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Adsorption of Ni2+, As5+, Pb2+ on carbon nanotubes modified biochars derived from sewage sludge and rice husks

Toti Paraskevi

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Summary

In the context of this thesis, the production of simple and carbon nanotubes modified biochars was initially studied, including their characterization, in order to investigate their capability as absorbent materials. Two different types of biomass were used: Rice husks (RH) and Sewage sludge (SS). Two concentrations of 0.1% and 1% carbon nanotubes were used for the modification of biochars, while the pyrolysis took place at two different temperatures, 400oC and 600oC. A total of 12 biochars were produced, 4 conventional and 8 advanced. The produced biochars were characterized in terms of their physicochemical properties, such as yield, ash, volatile solids, pH, electrical conductivity, elemental analysis. The point of zero charge, the cation exchange capacity, the bulk density, the total concentrations of metals in the solid materials were also determined. Further analysis, such as surface area measurement (BET), total organic carbon measurement and scanning electron microscopy imaging, was also performed. From the specific analyses it was concluded that the best biochars in terms of their physicochemical properties were the modified biochars derived from rice husks of 600οC, with the best being RH_CNT1_600. The results showed that the main parameters affecting biochars were the type of biomass and the pyrolysis temperature, while the modification with carbon nanotubes showed some significant changes in the specific surface area of the biochar and increased the functional surface positions. Kinetic adsorption experiments of three heavy metals were then performed, which were simultaneously dissolved in water, using the simple and modified biochars produced. In particular, the experiments showed whether these biocarbons are capable of removing Ni2+, As5+ and Pb2+ from an aqueous solution with contact times of 5, 30 and 60 minutes. Based on the results of the kinetic experiments, it was shown that all biochars removed to a significant extent all three metals. Specifically for Pb2+ the largest removals recorded were in excess of 95% in the first 5 minutes, for Ni2+ the removal levels were also very high, while As5+ was the metal that recorded the lowest removal rates. As for nickel, the removal from the biochars of rice husks in 60 minutes for both temperatures ranged from 82-98%, while from those of sewage sludge from 64-97%, with the most efficient biochar being the RH_CNT0.1_600. For arsenic the removal ranged from 30-66% for rice husks and for those of sewage sludge from 15-49%, with RH_CNT0.1_600 being the best. For lead the removals of rice husks at 60 minutes were 92-98% and for sewage sludge 94-99%, with RH_CNT0.1_600 being the best in terms of total removals.Also, leaching experiments were performed to determine the possible leaching of the heavy metals contained in the biochars in the aqueous solution, but the results of the measurements in all samples refuted it.Finally, the experimental adsorption results were simulated with two kinetic models, pseudo-1st and pseudo-2nd class, in order to identify the model that best resembles the experimental kinetic results of biochars for each metal. All three metals followed the pseudo-2nd class kinetic model, although in the case of lead both models described its kinetic adsorption with great accuracy. Non-linear regression was applied to the experimental data to calculate the model parameters.

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