ARTICLE pubs.acs.org/IECR
Removal of Lignin from Wastewater Generated by Mechanical Pulping Using Activated Charcoal and Fly Ash: Adsorption Kinetics Kerstin I. Andersson,*,†,‡ Marie Eriksson,‡ and Magnus Norgren† † ‡
Department of Natural Sciences, Engineering and Mathematics, FSCN, Mid Sweden University, SE-851 70 Sundsvall, Sweden SCA R&D Centre, SE-851 21 Sundsvall, Sweden ABSTRACT: The possible application of adsorption for the removal of lignin-related material found in wastewater generated by mechanical pulping was investigated. Activated charcoal and fly ash were used as adsorbents in batch experiments. The lignin-related material exhibited properties well-suited for adsorption onto both adsorbents, although the sorption capacity of activated charcoal exceeds that of fly ash. The experimental data were fitted to pseudo-first- and pseudo-second-order rate kinetic expressions, and an attempt was made to find the rate-limiting step involved in the adsorption processes. The results showed that lignin adsorption onto both activated charcoal and fly ash follows pseudo-second-order rate kinetics and that both boundary-layer diffusion and intraparticle diffusion are likely involved in the rate-limiting mechanisms. Adsorption is an interesting option in advanced wastewater treatment, and fly ash appears to be a suitable low-cost adsorbent for recalcitrant organic pollutants.
1. INTRODUCTION Thermomechanical pulping (TMP) and alkaline peroxide bleaching of pulp generate wastewater containing wood-derived compounds. To reduce the discharge of such compounds into recipient water bodies, biological treatment is normally applied to the wastewater. However, it has been shown that lignin resists biological treatment and significantly contributes to the residual amount of oxygen-consuming substances in treated wastewater.1,2 Such recalcitrant substances might necessitate the use of advanced wastewater treatment methods to satisfy stringent discharge requirements. Adsorption is a well-established method for treating industrial wastewater but seems to rarely be applied in the pulp and paper industry. Activated charcoal is the most widely applied and effective adsorbent,3 although its high material cost limits largescale application.3,4 Finding a low-cost and readily available adsorbent has therefore become of interest to many researchers, and fly ash is reported by many to exhibit properties suitable for adsorption.3,57 This study investigates the adsorption behavior of lignin-related substances found in mechanical pulping effluent. Activated charcoal and fly ash were compared with respect to adsorption capacity and process kinetics by various fitting models. 2. METHODS AND MATERIALS 2.1. Sampling. Wastewater samples were collected at the integrated SCA Ortviken mill in Sundsvall, Sweden, which produces lightweight coated (LWC) paper and newsprint from thermomechanical pulp (TMP). Approximately 55% of the produced TMP is bleached using alkali and hydrogen peroxide. The collected samples were mixed effluent from the TMP plant, bleaching plant, paper machines, and debarking and sludge dewatering processes. Samples were collected on two occasions for the experiments using activated charcoal and fly ash. 2.2. Fractionation of Wastewater. The wastewater was fractionated to isolate lignin by adsorption chromatography r 2011 American Chemical Society
using Amberlite XAD-8 resin (Fluka Chemie, Buchs, Switzerland), as described in detail by Pranovich et al.8 It has previously been demonstrated that this method can be used to isolate pure and unaltered lignin-related material from wastewater.1 The isolated material is henceforth referred to as lignin. 2.2.1. Absorptivity Coefficient by UV Absorption. The isolated lignin fractions from the two wastewater samples were characterized by absorption of UV radiation at 280 nm. The absorptivity coefficients varied between 0.0099 and 0.0114, with average values of 0.0105 and 0.0112 for the two samples. A previous chemical characterization of lignin isolated from wastewater collected at the same sampling point showed an absorptivity coefficient of 0.0129 L/(g cm).1 The similarities in absorptivity suggest that the lignin samples were alike in character and that the characterization presented in Andersson et al.1 can be applied to the lignin used here. 2.3. Adsorbents. Two adsorbents were used: activated charcoal and fly ash. The activated charcoal (Merck, Darmstadt, Germany) was of analytical grade with 90% of the particles smaller than 100 μm. The fly ash was collected from one of the mill’s steam-producing boilers fuelled with bark residues. The fly ash was sieved, and the
