDHs), also known as anionic clays or hydrotalcite(HT)-like materials have been widely investigated in recent years because of their promisingapplications in areas such as water treatment, nuclear waste treatment/storage， anionscavengers, neutralizers (antacids), filtration, polymer stabilizers, catalysts and catalystsupports, anion exchangers, adsorbents, electroactive, photoactive materials andpharmaceutics. Recently, significant interest has been devoted to the synthesis of LDHs withnew compositions by various kinds of modified method allowing improved applications, suchas ion-exchange method, melt intercalation, sol-gel method, situ growth method andco-precipitation method.In this study, we improved the co-precipitation method to prepare Ca-Al-LDHs thatsignificantly shortens the reaction time and reduced the reaction conditions. Hydrotalcites aresynthesized at improving and optimizing reactive routines, thus allowing wide and facilereactive conditions. The improved ultrasound technique method applied to the synthesis oflayered double hydroxides is simpler, easier to control, and consumes less energy andmaterials than traditional methods. After that, we synthesized hydrotalcite and hydroxyapatitecomposite (Ca-Al LDHs-HA) using the layered double hydoxides as the precursor inultrasonic condition. The study attempted to select a variety of different synthesizingprocesses with Ca-Al-LDHs as adsorbing material for the removal of chromium (Ⅵ) anduranium (Ⅵ) from water. Characterization methods such as XRD and SEM and removalefficiency (Adsorption,%) are typically used to select suitable adsorbent. Adsorption ofchromium (Ⅵ) and uranium (Ⅵ) nitrate on Ca-Al hydrotalcite (Ca-Al LDHs) and itscomposite (Ca-Al LDHs-HA) are investigated. The powdered materials are synthesized in amodified co-precipitation method under the ultrasonic treatment with different conditions forCr(Ⅵ) and U(Ⅵ) adsorption. Factors that affect the compounds synthesis such as temperature,reaction time, calcination and aging conditions are reported. The effects of various parameterson adsorption process including adsorption kinetics, isotherms and influencing factors such ascontact time, solution pH and effect of competing ions are studied. The Ca-Al LDHs andCA-Al LDHs-HA are effective in removing of chromium and uranium from an aqueoussolution over a wide pH range. A highly alkaline contaminated solution is found to have an impact on the removal of chromium and uranium, because of an increase in hydroxide. Thisinvestigation also showed that the equilibrium isotherm data are fitted well by Langmuirisotherm and pseudo-second-order equation. It is found that Ca-Al LDHs has the largestcapacity for both Cr(Ⅵ)and U(Ⅵ), adsorbing104.82mg/g and54.79mg/g, Ca-Al LDHs-HAhas the largest capacity for both Cr (Ⅵ) and U (Ⅵ), adsorbing86.86mg/g and46.92mg/g.Cr(Ⅵ) and U(Ⅵ) adsorption decreased obviously with pH levels outside the range of6~11.The effect of competing ions such as nitrate radical and chloridion was negligible.