Chromium(III) biosorption onto spent grains residual from brewing industry : equilibrium, kinetics and column studies

The use of industrial wastes for wastewater treatment as a strategy to their re-use and valorisation may provide important advances toward sustainability. The present work gives new insights into heavy metal biosorption onto low-cost biosorbents, studying chromium(III) biosorption onto spent grains residual from a Portuguese brewing industry both in batch and expanded bed column systems. Experimental studies involved unmodified spent grains and spent grains treated with NaOH. Metal uptake followed a rapid initial step, well described by the pseudo-second-order kinetic model up to 27 h, indicating chemisorption to be the rate-limiting step. Beyond this period intraparticle diffusion assumed an important role in the uptake global kinetics. The best fit for equilibrium data was obtained using the Langmuir model, with unmodified spent grains having the higher maximum uptake capacity (q max = 16.7 mg g1). In open system studies, using expanded bed columns, the best performance was also achieved with unmodified spent grains: Breakthrough time (C/C i = 0.25) and total saturation time (C/C i = 0.99) occurred after 58 and 199 h of operation, corresponding to the accumulation of 390 mg of chromium(III), 43.3 % of the total amount entering the column. These results suggest that alkali treatment does not improve spent grains uptake performance. Changes in biomass composition determined by Fourier transform infrared spectroscopy suggested hydroxyl groups and proteins to have an important role in chromium(III) biosorption. This study points out that unmodified spent grains can be successfully used as low-cost biosorbent for trivalent chromium.The authors would like to thank the Portuguese brewing industry UNICER for all the support and FCT (Fundacao para a Ciencia e a Tecnologia) financial support through the Grant PRAXIS XXI/BD/15945/98

Valorisation d’un sous-produit de l’industrie sucrière : mise en œuvre dans un procédé de traitement d’eaux usées industrielles

International audienceSugar beet pulp generated by sugar refining factories is a very cheap and available by-product. The aim of the present work is to develop a new dynamic process involving this low cost biosorbent in order to remove metal ions from aqueous solutions. As a preliminary step, a complete characterization of the polysaccharide has shown that sugar beet pulp contained about 20% galacturonic acid and 0.466 mmol.g-1 as carboxylic groups. The total cation exchange capacity (0.575 mmol.g-1) suggests good metal binding capacities. Then, batch adsorption studies were performed for several metal ions, namely Pb2+, Cu2+, Zn2+, Cd2+ and Ni2+ cations. For 8 10-4 M initial metal concentration, the initial sorption rates ranged from 0.063 mmol.g-1.min-1 for Pb2+ to 0.275 mmol.g-1.min-1 for Ni2+ ions. The equilibrium data fitted well with the Langmuir model and the maximum adsorption capacities ranged from 0.202 to 0.356 mmol.g-1 with the following affinity order: Pb2+ > Cu2+ > Zn2+ > Cd2+ > Ni2+. Ion exchange with Ca2+ ions neutralizing the carboxyl groups of the polysaccharide was found to be the predominant mechanism, added with complexation for Pb2+, Cu2+ and Zn2+ metal ions. The dynamic studies of Pb2+ and Cd2+ fixation onto the natural polysaccharide involve an adsorption reactor coupled with a microfiltration membrane in order to confine the particles. Due to their lower affinity for the biosorbent, Cd2+ ions were found to break through the process faster than Pb2+ cations. A mass balance model based on the Langmuir equilibrium isotherm was used to describe the adsorption process in transitory regime. This model successfully simulated the entire breakthrough curves whatever the operating conditions used. It provides a useful tool for process simulation and optimisation. Based on these results, it is demonstrated that the biosorbent studied represents an interesting low-cost solution for the treatment of metal polluted waters.L’objectif général de cette étude vise à valoriser un sous-produit de l’industrie sucrière dans le traitement des effluents chargés en ions métalliques. La pulpe de betterave est un polysaccharide naturel abondant, dont les fonctions de surface (CEC = 575 meq.g1) développent des propriétés de fixation intéressante vis-à-vis des ions métalliques en solution : les capacités maximales de fixation, déduites du modèle de Langmuir, s’échelonnent de 0,36 mmol.g1 pour le plomb à 0,22 mmol.g1 pour le cadmium. Tous les cations métalliques étudiés se fixent très rapidement sur la pulpe puisque l’équilibre, en réacteur discontinu parfaitement agité, est atteint après une heure de mise en contact entre le support et les ions en solution. Un procédé couplant un réacteur parfaitement agité à une séparation par membrane a été mis au point et un modèle de transfert de matière a été établi à partir des données obtenues en réacteur discontinu. Ce modèle a permis de décrire correctement les courbes de percée expérimentales, en prenant en compte les capacités de fixation différentes de chaque ion, leur concentration et la quantité d’adsorbant dans le réacteur. L’ensemble de ces résultats confirme la possibilité d’utiliser la pulpe de betterave comme adsorbant pour éliminer les ions métalliques présents dans les eaux usées