Calcium sulfate dihydrate (CaSO4•2H2O, CSD), gypsum is a by-product in the production of citric acid (citryogypsum). This by-product could neither be exploited nor distributed as a reactant because of its physical properties including those that are not equivalent to natural gypsum. Moreover, the mentioned citrogypsum has been continually increasing environmental problems. Therefore, this research aims at how to recycle gypsum that is synthesized by hydrothermal method at 95oC for 7 hrs under the atmospheric pressure via different solutions (MeOH, EtOH, PrOH, BuOH and Hexane). In order to produce alpha-calcium sulfate hemihydrate (α-CaSO4•0.5H2O, α-CSH) with improved physical properties that will be used for different industries. FT-IR reveals the chemical composition of crystal and the adsorption of methyl group on the surface. Besides, TGA thermogram shows the theoretical crystal water content of CSD and α-CSH 20.9 wt% and 6.2 wt% respectively. The DSC thermogram, shows that endothermic peaks at 151.2 oC and 168.5 oC. There were two steps of loss at 1.5 and 0.5 water molecule respectively. With SEM images of crystal shows the plate-like shape of citrogypsum, while α-CSH shows the hexagonal shape excluding hexane solution. Of all the results, the polarity of solution has an impact on the transition of CSD to α-CSH under this condition.

The acid purification of calcium citrate from citric acid production using sulfuric acid as a reagent resulted in citrogypsum waste. The identification of citrogypsum by XRD technique indicates that the main component consists of CaSO4∙2H2O (DH). Furthermore, the comparison of the colours between citrogypsum and natural gypsum are also different. Hence, this research mainly focused on the phase transformation of DH to α-CaSO4 ∙0.5H2O (α-HH) due to high strength and heat resistant. The preparation of α-HH carry out in different volume ratios of ethylene glycol (EG): water solutions at 95oC for 7 hrs under atmospheric pressure. The FT-IR spectra of DH and α-HH results reveal that the absorption frequencies at 1700 and 1800 cm-1 of –OH group are 0.5 and 1.5 water molecule in CaSO4 respectively. TGA thermograms show the theoretical crystal water content of DH approximately 20.1 wt% and the converted α-HH about 4-5 wt%. DSC thermograms of the citrogypsums show two endothermic peaks regarding to two steps of water molecule loss at 151.2oC and 168.5oC respectively. In addition, the α-HH shows exothermic peak at 238.7oC. The morphology of citrogypsum and α-HH are observed by SEM, showing the plate-like shape of citrogypsum and hexagonal shape of α-HH. Moreover, the products could be applied to several other industries for increasing the value and reducing the environmental concerns.

It describes the development of a marketable citrogypsum which is also a by-product of citric acid production from laboratory scale to pilot scale. The modification of b-phase citrogypsum was carried out in different sodium chloride solution at 95oC for 7 hours under atmospheric pressure to α-calcium sulfate hemihydrate (α-CaSO4∙0.5H2O, α-HH). This can be achieved in the laboratory scale experiment. The phase analysis of a citrogypsum was confirmed by XRD technique demonstrated that mostly made up of calcium sulfate dihydrate (CaSO4∙2H2O, DH). The dehydration and phase transformation of citrogypsum to α-HH were conducted by DSC thermograms which were presented two endothermic peaks in the range 150-180oC of citrogypsum and an exothermic peak at 290 to 300oC, resulting that the product being α-HH when the 4M and 5M sodium chloride solutions were used. The outcome products were presented in a plate-like shape of citrogypsum but a hexagonal shape of α-HH. The experiment scaled up for modification of DH up to 100% in a batch reactor at the same condition with the 4M sodium chloride solution. The results showed that α-HH was obtained within 15 to 60 min after that calcium sulfate anhydrous (CaSO4, AH) had been formed. The non-isothermal of DSC was an adapted method to investigate kinetics study of DH to α-HH transformation under the optimum condition with a model fitting dα/dt = -3k (α2) for predicting the process compared to the experiment values. In addition, the coefficient of determination (R2) from estimation and experiment value was 0.9940. Hence, the model equation was completely represent data.

Azo dyes are usually used in textile industry. However, they can cause water contamination, lead to water pollution, damage to aquatic lives and degenerate scenery due to their toxicity. These problems can be overcome by photocatalytic process in which the azo dyes are converted to CO2 and water. This research concentrates on effect of Bi2O3, BiOBr and BiOI contents on titanium dioxide substance (TiO2) for the photocatalytic process. In the study, photocatalysts were synthesized by sol-gel and wetness impregnation methods. They were studied in surface area by BET technique, chemical composition by FT-IR spectroscopy and optical properties by UV-DRS technique. Increase in bismuth content on TiO2 results in decreasing surface area. In FT-IR spectra, Ti-O-Ti stretching bands at 400-800 cm-1 were detected. The band gap energy of these photocatalysts is decreased when bismuth was doped. Since efficiency of CO2 and water conversion of the photocatalysts can be determined indirectly via determinaiton of decreasing Methyl Orange (MO) concentration, the lowest MO concentration was observed in the 4%Bi2O3T photocatalyst after 16 hours when compared to the other photocatalyst samples and Degussa P25. In other words, this photocatalyst efficiently converts the azo dyes to CO2 and water.