Experiments investigating lead adsorption by activated sawdust of different particle sizes of two timber species were conducted. The experimental data were fitted to isothermal and kinetic models. The optimum particle size was 0.85 mm for Khaya ivorensis and 1.18 mm for Pycanthus angolensis. The adsorption of lead by Khaya ivorensis and Pycanthus angolensis conformed to the Langmuir isotherm (0.83 ≤ R2 ≤ 0.96 and 0.86 ≤ R2 ≤ 0.98, respectively) and Freundlich isotherm (0.69 ≤ R2 ≤ 0.97 and 0.94 ≤ R2 ≤ 1.0, respectively). The adsorption process for the two species of timber was controlled by solute transport in the bulk liquid and intraparticle diffusion which was confirmed by good agreement of experimental data with pseudo-first-order kinetics (0.96 ≤ R2 ≤ 1.0 for Khaya ivorensis and 0.9 ≤ R2 ≤ 1.0 for Pycanthus angolensis) and the intraparticle diffusion model (0.9 ≤ R2 ≤ 0.99 for Khaya ivorensis and 0.84 ≤ R2 ≤ 0.97 for Pycanthus angolensis). A new kinetic model was developed with R2 of 0.93 ≤ R2 ≤ 0.99 for Khaya ivorensis and 0.88 ≤ R2 ≤ 1.0 for Pycanthus angolensis.

INTRODUCTION

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.

MATERIALS AND METHODS

Preparation of adsorbent and lead solution (adsorbate)

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.

Sorption isotherms

Equilibrium relationships between adsorbent and adsorbate are described by adsorption isotherms which are usually the ratio between the quantity adsorbed and that remaining in solution at fixed temperature at equilibrium. The quantity adsorbed is nearly always normalized by the mass of the adsorbent to allow comparison of different materials. The results of the adsorption experiment were fitted to Langmuir (Equation (1)) and Freundlich (Equation (2)) isotherms. 
formula
1
 
formula
2

Sorption kinetics

By analysing and comparing the adsorption efficiencies of the various particle sizes for each timber species, the optimum particle sizes were obtained and then used for kinetic studies. The optimum sizes of the two samples are PA(1.18 mm) and KI(0.85 mm). The sorption kinetics were investigated using various levels of lead concentration as follows: low concentration (10 mg/l), medium concentration (50 and 70 mg/l) and high concentration (130 mg/l). Two hundred milligrams (0.2 g) of adsorbent was introduced into the lead solution and agitated. Samples of the adsorbate were taken at predetermined time intervals. In order to describe the sorption rate and confirm the reaction mechanism of lead ion (Pb2+) five kinetic models were employed. The performance of the kinetic models was evaluated using R2 and absolute mean error (AME) values. The first-order kinetic model (Equation (3)) is based on the assumption that sorption of lead ions onto the activated sawdust was reversible and followed a first-order rate kinetics (Vinod 2002). The pseudo-first-order kinetic equation (Equation (4)) is the most widely used rate equation for sorption of a solute from a liquid solution based on solid capacity (Ho & Mckay 1999; Ho et al. 2000). The second-order equation (Equation (5)) is seldom used for describing the adsorption process, but it was used in this work for the sake of completeness and confirmation. The pseudo-second-order kinetic model (Equation (6)) is based on the assumption that the rate limiting step may be chemical adsorption involving valance forces through sharing or exchange of electrons between adsorbent and adsorbate (Bulut & Tez 2007). The intraparticle diffusion model (Equation (7)) is used if intraparticle diffusion is the rate limiting step (Attia et al. 2010). If the rate limiting step is intraparticle diffusion, a plot of solute adsorbed against square root of contact time should yield a straight line passing through the origin (Weber & Morris 1963). 
formula
3
 
formula
4
 
formula
5
 
formula
6
 
formula
7

RESULTS AND DISCUSSION

Effect of particle size

The adsorption efficiencies of Khaya ivorensis and Pycanthus angolensis are shown in Figure 1. For large particle sizes (2 mm), Khaya ivorensis performed better than Pycanthus angolensis. The difference in percentage removal of lead was more pronounced at low lead concentration with a percentage removal of 98.7% for Khaya ivorensis and 81.3% for Pycanthus angolensis after 3 hours of contact. However, at high concentration (130 mg/l), the removal efficiency was 50.3% for Khaya ivorensis and 46.6% for Pycanthus angolensis. The reduction in percentage removal as lead concentration increases can be attributed to saturation of adsorption sites in the adsorbent. The performance of Khaya ivorensis was higher than that of Pycanthus angolensis by an average of 16.8% for all concentrations studied. Figure 2 shows that the smallest particle sizes (0.3 mm for Khaya ivorensis and 0.6 mm for Pycanthus angolensis) exhibited the lowest efficiency of lead removal. Generally, Khaya ivorensis performed better than Pycanthus angolensis for particles of comparable sizes. Only particle size 1.18 mm stood out among Pycanthus angolensis particles. The optimum particle sizes for Khaya ivorensis and Pycanthus angolensis are 0.85 mm and 1.18 mm respectively with nearly 100% lead removal efficiency. The optimum adsorbent dosage is 0.133 g/mg of lead with 88% and 94% removal efficiency for Pycanthus angolensis and Khaya ivorensis respectively. The spent adsorbent can be regenerated by steam, pressure, vacuum or microwave. After several cycles of regeneration and use, the adsorbent can be safely disposed of by incineration and the resultant ash sequestered in concrete. Udueyo & Dashibil (2002) observed that partial replacement of Portland cement by up to 30% sawdust ash yields concrete of acceptable strength if given enough time to cure.
Figure 1

Adsorption efficiency of various particles sizes.

Figure 1

Adsorption efficiency of various particles sizes.

Figure 2

Comparison of composite overall efficiency.

Figure 2

Comparison of composite overall efficiency.

Adsorption isotherm

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.

Generally, Khaya ivorensis exhibited higher relative adsorption capacity and maximum adsorption capacity than Pycanthus angolensis (Table 1). The rate of adsorption of solute onto an adsorbent is dependent on the molecular size of the solute relative to the pore size distribution of the adsorbent as well as the electrokinetic properties of the adsorbent and the adsorbate (Weber & Morris 1963). The adsorption mechanism can be further understood by careful observation of the micrographs of the sawdust particles (Figure 3). Even before activation, Khaya ivorensis presented a well-defined network of interconnected pores, unlike Pycanthus angolensis whose pores can hardly be visualized. Activation further opened up the entrance to the pores of Khaya ivorensis, thereby making them more accessible to lead ions. After activation, Pycanthus angolensis presented a sponge-like surface without a well-defined network of pores. Khaya ivorensis seems more amenable to activation than Pycanthus angolensis because the activating agent can easily inundate the highly developed network of pores. Besides, the numerous pores mean increased surface areas of adsorbent in contact with the solute. Ndukwe et al. (2012) reported that the lignin content of Khaya ivorensis is 31.40% while that of Pycanthus angolensis is 28.35%. Heavy metals prefer binding to lignin and tannin than the cellulose backbone of sawdust. Fourier transform infrared analyses (4,000 to 400 cm−1) of the adsorbents before and after use confirm the presence of C = C, C = O, O-H (alcohol and carboxylic acids) and C-H functional groups in the two species of sawdust. The results further confirm the participation of the functional groups in the adsorption process by a shift in wavenumbers as summarized in Table 2. These shifts in wavenumbers indicate change in functional groups of adsorbents and surface properties as a result of lead adsorption onto the adsorbent surface.
Table 1

Freundlich and Langmuir isotherm coefficients

Type of SawdustParticle SizeFreundlich Isotherm
Langmuir Isotherm
KfnR2qmaxKLR2
Pycanthus angolensis 2.36 2.5 2.56 0.98 14.47 0.097 0.93 
1.5 1.88 0.99 18.42 0.041 0.91 
1.18 3.1 2.54 0.94 15.43 0.190 0.98 
0.71 2.13 2.50 1.00 14.21 0.062 0.94 
0.6 1.74 2.37 0.94 14.65 0.040 0.86 
Khaya ivorensis 4.17 3.24 0.96 17.86 0.096 0.92 
1.18 2.77 2.67 0.94 14.60 0.107 0.87 
0.85 3.71 2.88 0.83 15.49 0.210 0.97 
0.6 2.82 2.39 0.94 17.86 0.616 0.84 
0.3 1.15 1.90 0.94 19.61 0.020 0.69 
Type of SawdustParticle SizeFreundlich Isotherm
Langmuir Isotherm
KfnR2qmaxKLR2
Pycanthus angolensis 2.36 2.5 2.56 0.98 14.47 0.097 0.93 
1.5 1.88 0.99 18.42 0.041 0.91 
1.18 3.1 2.54 0.94 15.43 0.190 0.98 
0.71 2.13 2.50 1.00 14.21 0.062 0.94 
0.6 1.74 2.37 0.94 14.65 0.040 0.86 
Khaya ivorensis 4.17 3.24 0.96 17.86 0.096 0.92 
1.18 2.77 2.67 0.94 14.60 0.107 0.87 
0.85 3.71 2.88 0.83 15.49 0.210 0.97 
0.6 2.82 2.39 0.94 17.86 0.616 0.84 
0.3 1.15 1.90 0.94 19.61 0.020 0.69 
Table 2

Band position of functional groups for unused and spent adsorbent

AdsorbentBand Position (cm−1)
C = C (aromatic)C = C (alkanes)C = O (carboxylic)O-H (H bonded)O-H (carboxylic acid)C-H
KI Activated Carbon 1,536.04 1,659.36 – 3,404.28 3,045.3 2,983.45 
Spent KI Activated Carbon 1,536.04 1,632.54 1,717.46 3,412 3,095.48 2,987.4 
PI Activated Carbon 1,597 1,659.66 1,717.46 3,446.47 3,230.58 2,991.26 
Spent PI Activated Carbon – – – 3,458.32 3,265.32 3,045.30 
AdsorbentBand Position (cm−1)
C = C (aromatic)C = C (alkanes)C = O (carboxylic)O-H (H bonded)O-H (carboxylic acid)C-H
KI Activated Carbon 1,536.04 1,659.36 – 3,404.28 3,045.3 2,983.45 
Spent KI Activated Carbon 1,536.04 1,632.54 1,717.46 3,412 3,095.48 2,987.4 
PI Activated Carbon 1,597 1,659.66 1,717.46 3,446.47 3,230.58 2,991.26 
Spent PI Activated Carbon – – – 3,458.32 3,265.32 3,045.30 
Figure 3

Scanning electron microscope view of activated and non-activated sawdust.

Figure 3

Scanning electron microscope view of activated and non-activated sawdust.

Kinetics of lead adsorption

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%.

Table 3

Performance of kinetic models

Type of SawdustInitial Conc (C0) (mg/l)1st Order
2nd Order
Pseudo 2nd Order
Pseudo 1st Order
Intraparticle Diffusion
R2Calc C0R2Calc C0R2Calc C0R2Calc C0R2Calc C0
Khaya ivorensis 10 0.96 10.30 0.94 5.55 0.75 11.64 0.96 12.06 0.90 9.39 
50 0.97 44.24 0.99 57.651 0.95 55.26 1.0 49.63 0.98 52.01 
70 0.90 57.80 0.96 71.45 0.98 74.91 0.98 67.57 0.92 74.22 
130 0.94 115.50 0.97 116.56 0.97 138.05 1.0 124.01 0.99 133.14 
AME 10.8 18.053 10.0 7.4 4.6 
Pycanthus angolensis 10 0.95 5.78 0.94 5.80 0.97 11 0.95 12.30 0.84 11.44 
50 0.96 42.81 1.0 47.52 0.99 57.68 1.0 47.65 0.97 52.9 
70 0.88 79.52 0.94 51.32 0.82 91.11 0.90 75.85 0.92 73.71 
130 0.87 108.34 0.93 109.03 0.99 135.74 0.99 115.93 0.95 135.28 
AME 21.7 22.4 15.0 11.7 7.4 
Type of SawdustInitial Conc (C0) (mg/l)1st Order
2nd Order
Pseudo 2nd Order
Pseudo 1st Order
Intraparticle Diffusion
R2Calc C0R2Calc C0R2Calc C0R2Calc C0R2Calc C0
Khaya ivorensis 10 0.96 10.30 0.94 5.55 0.75 11.64 0.96 12.06 0.90 9.39 
50 0.97 44.24 0.99 57.651 0.95 55.26 1.0 49.63 0.98 52.01 
70 0.90 57.80 0.96 71.45 0.98 74.91 0.98 67.57 0.92 74.22 
130 0.94 115.50 0.97 116.56 0.97 138.05 1.0 124.01 0.99 133.14 
AME 10.8 18.053 10.0 7.4 4.6 
Pycanthus angolensis 10 0.95 5.78 0.94 5.80 0.97 11 0.95 12.30 0.84 11.44 
50 0.96 42.81 1.0 47.52 0.99 57.68 1.0 47.65 0.97 52.9 
70 0.88 79.52 0.94 51.32 0.82 91.11 0.90 75.85 0.92 73.71 
130 0.87 108.34 0.93 109.03 0.99 135.74 0.99 115.93 0.95 135.28 
AME 21.7 22.4 15.0 11.7 7.4 

As the concentration of lead solution increases, the adsorption process tends to be explained more by the intraparticle diffusion model. It was observed that at low concentrations of lead, the process basically follows first-order kinetics because of availability of sufficient sorption sites within the adsorbent. Hence, at low concentrations, the liquid phase transport becomes the limiting factor. The liquid phase transport process involves mostly transport in the bulk liquid phase, diffusion across the liquid film surrounding solid particles and diffusion in liquid-filled mesopores (Ho et al. 2000). However, the major limitations of these models is their failure to accommodate the time-dependent nature of the diffusion rate in batch adsorption. This anomaly was clearly manifested in the kinetic plots by a clear departure from a straight line after about 30 minutes. This same anomaly was observed by Bulut & Tez (2007). This departure could signify a decrease in mass transfer rate in the liquid phase and change in the rate controlling process. Despite the high performance of the intraparticle diffusion model, it was found that intraparticle diffusion is not the only rate controlling process because a backward extrapolation of the model did not pass through the origin. The adsorption process for porous solids can be separated into three stages: (i) mass transfer or boundary layer diffusion, (ii) sorption onto sites, and (iii) intraparticle diffusion inside the pore system (Attia et al. 2010). Ho et al. (2000) observed that intraparticle diffusion is often the rate-limiting step in many adsorption processes involving sorption of heavy metals by biosorbents. The equations for intraparticle diffusion indicate that kd values will vary with the square root of concentration (Ho et al. 2000). This requirement was met by the experimental data as demonstrated by Figure 4.
Figure 4

Intraparticle diffusion parameter versus concentration.

Figure 4

Intraparticle diffusion parameter versus concentration.

Finally, a new kinetic model was developed to account for both the concentration and time dependent nature of sorption rate. This model recognizes that the availability of sorption sites deceases with time (Equation (8)): 
formula
8
Integrating Equation (8) gives rise to Equation (9) which is taken herein as the new model. 
formula
9
The new model outperformed all the other models except the intraparticle diffusion model. The AME was 6.48 for Khaya ivorensis and 10.21 for Pycanthus angolensis as against 4.64 for Khaya ivorensis and 7.39 for Pycanthus angolensis exhibited by the intraparticle diffusion model. However, the new model had higher R2 values ranging from 0.88 to 1.0 for Khaya ivorensis and 0.93 to 0.99 for Pycanthus angolensis as against 0.84 to 0.95 for Khaya ivorensis and 0.9 to 0.99 for Pycanthus angolensis, respectively, obtained for the intraparticle diffusion model. Figure 5 shows that the adsorption data was in good agreement with the new model. It can be clearly observed that the slope of the plot of LnC versus square root of time decreases as concentration of lead decreases signifying the diminishing rate of intraparticle diffusion in dilute solution.
Figure 5

Plots of kinetic models.

Figure 5

Plots of kinetic models.

CONCLUSION

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.

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