CHROMIUM PHOSPHATE SERIES

Layered double hydroxides (LDH) are considered one of the most promising inorganic functional materials, due to their special layered structure, large surface area and low-cost. A wide range of physical and chemical processes is available to eliminate Cr (VI) and P from wastewater, such as: liquid–liquid extraction, hybrid anion exchange–preci-pitation,10,18,19 and adsorption.7,16 Among these potential separation technologies, adsorption is one of the most pro-mising because of its initially low cost, flexibility, and ease of operation.20-24 So the search for ideal adsorbents with high adsorption capacity becomes necessary.

To mitigate the adverse effects of P, an appropriate removal technology is required. These substances will adversely affect the quality of water and soil: they pose a significant threat to environmental security. Compared to their permissible limits, industrial and agricultural effluents contain much higher concentrations of Cr (VI) and P. With the rapid development of industrialisation and urbanisation in recent years, large amounts of Cr (VI) and P were produced in wastewater. The recommended limit of Cr (VI) in drinking water is 0.05 mg/L.2,9,10 Methods for the removal of Cr (VI) involve chemical precipitation, electrolysis, biochemical process, and adsorption.11,12 Phosphorus is one of the major nutrients for plant growth and it comes from agricultural fertilisers, soaps, and detergents with formulations contain-ing sodium tripolyphosphates, atmospheric nitrogen, erosion of soils containing nutrients, and discharge from sewage treatment plants.7,13 Though it is an essential macronutrient responsible for healthy plant growth, concentration in an excess of the described limit can result in the eutrophication of lakes, rivers, and seas.14-16 The permissible limit for phos-phate in drinking water is only 1.0 mg/L.7 Therefore, high concentrations of phosphate (P) must be removed from wastewater before its discharge.17 There are also numerous methods used for removing phosphate, such as: chemical precipitation, biological processes, adsorption, and crystalli-zation.15 Cr (VI) is one of the most dangerous heavy metals because of its strong oxidizability and high toxicity.3-5 As is known Cr (VI) is mainly released to the environment by textile processing, leather tanning, mining operations, water-cooling, electroplating and pigment manu-facturing,6-8 which may cause environmental and health problems. However, the suppression of Cr (VI) adsorption was obvious when the initial concentration of P was up to 10 mg/L with a concentration of 0.5 g/L of CLDH.Ĭhromium (Cr) is a major pollutant in wastewater, and it can be found generally in rocks, soil, plants, animals, and gases.1,2 The most common oxidation states of chromium are Cr (III) and Cr (VI). The presence of Cr (VI) had no significant influence on P adsorption. Competitive adsorption between Cr (VI) and P existed in the binary system. The experimental data were best fitted by a pseudo-second-order kinetics model. It was found that the initial pH was approximately 6 and it took 24 h to reach equilibrium when P and Cr (VI) were added simultaneously. The experimental data were close to the theoretical adsorption capacity given by the Langmuir isotherm, the calculating adsorption capacities of Cr (VI) and P were up to 70.42 mg/g and 97.09 mg/g, respectively. In this study, CLDH exhibited a high adsorption capacity for Cr (VI) and P in a single system.

CHROMIUM PHOSPHATE SERIES

A series of batch experiments were performed to study the influence of various experimental parameters. The adsorption characteristics of chromium (VI) and phosphate on calcined Mg-Al- $CO_3$ layered double hydroxides (CLDH) were investigated in single and binary systems.

Chromium phosphate