Adsorption behavior of hydrothermally treated municipal sludge & pulp and paper industry sludge
Introduction
In wastewater and drinking water treatment activated carbon has widely been used as an adsorbent because of its efficiency to remove organic compounds, metals and other inorganic pollutants. Activated carbon refers to carbon-based materials with characteristics such as highly developed porosity, large surface area, variable surface chemistry, and high degree of surface reactivity (Bhatnagar et al., 2012).
Despite of wide use of activated carbon its exploitation is sometimes restricted due to high cost, which depends on availability of the raw material, processing requirement, treatment conditions, and both recycle and lifetime issues (Gupta et al., 2009). Therefore, researchers are looking for new low-cost alternatives for activated carbon where raw materials comprise of natural materials (wood, peat, lignite, clay etc.), as industrial, agricultural or domestic wastes and byproducts. Replacement of activated carbon by low-cost adsorbent would reduce the production cost of adsorbents. In addition exploitation of waste materials as feedstock would reduce partly the cost of waste disposal. In the view of low production cost, regeneration of the material would not be necessary (Babel and Kurniawan, 2003, Bhatnagar and Sillanpää, 2010, Dias et al., 2007).
Hydrothermal carbonization (HTC) is an unconventional method for the production of carbonaceous material. Process is attractive due to its green aspect, simplicity, low-cost, and energy and CO2 efficiency (Titirici et al., 2012). Major advantage of HTC over other methods for the production of carbonaceous material is converting wet input material into solids without the need for energy-intensive drying before and during the process. This expands the feedstock choice to high water content raw materials such as wet animal manures, human waste, sewage sludge, municipal solid waste (MSW), aqua culture and algal residues (Libra et al., 2011).
Reaction takes place in an aqueous environment at the moderate temperature (180–250 °C) and pressure in a closed system. Pressure formation is self-generated and mostly related to water steam but also raw material and design of vessel. At process temperature around 220 °C corresponding pressure has been reported to be approximately 20 bar. Overall HTC reaction is a combination of several simultaneous reactions. Reaction mechanism includes dehydration, decarboxylation, condensation polymerization and aromatization. However, the reaction mechanism is not understood in detail (Funke and Ziegler, 2010). HTC process results higher solid yields, more water soluble organics, and fewer gases compared to dry pyrolysis, which is the most commonly used method for the production of adsorbents.
Hydrochar resembles natural coal with respect to the type of chemical bonds, their relative quantity, and chemical composition. It has also higher hydrogen/carbon and oxygen/carbon ratios than carbonaceous material from dry pyrolysis (Libra et al., 2011). Significant differences in structural studies of hydrochars produced from small pure carbohydrates and complex biomass was not noticed. These hydrochars showed similarities with respect to morphology and local structural connectivity (Titirici and Antonietti, 2010).
In this study suitability of hydrochar produced by hydrothermal carbonization for adsorption of various metals was investigated. Adsorption behaviors of these carbonaceous materials were examined towards both cationic and anionic metal ions. Furthermore equilibrium behavior and adsorption kinetics of prepared hydrochars were investigated.
Section snippets
Materials and physicochemical characterization
Anaerobically digested municipal sludge (total solids 3.5 wt.%) (Mikkeli wastewater treatment plant, Finland) and pulp and paper industrial sludge (Dry solids (total solids 38 wt.%) Ekokem) were used as a starting material. Pulp and paper industry sludge consisted of both fiber rich primary sludge and biological secondary sludge.
Heavy metals Pb(II), As(III), As(V), Cr(III) and Cr(VI) were used for the adsorption studies. All metals were of analytical grade. 1000 mg l−1 stock solutions were prepared
Characterization
Adsorption process is adsorbent specific and depends strongly on the characteristics of the adsorbent. In this study the structural properties of the adsorbent were examined by determining surface area, pore volume, isoelectric point (pHIEP) and organic elemental composition of adsorbents. Surface morphology was imaged by using SEM and structural composition of hydrochars was examined by using FTIR.
Both hydrochars presented weakly developed surface area and pore volume, which is common for
Conclusion
Hydrochars prepared from anaerobically digested sludge (ADS) and industrial sludge (INS) via hydrothermal carbonization indicated significant adsorption efficiency towards Pb(II). ADS removed small amounts of Cr(VI), As(V) and As(III) as well. Adsorption of INS towards anionic compounds was low. Pb(II) adsorption (qm = 11 mg g−1) by ADS and INS was comparable, however, adsorption capacity of INS was slightly higher. Both hydrochars followed the Sips adsorption isotherm model, which allowed the best
Acknowledgements
Authors are grateful to Finnish Funding Agency for Technology and Innovation (TEKES) for financial support. Emmanuelle Castagnoli is thanked for her valuable contribution to the experiments.
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