An efficient biosorption of direct dyes from industrial wastewaters using pretreated sugarcane bagasse

Introduction -Color is the first contaminant to be recognized in the industry, even small amounts of dyes in water is highly visible. -Owing to the low biodegradation of dyes, the conventional biological treatment processes are not very effective in dye removal and physical or chemical processes include limitations. -Currently, there is a growing interest in using low cost sorbents such as agricultural waste products like sawdust wheat straw, coir pith and others. -Direct dyes are characterized with their high conjugated molecular structure and they contain one or more anionic sulphate groups.

Dyes structure Direct yellow 12

Evaluation of acid treated bagasse Four acid treated bagasse, namely bagasse propionic, oleic, palmitic and stearic acids were evaluated for direct dye removal. Bagasse propionic acid was the best, while bagasse stearic acid was the least.

Effect of initial pH The maximum biosorption of direct yellow 12(pH 3.2) and direct red 81(pH 2.5). At lower pH, bagasse surface is +ve charged, attaction to –ve charged dyes. Direct yellow 12 Direct red 81

Effect of pH (contd.) The protonated groups of the propoinic acid pretreated bagasse are mainly the carboxlic group of the acid (-CO-OH 2 +), (OH 2 + ) of the bagasse. The deprotonated groups of the two dyes are the sulphonate groups (-SO 3 - ). Increase of the pH values leads to an increase of the –ve charged sites, repulsion with the –ve dye.

Effect of contact time The effect of contact time was studied and the following results were found: The uptake of both dyes was rapid in the first 20 minutes and after 75 minutes the amounts of both dyes were almost constant. The two curves of contact time were single, smooth and continuous until reaching saturation, suggesting possible monolayer coverage of both dyes on the surface of modified bagasse.

contact time for direct yellow 12 contact time for direct red 81

Effect of contact time (contd.) -The first rapid uptake can be rationalized as a rapid attachment of both dyes to the sorbent surface. -Or due to the large number of vacant sites available.

As the initial concentration of the dye increases, the sorption capacity linearly increases (Effect of initial dye concentration) Sorption capacity (mg/L) of direct yellow 12(left) &direct red 81(right) increases lineally with initial dye increase until 500 mg/l Direct yellow 12 Direct red 81

The percentage removal increases as the bagasse weight varied from 0.2-2g due to availability of active sites (Effect of sorbent dose) Direct yellow 12 Direct red 81 The adsorption density (amount adsorbed per unit mass) decreases with increase in sorbent dose due to : Particle aggregation which decreases the total surface area and an increase in diffusional path length. Particle interaction may desorb some dyes loosely bound to bagasse

Direct yellow 12 & Direct red 81 (Effect of particle size) (0.25, 0.5, 0.75, 1mm) The increase in particle size decreases% removal, surface area increase with size decrease,for large size diffusional resistance to mass transfer &diff.path length increases,blocked parts of the particles not available.

Freundlich isotherm (left) log q e = log K F + (1/n) log C e (direct y12) q e (mg/g), C e (mg/L) rel. dye,K F, 1/n related to adsorpt. capac Langmuir isotherm (right) C e /q e = ( 1/a Q m ) + ( Ce/ Q m ) (direc.red 81) Q m is the max. adsorpt. capacity, “a” is a constant.

Adsorption kinetic representation: (left) pseudo-first order plot for direct yellow 12 and (right) pseudo-second order plot for direct red 81 Direct yellow 12 Direct red 81

q t = K id t 1/2 + C, K id is the intra-particle diffusion rate constant, q t (mg g -1 ) is the amount of dye adsorbed at time t (min). 2 intersecting lines,1 st, fast diffusion and adsorption of the dye on the macropores surface of bagasse, 2 nd, slow, micropores. Pass not from the origin i.e other processes involved. Linearity means intra- particle diff. contribution.

Desorption studies: A sample of both dyes was agitated with distilled water adjusted to different pH values. The dye was separated and estimated. The percent desorption increases as the pH of the medium increases which is just the opposite of the pH effect.

Comparison between Qm for direct red 81 and Qm for othe direct dyes Sorbent Dye Qm Banana pith……… … Direct red………… 5.92 Dead fungus Aspergillus… Direct red AC orange peel Direct red This work Direct red ………………………………………………………………………………… Direct red 81 shows higher adsorption capacity on bagasse than the above mentioned related direct dyes on other sorbents.

Conclusion Based on the above findings and arguments, it is to be concluded that propionic acid pretreated bagasse could be employed as an effective sorbent for designing and fabricating an economically cheap treatment process for the removal of direct dyes from dilute industrial effluents.

Acknowledgment The authors would like to thank the financial support from Science and Technology Development Fund (STDF),Ministry of Higher Education, Egypt.