Reactive blue 5G dye removal in a fixed-bed column packed with Dowex Optipore SD-2 adsorbent was modelled. Three mathematical models were tested in order to determine the limiting step of the mass transfer of the dye adsorption process onto the adsorbent. The mass transfer resistance was considered to be a criterion for the determination of the difference between models. The models contained information about the external, internal, or surface adsorption limiting step. In the model development procedure, two hypotheses were applied to describe the internal mass transfer resistance. First, the mass transfer coefficient constant was considered. Second, the mass transfer coefficient was considered as a function of the dye concentration in the adsorbent. The experimental breakthrough curves were obtained for different particle diameters of the adsorbent, flow rates, and feed dye concentrations in order to evaluate the predictive power of the models. The values of the mass transfer parameters of the mathematical models were estimated by using the downhill simplex optimization method. The results showed that the model that considered internal resistance with a variable mass transfer coefficient was more flexible than the other ones and this model described the dynamics of the adsorption process of the dye in the fixed-bed column better. Hence, this model can be used for optimization and column design purposes for the investigated systems and similar ones.

Neste trabalho foram realizados experimentos para avaliação do efeito do pH da solução (1 – 12), da temperatura do sistema (20, 30, 40 e 50°C) e do diâmetro das partículas de adsorvente (0,01 - 0,12; 0,05 - 0,06; 0,06 - 0,12 cm) na remoção do corante Azul Reativo 5G utilizando o adsorvente comercial DowexTM OptiporeTM SD-2. Os experimentos foram realizados em sistema fechado e batelada colocando-se 50 mL de solução de corante com concentração de 150 mg L-1 em contato com 0,005 g de adsorvente sob agitação constante (120 rpm) por um período de 96 horas. Além disto, foi determinado o ponto de carga zero do adsorvente (pHPCZ) pelo método de titulação potenciométrica. As condições experimentais que apresentaram os melhores resultados em relação à quantidade de corante adsorvido foram: pH da solução igual a 2, temperatura do sistema de 30°C e partículas de adsorvente com diâmetro entre 0,01 e 0,12 cm.  Os valores obtidos para os parâmetros termodinâmicos indicam que o processo é exotérmico, espontâneo e possui afinidade entre adsorvente/adsorvato. O pHPCZ do adsorvente ficou entre 4 e 10. Em todos os testes realizados, o adsorvente Dowex™ Optipore™ SD-2 apresentou alta capacidade de adsorção do corante Azul Reativo 5G (superior a 100 mg g-1) sendo que nas melhores condições, entre as avaliadas, a capacidade de adsorção foi superior a 230 mg g-1. Desta forma, o adsorvente estudado apresenta grande potencial para ser utilizado em sistemas de tratamento de efluentes têxteis.

Stevia rebaudiana Bertoni is plant used for producing a natural sweetener. After extraction from leaves, stevia extract is dark in colour and must be clarified for better acceptance by consumers. Adsorption processes are widely employed in the clarification because they do not use organic solvents. In the present work, the use of alginate beads as an adsorbent was investigated. Experiments were carried out in a batch system in which the aqueous stevia extract was mixed with the alginate beads previously prepared by ionotropic gelation. The effect of pH on the adsorption process was evaluated and the performance of the beads was examined by colorimetric analysis (colour removal, turbidity removal and adsorption of stevia steviol glycosides), as well as by refractive analysis for determining the adsorption of soluble solids. The results showed that the alginate beads were capable of adsorbing both the stevia steviol glycosides and the pigments that colour the extract. The best results were observed at a pH value of 2.0.

Stevia is used as a sweetener due to its low calorific value and its taste, which is very similar to that of sucrose. After extraction from dried leaves, stevia extract is dark in colour, and therefore needs to be whitened to increase acceptance by consumers. In this study we tested chitosan, a cationic polyelectrolyte, as flocculant agent for the whitening of the Stevia extract. Positive charges of chitosan can interact electrostatically with a counter-ion, sodium tripolyphosphate (TPP), and then chitosan precipitates. A factorial design was used to study the whitening process, in which Glycosides Removal, Colour Removal, Turbidity Removal and Soluble Solids Removal were evaluated. The studied factors were Chitosan Mass and pH of the TPP solution. The results showed that chitosan is a good flocculant agent, being able to flocculate both the glycosides and the pigments that make the extract coloured.

Stevia is used as a sweetener due to its low calorific value and its taste, which is very similar to that of sucrose. Stevia extract is dark in colour and needs to be clarified for better acceptance by consumers. The adsorption processes widely used in clarification do not use organic solvents. Alginate beads have been used as the adsorbent material in the present work. Experiments were carried out in batch mode in which the aqueous stevia extract was mixed with the alginate beads prepared previously by ionotropic gelation. A factorial design was used to study the process of clarification, in which colour removal and the percentage of stevioside adsorbed were evaluated. The studied factors were temperature, pH of the aqueous stevia extract and adsorption time. The results showed that alginate beads are capable of adsorbing both the steviosides and the pigments that give colour to the extract. The experimental design showed that only the pH and contact time were important variables in determining the percentage clarification, in contrast to the other responses which were not affected by the various factors studied. An equation describing the percentage clarification as a function of pH and time is proposed.