Effectiveness of Cupressus sempervirens cones as biosorbent for the removal of basic dyes from aqueous solutions in batch and dynamic modes

Abstract

The feasibility of using cypress cone chips from Cupressus sempervirens as a low-cost biosorbent for the removal of two representative basic dyes, methylene blue (MB) and rhodamine B (RhB), from aqueous solutions was investigated in batch and continuous modes. Dyes biosorption was strongly dependent on the solution’s pH. Sorption kinetics was determined and properly described by the pseudo-second-order rate model. Experimental equilibrium isotherms fitted the Langmuir model, showing maximum biosorption capacities of 0.62 mmol/g for MB and 0.24 mmol/g for RhB. Competitive experiments from a binary solution of the dyes demonstrated the preference of the cone chips for biosorbing MB. Very low desorption efficiencies were obtained for both dyes. Dynamic experiments showed that the breakthrough time was three times higher for MB biosorption than for RhB for the same conditions. Breakthrough curves were properly represented by a mathematical model.

Introduction

The sustainable removal of dyes from textile wastewater still represents a major environmental challenge. Large amounts of water and chemicals are used to colour fabrics in textile industries. Spent dye baths in discontinuous dyeing, residual dye liquors and water from washing operations always contain a percentage of unfixed dye, which varies considerably depending on dye–fiber affinity and dyeing process parameters. These effluents may constitute a severe hazard if they are released into water bodies without previous treatment (Crini, 2006). Dyes are the first contaminants to be visually recognized, but one of the most difficult to remove because of their synthetic origin and mainly complex aromatic molecules (Banat et al., 1996). Although a great proportion of dyes is not directly or highly toxic for living organisms, high colouration added to the discharge water courses can suppress photosynthetic processes and generate high chemical and biological oxygen demand (Aksu, 2005).

Dyes exhibit considerable structural diversity and are classified by their chemical structure, application to the fiber type and/or solubility (Gupta and Suhas, 2009). Basic dyes are cationic due to the positive charge delocalized throughout the chromophoric system and have affinity towards materials with negatively charged functional groups (Sachdeva and Kumar, 2009), such as wool, silk, nylon and acrylics, where bright dyeing is the prime consideration. Among basic dyes, methylene blue (MB) is the most common water-soluble dye, widely used for dyeing leather, printing calico, cotton and tannin, and for medicinal purposes in its purified zinc-free form (Gupta et al., 2004). Although this dye is not highly toxic to human being, it can cause eye/skin irritation, and systemic effects including cyanosis and blood changes (Tsai et al., 2009). Rhodamine B (RhB) is also a fluorescent basic dye used for dyeing cotton, silk, paper, bamboo, weed and leather, and for preparing carbon paper, ball pen, stamp pad inks and paints (Sarma et al., 2008). It exists in three monomeric molecular forms, as cation, zwitterion and lactone, but the lactone form does not contribute to colour (Deshpande and Kumar, 2002). Though RhB is not strongly hazardous either, acute ingestion of the dye may irritate the gastrointestinal tract and cause toxic effects to skin, eyes and respiratory tract (Lata et al., 2008).

Several methods have been developed to remove dyes from wastewaters, varying in effectiveness and cost. Adsorption using activated carbons is widely recognized as effective for the removal of cationic contaminants from wastewaters, mainly heavy metals and dyes (De Celis et al., 2009, El Qada et al., 2008, Senthilkumaar et al., 2006, Tan et al., 2008). However, the cost of this adsorbent has led to the search for new, less expensive alternatives for the treatment of polluted waters. In this context, biosorption, namely the passive uptake of toxicants by dead/inactive biological materials or by materials derived from biological sources (Vijayaraghavan and Yun, 2008a), has gained the interest of many researchers in recent years (Crini, 2006, Demirbas, 2008). In particular, agro-forest lignocellulosic residues, mostly wood sawdust and bark from tree species have shown good biosorption capacities for some basic dyes (Batzias and Sidiras, 2004, Garg et al., 2003, Janoš et al., 2009). Female cones from conifer trees, that are abundant in wood processing fields and may constitute another promising lignocellulosic biosorbent, have been applied more recently for biosorption of heavy metals (Argun et al., 2008, Malkoc, 2006, Nuhoglu and Oguz, 2004, Ucum et al., 2003). However, their potentialities for dye biosorption have been almost unexplored (Akar et al., 2008). Uses of conifer cones have been limited to domestic fuel in some rural areas, extraction of essential oils for therapeutic purposes when they are still unripe, and on seasonal decoration (Haykiri-Acma and Yaman, 2007). On the other hand, biosorption studies are usually restricted to experiments in batch mode, that are useful to elucidate biosorption capacities parameters for dye removal although they lack of applicability when scaling up is being considered for real wastewater treatment systems. For this purpose, research in dynamic conditions using small-scale continuous flow operation columns, is more suitable to predict the biosorption performance of the material investigated (Aksu, 2005, Vijayaraghavan and Yun, 2008b).

In this framework, the aim of the present work is to examine the feasibility of using natural cypress cone chips from Cupressus sempervirens as a low-cost biosorbent for the removal of two representative basic dyes (MB and RhB) from aqueous solutions, in batch and continuous modes. Effects of various operational parameters were examined, as well as the interaction between the dyes in a competitive study for their biosorption on this material. Kinetic and equilibrium biosorption parameters were obtained from the application of mathematical models. Finally, continuous removal of both dyes was assessed from experiments in an up-flow column.