Natural argentinian diatomites: Efficient Green Catalysts for Fenton-like oxidation of Orange II

N INCHAURRONDO; JOSEP FONT; P HAURE

Praga

Congreso; 21st International Congress of Chemical and Process Engineering (CHISA); 2014

Diatomite occurs as abundant sedimentary accumulations of siliceous skeletons (frustules) of microscopic aquatic plants known as diatoms. Due to its extremely porous structure, low density and high surface area, they are used for adsorption of organic and inorganic chemicals and as a catalyst support for several reactions. However, the use of diatomite as a catalyst rather as a catalyst support has not been extensively studied and the presence of a high percentage of iron would enable its use as a heterogeneous Fenton-like catalyst without major modifications. In this context, the aim of this study is to investigate the performance of natural argentinian diatomite as a Fenton-like catalyst in the degradation of an azo-compound, Orange II (OII). The diatomite sample was obtained from the deposit of diatomites in Ing. Jacobacci, Río Negro, Argentina. The raw diatomite was sieved (mesh 2.38-2.83 mm) and calcined at 500, 700 and 1000°C. The characterization by FTIR, XRF, XRD and EDX indicated the presence of clay minerals. According to the XRD spectra diatomites are mainly composed by amorphous silica and calcinations led to the formation of cristobalite. SEM micrographs show the structure of the fossilized diatoms, which present a regular array of submicron pores (250-500 nm). The thermal treatment eliminated some impurities on the surface, the skeletal structure of diatoms partially collapse at higher temperatures, but the original geometry was mostly preserved. Upon calcinations, the Fe percentage increased from 0.17 % (raw) to 0.5 % (1000 °C) and the shrink and hardening of individual particles was generated. This may have contributed to the gradual reduction of the surface area from 151.1 m2/g (raw) to 87.2 m2/g (700°C). Diatomite samples were used in the Fenton-like oxidation of OII (100 mg/L) with the stoichiometric requirement of H2O2, in a batch reactor at 50-80 °C, with initial pH 3, reaction time of 4 hours and a diatomite mass equivalent to 100 mg/L of Fe in the reaction media. OII adsorption assays presented negligible OII and TOC conversions. In spite of showing slightly higher conversions, samples calcined at 500°C released impurities into the reaction media, not removed during calcination, inhibiting their use. According to these results, the optimum calcination temperature is 700 °C, the impurities that interfere with the reaction are removed and the loss of surface area is lower than in samples calcined at 1000 °C, which also presented an important induction period during the oxidation assays. In the oxidation tests performed at 50 °C, with samples calcined at 700°C, the level of mineralization was very low (10 %) so it was necessary to increase the temperature up to 60 and 70 °C to obtain satisfactory TOC conversions (56-67 %) and complete discoloration. At 80°C the thermal decomposition of H2O2 yielded lower TOC final conversions (56 %). The leaching of Fe measured in the supernatant was lower than 1 mg/L (1%). The diatomite catalyst presented satisfactory results comparable or even better than those reported in literature, using an OII concentration higher than the commonly evaluated. Diatomites are an attractive and innovating option as it is a low cost material, widely available from a sustainable source and the presence of iron would enable its use without major modifications.