A comparative study of lead adsorption by activated Khaya Ivoresnsis and Pycanthus Angolensis sawdust

Heavy metals are not biodegradable and tend to persist in water and accumulate in living organisms, causing various health disorders. Lead has been identified as one of the most toxic heavy metals that has dominant long-term negative impacts on health, causing hepatitis, anaemia, nephritic syndrome, brain damage, mental deficiency, cancers, reduced fertility, kidney failure, autoimmunity and Alzheimer's disease (Lin et al. 1996; Brooks et al. 2010). Lead remains one of the most widely used industrial metals and its presence in water can be traced to effluents from metal finishing and electroplating, mining and operations, textile activities and nuclear power generation (Honda et al. 2007; Akpor et al. 2014). The most recent case of water contamination by lead of up to 13,200 ppb in Flint, Michigan is a wake-up call to focus attention on this ubiquitous heavy metal. In a related incident, 28 children under the age of five died as a result of drinking water contaminated by lead from gold mining activities in Nigeria. In June 2015, a panel of enquiry found lead concentrations exceeding World Health Organization limits in the Hong Kong public water supply. Lead can be removed from water by chemical precipitation, ion exchange, and membrane techniques. However, these methods are plagued by high capital and operational cost (Chiban et al. 2012), hence the need for a low-cost approach.

Results from several studies have proved that sawdust has good adsorption properties and can be employed in large-scale removal of contaminants from water (Mane & Vijay-Babu 2007; Kalavathy et al. 2009). Though sawdust has long been recognized as a good, low-cost adsorbent, large quantities of sawdust continue to be improperly disposed of worldwide. In general, an adsorbent can be assumed as ‘low cost’ if it requires little processing, is abundant in nature, or is a by-product or waste material from industry (Bailey et al. 1999). Abundantly available low-cost adsorbent can be disposed after the first cycle of use, without the need for the expensive process of regeneration (Bailey et al. 1999). Apart from being in continuous supply and abundantly available, sawdust is a free resource in developing countries where it is constantly disposed of along with municipal solid waste. Just like other carbon-based waste products, sawdust is mostly composed of lignin and cellulose, as well as other polar poly functional groups that are able to bind heavy metals and form complexes with the metal ion in solution (Bulut & Tez 2007; Begum & Alhaji 2013; Gad et al. 2013). The high lignin content of sawdust makes it an excellent adsorbent for metals (Bryant et al. 1992). This research was aimed at ascertaining the effectiveness of sawdust of various particle sizes and timber species in the removal of lead from water. The novelty of this research lies in the selection of sawdust of known specific timber species unlike several other studies that used composite sawdust, segregation into various ranges of particle sizes in order to ascertain their effect, and the development of a new kinetic model to account for decrease in sorption rate with time.

Two timber species (Pycanthus angolensis commonly called ‘Akwa mmiri’ and Khaya ivorensis commonly called Mahogany) were selected from twelve candidate timber species because of their abundance and prevalence of use in woodworks in Nigeria. The two timber species have widely different characteristics. Detailed physico-chemical characteristics of the two species have been presented by Ejikeme et al. (2014). The two sawdust samples were obtained on request from a timber mill in Enugu Timber Market. The sawdust samples were sieved into different sizes. The sizes of Pycanthus angolensis sawdust used in this work are 2.36 mm, 2.00 mm, 1.18 mm, 0.71 mm, and 0.60 mm which were denoted as PA(2.36 mm), PA(2.00 mm), PA(1.18 mm), PA(0.71 mm), and PA(0.60 mm) respectively; while the sizes of Khaya ivorensis sawdust used were 2.00 mm, 1.18 mm, 0.85 mm, 0.60 mm and 0.30 mm denoted as KI(2.00 mm), KI(1.18 mm), KI(0.85 mm), KI(0.60 mm) and KI(0.30 mm) respectively. The slight variation in particle sizes used was as a result of the different ranges of particle sizes between Khaya ivorensis and Pycanthus angolensis. These samples were carbonized in the oven at a temperature of 250 °C, activated by mixing with 0.5 mol of HCl and soaked for 2 hours. The slurry was washed with deionized water repeatedly until neutral pH was reached. It was then oven dried for 3 hours at a temperature of 103 °C. Lead solutions of concentrations 130 mg/l, 110 mg/l, 90 mg/l, 70 mg/l, 50 mg/l, 30 mg/l and 10 mg/l were prepared by adding 13 ml, 11 ml, 9 ml, 7 ml, 5 ml, 3 ml and 1 ml of standard lead solution of 100 mg/l into 100 ml of deionized water. Two hundred milligrams (0.2 g) of adsorbent was weighed out and added to the lead solution. Thereafter, the solution was put in a shaker and was agitated for 3 hours. The concentration of lead remaining in the solution after 3 hours was determined using an ultraviolet (UV) spectrophotometer (Uv-1800 Shimadzu, Japan) which had previously been calibrated by determining the absorbance for predetermined concentrations of lead solution.

The adsorption isotherm describes how target species are distributed between liquid and solid phases at equilibrium (Attia et al. 2010). The adsorption of lead by Khaya ivorensis was found to follow the Freundlich isotherm for all particle sizes with R² values ranging from 0.83 to 0.96. The process also fitted the Langmuir isotherm, especially for large and intermediate particles (0.6 mm to 2.0 mm) with R² values ranging between 0.86 and 0.97. However, finer particles (0.3 mm) had an R² value of 0.69 which is low compared to others. The adsorption of lead by Pycanthus angolensis was better described by the Langmuir isotherm for all ranges of particle size studied. The R² ranged from 0.94 to 1.0 with particle size 0.7 mm having the highest R² of 1.0. Though the Langmuir isotherm also fitted the adsorption of lead by Pycanthus angolensis, the R² values (0.86 to 0.98) were generally lower than those of the Freundlich isotherm for sawdust of comparable particle sizes. It was however observed that the Langmuir isotherm did not perform very well with respect to the finest Pycanthus angolensis particles of 0.6 mm. From the foregoing, it can be surmised that the adsorption of lead by sawdust can best be described by the Freundlich isotherm which recognizes surface heterogeneity and molecular interaction. The smallest particle sizes (0.3 mm for Pycanthus angolensis and 0.6 mm for Khaya ivorensis), having the lowest overall removal efficiencies, fitted the Freundlich isotherm better than the Langmuir isotherm.

Sorption kinetics yield the solute uptake rate which determines the residence time required for completion of the sorption process, as well as providing information regarding the reaction mechanism (Ho et al. 2000). Because the sorption mechanism may change during the entire period of the sorption process, it is necessary to test several models over the entire sorption range in order to identify the correct sorption model. The experimental data were tested on existing kinetic models. All the kinetic models performed well with R² values ranging from 0.75 to 1.0 for Khaya ivorensis and 0.84 to 1.0 for Pycanthus angolensis. At low concentrations of lead, the first-order kinetic and the intraparticle diffusion models gave the best results for both Khaya ivorensis and Pycanthus angolensis. For moderate concentrations of lead (50 mg/l and 70 mg/l), the pseudo-first-order and the intraparticle diffusion models gave the best results for both species. The intraparticle diffusion model, with AME of 4.64 for Khaya ivorensis and 7.39 for Pycanthus angolensis, followed by the pseudo-first-order kinetic model, with AME of 7.35 for Khaya ivorensis and 11.72 for Pycanthus angolensis, outperformed the other kinetic models. Several researchers also reported that the pseudo-first order kinetic model is suitable for describing the adsorption of heavy metals by sawdust (Kobya 2004; Attia et al. 2010). As can be seen from Table 3, Khaya ivorensis had lower AME than Pycanthus angolensis for all the models, suggesting that the adsorption of lead by Khaya ivorensis is more amenable to kinetic interpretations than Pycanthus angolensis. The relatively high performance of the pseudo-first-order kinetic model shows that the process is not only controlled by the concentration of lead, which in turn determines the rate of transport in the bulk liquid phase, but also by the characteristics of the adsorbent. Aharoni & Sparks (1991) noted that when the chemical reaction in the solid phase is rapid, the liquid phase transport process becomes the rate controlling reaction. Unlike the first-order and second-order models, that assume that rate of sorption is solely dependent on the solute concentration, the pseudo kinetic models suggest that the rate of sorption of the solute as a mass fraction of the adsorbent is directly proportional to the availability of active sorption sites. Invariably the first- and second-order pseudo kinetic models take into cognisance the adsorption capacity of the adsorbent. It was observed that the first- and second-order models almost consistently underestimated the initial lead concentration. This signifies an inability to account for a fraction of lead adsorbed. On the other hand, the pseudo-second-order kinetic models consistently overestimated lead concentration by up to 30%.

Activated sawdust of both Khaya ivorensis and Pycanthus angolensis can be effectively applied in the removal of lead from water, though the former has proved to be a better option. Khaya ivorensis exhibited higher adsorption capacity ranging between 14.60 and 19.60 mg/g than Pycanthus angolensis (14.21–18.42 mg/g). Intermediate particle sizes ranging between 0.85 mm and 1.18 mm gave the best adsorption efficiency for the two timber species, confirming the significance of particle size in the adsorption process using activated sawdust. Khaya ivorensis conforms to the Freundlich isotherm with R² ranging between 0.94 and 1.0 while Pycanthus angolensis conforms to the Langmuir isotherm with R² values ranging between 0.86 and 0.98. The adsorption of lead by both Khaya ivorensis and Pycanthus angolensis sawdust is controlled by solute transport in the bulk liquid and intraparticle diffusion. The concentration of lead in both large- and small-scale industrial effluents can be attenuated by activated sawdust of these timber species.