Potentiality of lignin from the Kraft pulping process for removal of trace nickel from wastewater: Effect of demineralisation
Introduction
The spent pulping liquors (black liquors) from the Kraft process employed in the pulp and paper industry, involving aqueous solution of sodium hydroxide and sodium sulphide to extract cellulose by dissolution of lignin binding the cellulose fibers together in woody matrix, are known to adversely impact treatment facilities and aquatic life. About 90–95% of the reactive lignin biopolymer is solubilized to oligomers that contribute to the dark brown colour and pollution load (U.S. EPA, 1997, Mohan et al., 2006). Around 63 × 104 metric tons/year of lignin are produced worldwide by pulping (Mohan et al., 2006), mostly employed as in-house low grade fuel. Larger plant capacities and optimization of the pulping process to improve cost effectiveness lead to more by-product lignin than the amount needed for energy requirements. Accordingly, use and/or conversion of surplus lignin from black liquors to added-value products might represent a profitable alternative to incineration (Fierro et al., 2005), and studies for different novel applications have been reported (Velásquez et al., 2003, Braun et al., 2005, Li and Sarkanen, 2005, El Mansouri and Salvadó, 2006, Baumlin et al., 2006, El Mansouri et al., 2007, Ramírez et al., 2007).
In particular, utilization of Kraft lignin as a biosorbent of trace metals from wastewater (Crist et al., 2003, Mohan et al., 2006, Pérez et al., 2006) or as a precursor for conversion into activated carbons (González-Serrano et al., 2004, Montané et al., 2005, Fierro et al., 2006, Fierro et al., 2007, Guo and Rockstraw, 2006, Suhas et al., 2007, Cotoruelo et al., 2007) constitutes an attractive option due to growing concern about water pollution by heavy metals. Biosorption, namely the use of natural materials or industrial and agricultural wastes to passively remove metal species from aqueous media, has gained special attention in recent years because it offers an efficient and cost-effective alternative to traditional remediation and decontamination technologies for large-scale tertiary wastewater treatment (Cochrane et al., 2006, Volesky, 2007).
Even though some studies have been earlier devoted to the potential use of Kraft lignin as a biosorbent of toxic metals, information is limited and, therefore, a thorough knowledge has not yet been attained. Accordingly, it is still difficult to predict the metal-binding ability of a Kraft lignin from reported results, especially because of the influence of the extent of delignification and pulping conditions on lignin structure (Baptista et al., 2006). Besides, previous own works concerning metal sorption by different lignocellulosic materials (Basso et al., 2002a, Basso et al., 2004, Cukierman, 2007) suggest that lignin is the main component responsible for metal sorption capacity, the materials with greater content of lignin attaining a higher effectiveness. These studies also indicated the same trend for those materials with lower ash content. Therefore, minerals inherently present in lignin could play a role in metal sorption capacity. To our knowledge, the effect of demineralisation on metal sorption ability of lignin has not been examined earlier, at least in the open literature.
In addition, the capability of a particular lignin as metal biosorbent is also conditioned by the targeted metal species and environmental factors involved in metal sorption measurements. In this sense, earlier research concerned with raw or modified Kraft lignin as a biosorbent of toxic metals has mainly focussed on Pb(II), Cd(II), Cu(II), Zn(II), Cr(III) and Cr(V) ions (Crist et al., 2003, Mohan et al., 2006). Nickel sorption by Kraft lignin has been almost unexplored (Pérez et al., 2006). Nickel is present in raw wastewater streams from electroplating industries, nonferrous metals mineral processing, dye industries, porcelain enameling, and steam-electric power plants. Removal of nickel to attain the permissible limits is critical since it is a known carcinogen and can cause serious problems in humans, such as dermatitis, allergic sensitization, and lung and nervous system damages (Basso et al., 2002a, Malkoc, 2006, Hanif et al., 2007).
Within this context, the present study investigates the feasibility of using an industrial raw Kraft lignin for the removal of Ni(II) ions from model dilute solutions, mimicking low metal concentration wastewater. The influence of demineralisation of the raw lignin on its metal sorption ability is especially explored by employing samples previously treated with acid. The effectiveness of both the raw and demineralised lignins in removing Ni(II) ions is first examined at fixed, pre-established equilibrium conditions, and compared with results earlier obtained for two different lignocellulosic biomasses in identical conditions. The influence of the solution’s pH on the metal equilibrium sorption for the raw lignin is then examined. Likewise, sorption kinetics and equilibrium isotherms for the raw and demineralised lignins are determined and modelled. The results are interpreted in terms of physicochemical features characterizing the lignin samples as determined by several complementary techniques.
Section snippets
Materials conditioning and characterization
A raw Kraft lignin (RKL) in powder form (particle diameter between 27 and 70 μm) from Lignotech Ibérica (Torrelavega, Cantabria, Spain) was used. Demineralisation was carried out by treating the raw lignin with sulphuric acid solution (1 wt%) for 1 h, in a proportion of 20 mL of solution per gram of sample, followed by washing extensively with deionized water until neutral pH in washing water was attained. The demineralised lignin is denoted as DKL.
Ash content and elemental composition of both the
Results and discussion
Ash content and elemental composition determined for both the raw Kraft lignin and the demineralised one are reported in Table 1. The data in Table 1 shows that the acid treatment of the RKL led to a pronounced reduction in ash content, in agreement with other results reported in the literature (El Mansouri and Salvadó, 2006), though it only promoted slight variations in elemental composition.
FT-IR spectra for the raw and demineralised lignin samples are illustrated in Fig. 1. The raw and
Conclusions
The potential of an industrial raw Kraft lignin for the removal of Ni(II) ions from model dilute solutions was ascertained. The effect of demineralisation of the raw lignin by a mild acid treatment on nickel sorption performance was also explored. Under identical pre-established equilibrium conditions, the industrial raw lignin attained an effectiveness in removing Ni(II) ions of 90% for the highest dosage investigated (1 g/100 mL). Demineralisation led to improve effectiveness, with the
Acknowledgements
Financial support from Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Universidad de Buenos Aires (UBA), Agencia Nacional de Promoción Científica y Tecnológica (ANPCYT-FONCYT) from Argentina, and from the European Commission - Alfa Programme (Project ALFA II 0412 FA FI), is gratefully acknowledged. M. Betancur acknowledges the mobility grant of the EU-Alfa Programme to carry out research activities at the University of Buenos Aires, Argentina. Thanks are also extended to
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