IJUP08 - Universidade do Porto

Low-cost materials as adsorbents for Textile Wastewater
Treatment
S. Santos, R. Deucher and R. Boaventura
Department of Chemical Engineering, Faculty of Engineering, University of Porto, Portugal.
Dyeing processes generate high volumes of colored and toxic effluents. This kind of
effluents is very difficult to treat. Conventional biological and physico-chemical treatments
are not effective enough to remove color from wastewaters. Adsorption has been
considered a simple and economical method. The use of activated carbon, the most widely
used adsorbent, is limited by the high prices associated with its replacement and
regeneration. The goal of this work is to investigate the utilization of natural and waste
materials as alternatives to activated carbon on the decolorization of textile effluents.
Several low-cost materials were firstly selected for the study: a Portuguese bentonitic clay
from Alentejo, a commercial sepiolite clay, a calcinated waste sludge from a water
treatment plant and an hydroxide metal waste sludge generated in a metal plating industry.
Preliminary adsorption tests were conducted with these potential adsorbents and different
kinds of dyes. The percentage of color removal was determined by absorbance
measurement after one hour of contact time between colored solutions (30 mg/L of dye)
and adsorbents (1.0 g/50 mL). Clay materials showed to be very effective for basic dyes
removal (a total color removal was perceived). Sepiolite also showed a good performance
for direct and sulfurous dyes adsorption. Waste sludge from the water treatment plant
exhibited poor adsorption properties, but a good capacity for some basic and direct dyes
was obtained. The metal hydroxide sludge, tested only with one dye of each class, showed
to be a very effective material for this kind of application, namely for reactive, direct and
acid dyes (100% color removal).
Adsorption isotherms for some of the most promising dye/adsorbent systems were
determined at neutral pH and 25 ºC and results were fitted to Langmuir and Freundlich
models. Values in the range of 89.9 mg/g (Blue Reactive dye/hydroxide metal waste
sludge) and 232 mg/g (Red Basic dye/bentonitic clay) were obtained as monolayer’s
adsorption capacities given by Langmuir fittings.
Further studies were conducted for Blue Reactive dye adsorption onto the hydroxide metal
waste sludge. The influence of salts and dyeing auxiliary products (which appeared
commonly in textile effluents) on the performance of adsorption was investigated. The
results indicate that NaCl does not interfere significantly in the adsorption. Dyeing
auxiliary chemicals and mainly Na2CO3 were responsible for a considerable decrease in the
adsorbed amounts.
The use of this kind of clays and the hydroxide metal waste sludge was then found to be
very promising, respectively for basic and anionic textile dyes removal. Adsorption
capacities obtained in this work are acceptable, in comparison with values reported in
literature for commercial activated carbons and other alternative materials.
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