Treatment of metal-contaminated wastewater: A comparison of low-cost biosorbents

Highlights

• Biosorption is highly pH and metal dependent.

• Biosorption is less temperature dependent.

• Competitive metal binding can adversely affect the use of seaweed for certain metals.

Abstract

This study aimed to identify some optimum adsorption conditions for the use of low-cost adsorbent, seaweed (Ascophyllum nodosum), sawdust and reed plant (Phragmites australis) root, in the treatment of metal contaminated wastewater for the removal of cadmium, chromium and lead. The effect of pH on the absorption capacity of each of these biosorbents was found to be significant and dependent on the metal being removed. Post-adsorption FTIR analysis showed significant binding activities at the nitro NO groups site in all biosorbents, especially for lead. Competitive metal binding was found to have possibly affected the adsorption capacity for chromium by A. nodosum more than it affected sawdust and P. australis root. Adsorption is believed to take place mainly by ion exchange particularly at low pH values. P. australis root exhibited the highest adsorption for chromium at pH 2, cadmium at pH 10 and lead at pH 7. A. nodosum seaweed species demonstrated the highest adsorption capacity of the three biosorbents used in the study, for cadmium at pH 7 and for lead at pH 2. Sawdust proved to be an efficient biosorbent for lead removal only at pH 7 and 10. No significant effect of temperature on adsorption capacity was observed, particularly for cadmium and lead removal.

Introduction

Heavy metal contamination of industrial effluents has been an issue of environmental and public health concern since the advent of industrialisation, owing to their non-biodegradable, toxic and bio-accumulative nature (Bailey et al., 1999, Nomanbhay and Palanisamy, 2005). Indicative metals are mostly transition metals like cadmium, copper, zinc, nickel, chromium, as well as other heavy metals such as lead, aluminium, and mercury, amongst others that pose significant risk to soil, water and air environments, their inhabitants and human health via the food chain (Oboh et al., 2009). On discharge of effluents from industries like mining, textile, tannery, metal-plating, petro-chemical, battery and fertilizer production, these metals are deposited in soil, aquatic life-forms and tissues, which form part of the human food-chain. Thus, the toxicity, bio-accumulation and persistence of these metals are transmitted through the food chain and the environment to cause environmental and human health problems (Kanamadi et al., 2006). This situation is most prevalent in tropical developing countries where the enforcement of industrial effluent discharge limits is yet to be accorded the priority it deserves, in contrast to developed countries where stringent environmental quality standards to control pollution by industrial effluents and protect the environment are being adopted and enforced (Dan'Azumi and Bichi, 2010).

Conventional treatment methods have been found to be very expensive and difficult to maintain due to high capital and operational costs as well as extra cost of treating the resultant sludge/secondary waste before disposal as it also poses hazards and pollution risks to the environment (Kumar, 2006). Due to these challenges associated with the conventional methods, growing interest and research into the use of low-cost adsorbents as preferred alternatives to the conventional methods has yielded various successful applications of these low-cost, naturally–occurring and readily available organic adsorbents in the treatment of metal-contaminated industrial effluents using adsorption processes, as reported in various studies (Oboh et al., 2009). The distinct advantages of biosorbents include: low cost, high efficiency, reduced production of chemical or biological sludge, possibility of regeneration of biosorbents and metal recovery (Sud et al., 2008).

Seaweed, sawdust and reed plant root are some of the low-cost readily available organic residues, found naturally in the environment and are considered waste or by-products. Ascophyllum nodosum is considered an effective seaweed for metal adsorption (Romera et al., 2006) and macroscopically produces biosorbent particles, conferring on it a better metal-binding capacity than most organic or inorganic adsorbents (Freitas et al., 2006). Seaweed is reportedly rich in three main polysaccharides – laminaran, fucoidin and alginate, which are rich in anionic carboxylate and sulphate ready for binding at neutral pH (Alluri et al., 2007). The alginate polysaccharide is responsible for its ion-exchange capacity due to its high concentration of carboxyl groups (Mehta and Gaur, 2005, Freitas et al., 2009). The binding surface of seaweed is rich in functional groups like carboxyl, hydroxyl, amine, imidazole, phosphate, sulfhydryl and sulphate groups. However, the carboxyl and sulphate groups are reputably the most active groups in the binding of metals during adsorption (Romera et al., 2007).

Sawdust, a by-product of the timber industry, which is considered waste except when it is used as packaging material (Vinodhini and Das, 2010) or as cooking fuel, is abundantly available to the point of constituting disposal challenges in timber mills, especially in developing countries. Sawdust consists of three dominant components: cellulose, lignocellulose and lignin, based on which its functional groups reputed to be actively involved in adsorption include CCH, CC, C–OH and C–O–C groups (Abdel-Ghani et al., 2007). Its main mechanisms of adsorption are ion exchange and hydrogen binding (Shukla et al., 2002), as well as, chelation and complexation reactions (Asadi et al., 2008).

Reed plant, Phragmites australis, is a perennial helophyte grass commonly found in tropical and temperate wetlands where it grows rapidly and exhibits outstanding tolerance of high metal concentrations (Batty, 2003). It is the most commonly employed plant species in the treatment and polishing of wastewater in constructed wetlands where it enhances the break-down organic pollutants by adsorption, accumulation and oxidation via aeration systems around the root zones (Lee and Scholz, 2007). Reed plant contains high concentrations of lignin and cellulose, two components believed to play important roles in its adsorptive capacity for various heavy metals from solution (Southichak et al., 2006). Carboxylic, carboxylate, lignin and lignin ester aromatic C–C groups have been revealed to actively contribute in adsorptive capacity (Southichak et al., 2006), irrespective of contrasting research reports on its suitability as a biosorbent (Batty, 2003) or otherwise (Lee and Scholz, 2007).

The aim of this study is to identify optimum adsorption conditions for the use of seaweed (Ascophyllum nodosum), sawdust and reed plant (Phragmites australis) root in the removal of cadmium, chromium and lead from metal-contaminated wastewaters. Cadmium and lead constitute two of the three heavy metals listed by the World Health Organisation (WHO) as the ‘Big Three’ toxic metals, along with Mercury, while chromium was selected because of its ubiquitous presence in various industrial effluents, which are of interest in this study.