sedimentation and sorption onto Pinus sylvestris sawdust

Bioresource Technology 100 (2009) 235–243

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Lead and vanadium removal from a real industrial wastewater by gravitational settling/sedimentation and sorption onto Pinus sylvestris sawdust F. Kaczala a,b, M. Marques a,c,*, W. Hogland a a

School of Pure and Applied Natural Sciences, Faculty of Natural Sciences and Technology, University of Kalmar, Landgången 3, Kalmar 391 82, Sweden The CAPES Foundation, Brazil Ministry of Education, Brazil c Department of Sanitary and Environmental Engineering, Rio de Janeiro State University-UERJ, Rio de Janeiro, Brazil b

a r t i c l e

i n f o

Article history: Received 3 November 2007 Received in revised form 27 May 2008 Accepted 29 May 2008 Available online 27 July 2008 Keywords: Sorption kinetics Sorption isotherms Heavy metals Wood industry Low-cost adsorbents

a b s t r a c t Batch sorption with untreated Pinus sylvestris sawdust after settling/sedimentation phase to remove vanadium and lead from a real industrial wastewater was investigated using different adsorbent doses, initial pH, and contact time. The development of pH along the sorption test and a parallel investigation of metals release from sawdust in distilled water were carried out. In order to evaluate kinetic parameters and equilibrium isotherms, Lagergren first-order, pseudo-second-order, intra-particle diffusion and Freundlich models were explored. When the initial pH was reduced from 7.4 to 4.0, the sorption efficiency increased from 32% to 99% for Pb and from 43% to 95% for V. Whereas, V removal was positively correlated with the adsorbent dose, Pb removal was not. The sorption process was best described by pseudo-second-order kinetics. According to Freundlich parameters (Kf and n) sawdust presented unfavourable intensity for sorption of V. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Heavy metals are found in wastewater discharged from industries such as mining, smelters, electroplating, dyes, textiles, tanneries, oil refineries, but also in stormwater runoffs from urban and agricultural areas where important sources for metal loads to soils includes atmospheric deposition, manure and fertilizers. Several biological studies have associated metals, such as lead, cadmium, copper, zinc, mercury, vanadium and arsenate with toxic effects and the ability to produce reactive oxygen species (ROS), resulting in lipid peroxidation and antioxidant enzymes alterations, leading to oxidative stress (Hu, 2000). Some of them, such as Pb, accumulate in the tissues of marine organisms, being conveyed through the food chain to humans (Storelli, 2008). There is a lack of scientific investigation addressing V toxicity and controversy regarding several toxic effects described in animals and humans exist (Srivastava, 2000); there are few publications addressing technologies to remove V from contaminated waters and further investigation is needed in order to bridge existing gaps. The most widely applied methods for metal removal from wastewaters are: chemical and electrochemical precipitation

* Corresponding author. Address: School of Pure and Applied Natural Sciences, Faculty of Natural Sciences and Technology, University of Kalmar, Landgången 3, Kalmar 391 82, Sweden. Tel.: +46 (0) 480 44 6142; fax: +46 (0) 480 44 7305. E-mail addresses: [email protected] (F. Kaczala), [email protected] (M. Marques). 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.05.055

(Ayyappan et al., 2005; Li et al., 2007); cationic and anionic ion-exchange resins (Shukla et al., 2002); membrane filtration and sorption (Shukla et al., 2002). The cost effectiveness of most methods can be questionable. Whereas, some methods such as ion-exchange are costly, others such as precipitation techniques have problems for disposal of metal-containing sludge. Sorption methods are considered flexible, easy to operate, with much less sludge disposal problems. Besides different types of commercial adsorbents such as granular activated carbon-GAC or powdered activated carbon-PAC, several low-cost adsorbents able to sequester heavy metals from contaminated waters have been extensively studied, many of them, being generated by forestry and agricultural activities (Ayyappan et al., 2005), with advantageous chemical characteristics (Vásquez et al., 1994). Efficient removal of Cu, Zn, As, Cd, Cr, Fe, Pb, Hg and Ni has been achieved with low-cost adsorbents in bench scale such as: biomass waste from biological wastewater treatment system (Al-Qodah, 2006; Chang et al., 2006); cotton boll (Duygu Ozsoy and Kumbur, 2006); pollens (Ucun et al., 2003); teak leaves powder (King et al., 2006); black gram husk (Saeed and Iqbal, 2003); grape stalk (Martínez et al., 2006); peat (Brown et al., 2000); fly ash (Wang and Wu, 2006); wood barks (Genç-Fuhrman et al., 2007; Shin et al., 2007) and; sawdust (Sciban et al., 2007). Among low-cost adsorbents, sawdust has been considered the most promising, due to abundance and availability mostly in wood-based industries (Shukla et al., 2002).

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Whereas, an extensive number of sorption studies in synthetic spiked waters is found in the literature (usually distillate water plus one or two metal under investigation) many of them having sawdust as adsorbent, the conclusions obtained from these studies can only be applied to real wastewaters to a very limited extent (Sciban et al., 2007). Industrial wastewaters usually contain a broad range of different cations as well as other inorganic and organic contaminants that certainly interfere in sorption mechanisms, including competitive sorption among metals, and also among metals and inorganic-organic pollutants, such as Cl, 3  þ 2+ 2+ + SO2 4 , PO4 , Ca , Mg , NO3 , NH4 , Na and others (Kalmykova, 2006; Raji and Anirudhan, 1998). Laboratory tests with metal-artificial solutions can, for instance, overestimate the performance of an adsorbent. Therefore, specific experiments using real waters are necessary if the final objective is to assess the potential for real wastewater treatment schemes (Kalmykova, 2006). Additionally, very few studies take into account the metal release from natural sawdust, which might result in an underestimation of the adsorbent’s removal efficiency. In the present study, a low-cost and fast treatment scheme to remove Pb and V from a real industrial wastewater stream generated by a wood floor industry during equipment and machinery washing procedures, combining gravitational settling/sedimentation in a first step with sawdust sorption was tested in laboratory scale. The goal was to bring knowledge to the development and design of an on-site treatment plant, starting with a pilot scale version. The results here presented focus on sawdust sorption processes.

COD according to DIN 38409H41-1 and TOC according to DIN 38403-H3. 2.4. Sawdust batch sorption tests 2.4.1. Adsorbent preparation Pinus sylvestris sawdust supplied by the same company that generated the wastewater to be treated was passed through a #10 sieve to obtain particles with 62 mm. The adsorbent was tested in its natural condition without washing or any chemical pre-treatment. Such strategy was chosen in order to test the suitability of this material under the simplest and practical operational conditions to be carried out later on by the company. 2.4.2. Batch tests ´ s were carried out: (i) Two batch tests with different initial pH the natural pH 7.4 found in the wastewater mixture after sedimentation with no further adjustment and (ii) the pH 4.0, based on literature information. A third test (blank test) with (iii) sawdust in distilled water (initial pH 6.5) intended to check an eventual release of metals from sawdust. (i) Initial pH without adjustment: Keeping the natural wastewater pH of 7.4 six adsorbent doses were tested: 1.25, 2.5, 12.5, 25, 50 and 75 g L1. Each batch test was performed in a 600 mL beaker. After adding the sawdust into the beaker, 400 mL of wastewater collected from the Imhoff cone (after 24 h of sedimentation) assumed as the initial concentration (C0, t = 0) for the sorption phase was poured and the system was kept under constant agitation (700 rpm) with a magnetic stirrer at room temperature (23 °C). It is assumed that by using an agitation speed as high as 700 rpm, no major constraints would be posed by the sampling procedure over the study results. Samples from the water phase (30 mL each) were carefully withdrawn in order to keep the same liquid/solid ratio (L/m) at 5, 11, 17, 22, 40, 80 and 160 min. Every time a sample was taken, the pH and water temperature were registered with a Digital pH meter-WTW Multi 340i. Each 30 mL sample was passed through vacuum filtration (GF/C WHATMANÒ 0.45 lm microfiber filters) in order to obtain solid–liquid separation. (ii) Initial pH adjusted: For the second batch test, the initial pH was adjusted to 4.0 with HCl (0.1 M) immediately after 48 h of sedimentation. Based on the results obtained during the first batch test, the sawdust dose of 25 g L1 was found to be the most efficient one if reduction of both metals is considered and therefore, it was selected for the second batch test with initial pH adjusted. This batch test was performed following the same procedures described for the first. Samples from the water phase (30 mL each) were withdrawn at different contact times (5, 17, 40, 80 and 160 min). (iii) Blank tests: In order to check an eventual release of heavy metals from the sawdust, a leaching test was carried out using distilled water. This approach is different from desorption tests widely reported (Al-Asheh and Duvnjak, 1997;

2. Methods 2.1. Wastewater A mixture of two wastewater streams generated during floor washing procedures at two different areas within the wood floor industry AB Gustaf Kähr in Nybro, South of Sweden was studied. Both wastewater streams were collected immediately after generation, brought to the laboratory and mixed in a shaker (1 L) in the proportion of 1:1 (v:v). The characterization of the wastewater is found in Table 1. 2.2. Gravitational settling/sedimentation Settling/sedimentation tests were carried out in order to simulate a primary treatment. The two-stream wastewater mixture was poured into an Imhoff Cone. Retention times of 24 and 48 h were investigated. All samples were stored at 4 °C in the laboratory for further analysis. Samples had pH and temperature registered with a Digital pH meter (WTW Multi 340i). 2.3. Analytical methods The metals As, Ba, Pb, Cd, Cr, Co, Fe, Cu, Mn, Ni, V and Zn were analyzed according to the ISO 17297-m method using an ICP-MS,

Table 1 Wastewater characteristics after mixture of two streams Variable Unit

pH

SMa mL L1

Fe mg L1

Zn

Mn

Ba

Cu

Pb lg L1

Cr

Ni

Co

V

As

Cd

CODb TOCb mg L1 (1000)

Mean St.dev.c

7.4 0.03

100 10

31.3 22.7

14.6 6.4

2.4 0.7

2.0 0.3

1.0 0.38

635 65

245 25

140 20

100.5 19.5

73 6

35 6

8.65 0.35

15–66

a b c

SM = Settleable matter. Minimum and maximum values. St.dev. = Standard deviation.

3.5–15

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Amuda et al., 2007; Raji and Anirudhan, 1998). Adsorbent doses selected were 25 and 50 g L1. The procedure followed the same steps described for the previous experiments. Samples were withdrawn at different contact times (5, 11, 17, 40, 80 and 160 min).

2.6.3. Intra particle diffusion model Intra-particle diffusion/transport process in the current experiment was explored by using Weber–Morris equation (6) (Weber and Morris, 1963) as described by (Kavitha and Namasivayam, 2007; Kula et al., 2008). 1

qt ¼ ki t 2 þ c; 2.5. Mass balance

ð6Þ 1

Assuming that sorption processes are responsible for the reduction of target heavy metals concentration in the liquid-phase, the following mass balance approach was used during the agitation period:

ðLt Þ ðC o  C t Þ ¼ ðmt Þðqt  qo Þ;

ð1Þ

where 0 < t < 160 min; Lt is the volume of solution at time t (L); C0 is the liquid-phase concentration at time t = 0 (lg L1); Ct is the liquid-phase concentration at time t (lg L1); mt is the adsorbent mass at time t (g); q0 is the solid phase concentration at time t = 0 (lg g1); and qt is the solid phase concentration at time t (lg g1). The mass balance was calculated both for: (i) a constant liquid/ solid ratio (L/m) along the agitation period in each batch (with each adsorbent dose) since the liquid mass as well as the adsorbent content is reduced after each sample is taken along the experiment and; (ii) the use of a solution with the same initial concentration (C0 = constant).

where qt is the amount adsorbed at time t (lg g ); t contact time (min), ki is the intra-particle diffusion rate constant (lg g1 min0.5). ki and C are calculated by plotting the function (Eq. (6)). 2.7. Sorption isotherms 2.7.1. Freundlich model Equilibrium isotherms indicate the capacity of a specific adsorbent at different experimental conditions (Ayyappan et al., 2005). In the present investigation, Freundlich model was applied, since this model has been considered particularly suitable for low to intermediate concentration ranges (Mohan et al., 2007). The Freundlich model in its linear form is expressed as:

log qe ¼ log kf þ

1 log C e ; n

ð7Þ

where Ce is the equilibrium concentration (lg g1); qe is the adsorbed amount at equilibrium (lg g1); 1/n and Kf are Freundlich constants. Kf and n are experimentally calculated by plotting the function (Eq. (7)).

2.6. Kinetic modelling The kinetics of Pb and V sorption were subjected to the Lagergren first-order according to Lagergren (1898) as described by (Ho and McKay, 1999) pseudo-second-order (Ho and McKay, 2000) and intra-particle diffusion models according to Weber and Morris (1963) as described by (Kavitha and Namasivayam, 2007; Kula et al., 2008). The explored models were followed according to Eqs. (2), (4), and (6), respectively. 2.6.1. Lagergren first-order equation The most popular sorption kinetics equation is expressed by:

k1 t; 2:303

ð3Þ

where qt is the amount of sorption at time t (min) (lg g1); K1 the rate constant of the equation (L min1) and; qe is sorption capacity at equilibrium (lg g1). K1 (min1) is calculated by plotting the function (Eq. (3)). 2.6.2. Pseudo-second-order equation(Ho and McKay, 2000) The second-order equation is presented in the following form:

ð4Þ

which after definite integration applying the conditions qt = 0 at t = 0 and qt = qt at t = t the equation becomes:

t 1 1 ¼ þ t; qt k2 q2e qe

Gravitational settling/sedimentation for a 24 h period resulted in a reduction of 85% and 93% of V and Pb, respectively. By increasing the period up to 48 h, no further reduction was observed: V and Pb were reduced in 88% and 90% respectively, compared to the initial concentration.

ð2Þ

which after definite integration applying the conditions qt = 0 at t = 0 and qt = qt at t = t the equation becomes:

dq ¼ K 2 ðqe  qt Þ2 dt

3.1. Settling/sedimentation

3.2. pH Development during batch tests

dqt ¼ k1 ðqe  qt Þ dt

logðqe  qt Þ ¼ log qe 

3. Results and discussions

ð5Þ

where K2 is the rate constant of the second-order equation (g lg1 min1); qt the amount of sorption (lg g1) at time t (min); and qe is the amount of sorption at equilibrium time (lg g1). K2 (g mg1 min1) is calculated by plotting the function (Eq. (5)).

Regardless the fact that pH is a well-known parameter that plays an important role during metals uptake from aqueous solution through sorption (Acar and Eren, 2006; Jang et al., 2005), surprisingly, very seldom the monitoring and the development of pH along sorption tests are reported. After literature survey (manuscript under preparation) the authors observed that among 70 scientific papers published between 1977 and 2007 about sorption onto non-conventional adsorbents, only five recorded the pH development, even though, without further discussions. According to the editor of the scientific journal Separation and Purification Technology (Tien, 2007), in most manuscripts about sorption studies submitted during the last two years to that journal, the pH effect was invariably examined only in terms of the initial value in the aqueous solution; no paper submitted mentioned the pH development during the course of the experiment. In this current investigation, during the sorption test with an initial pH 7.4, a pH decrease was observed in the first 5 min in the presence of high adsorbent amounts (12.5; 25; 50 and 75 g L1), followed by a conspicuous tendency for increase along the remained period of contact time. Furthermore, it was observed that the lower is the sawdust dose; the lesser is the decay during the first 5 min and the higher is the final pH reached after 160 min. In the presence of 12 g L1 of sawdust, the pH slightly decreased from 7.4 down to 7.18 with a further increase up to pH 8.02 by the end of the experiment (160 min of contact time). On the other hand, with

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the highest amount of sawdust in the water (75 g L1), the initial pH 7.4 decreased down to 6.37, followed by a slight increase up to pH 6.75 reached in 160 min. The initial pH decrease observed is in conformity with the investigation reported by Ho (2005) in which, by testing Pb removal by lignin-based adsorbent from tree fern, an increase of the hydrogen ions concentration during the first 3 min of reaction was observed. Ho (2005) pointed out that such increase of hydrogen ions might be related with deprotonation of acidic groups present in lignin, fact that might have occurred in the current study. According to Memon et al. (2007), deprotonation of carboxyl groups present in sawdust cell walls takes place when pH is equal or higher than 4.0. Consequently, sawdust becomes negatively charged, being able to bind positively charged metal ions. However, by raising pH above 5.0, Pb precipitation takes place as a consequence of OH ions (Ayyappan et al., 2005; Yu et al., 2001). Regarding V sorption dynamic, there is a lack of scientific literature, but a recent study demonstrated that the best pH range for V sorption by commercial TiO2 and iron-based adsorbents was between 3.5 and 4.0 (Naeem et al., 2007). When the initial pH was adjusted to 4.0, almost no pH variation along the experiment was observed, with the pH being kept the range of 4.0–4.1. Changes in pH (with initial pH ranging from 1 to 10) during a batch test of two hours with different types of adsorbents, including sawdust, has been previously described (Dakiky et al., 2002). The increase has been attributed to the hydrolysis of adsorbent functional groups, resulting in positively charged sites (Dakiky et al., 2002). As a consequence, lower Pb removal efficiency is expected, once electrostatic repulsion of these positive metal ions by positively charged surfaces are likely to occur, condition that might explain the results obtained in the present investigation. The presence of Pb precipitates cations, such as Pb(OH)+ and Pb2+ in pHs close to neutral value (Taty–Costodes et al., 2003). On the other hand, V removal up to 61% in 50 and 75 g L 1 of sawdust doses occurred because anionic forms of V are likely to prevail in pH ranges from 4.0 to 11.0, with consequent favourable electrostatic sorption by positively charged surfaces (Naeem et al., 2007). 3.3. Effects of initial pH on sorption efficiency A remarkable increase of the removal efficiency from 32% to 99% for Pb and from 43% to 95% for V was observed after 160 min when the initial pH was reduced from 7.4 to 4.0. Moreover, with adjustment of initial pH to 4.0, the equilibrium state was reached after 40 min of contact time. The sorption efficiency under the same contact time (40 min) during the sorption test with initial pH 7.4 (no adjustment), was much lower: 21% for Pb and 37% for V. The optimum initial pH for Pb sorption described in the literature is pH 4.0 using cone biomass of P. sylvestris (Ucun et al., 2003) and pH 5.0 using maple sawdust (Yu et al., 2001) and activated sawdust (Ayyappan et al., 2005). 3.4. Adsorbent dose The removal efficiency obtained for V and Pb with the initial pH 7.4 at different adsorbent doses, as well as metals adsorbed per adsorbent unit mass (sorption capacity) is shown in Fig. 1. V removal was positively correlated (R2 = 0.8798) with the adsorbent dose, as usually reported in the literature regarding different metals. Removal efficiency for V increased up to 60% at sawdust dose of 50 g L1 and then, remained constant regardless the increase in the adsorbent dose (75 g L1). Positive correlation between adsorbent dose and metals removal efficiency is related to increasing surface area of available exchangeable sites (Acar and Eren, 2006; Argun et al., 2007; Yu et al., 2000). Differently from V, Pb removal was negatively correlated (R2 = 0.7047) with the

Fig. 1. Effect of adsorbent dose on removal efficiency (%) and sawdust sorption capacity for Pb and V. Initial pH 7.4; contact time = 160 min; Pb C0 = 19 lg L1, V C0 = 10 lg L1.

adsorbent dose. Removal efficiency of Pb decreased from 42% at the lowest sawdust dose (1.25 g L1) to 5% at the highest sawdust dose (75 g L1). This apparently contradictory result might be a consequence of Pb release from untreated sawdust. During the blank test with distilled water, it was observed that whereas 25 g L1 of sawdust did not increase Pb concentration in aqueous phase after 160 min of contact time, an increase of 140% of Pb by adding 50 g L1 of sawdust in distilled water following positive correlation between% of increase and contact time (R2 = 0.8646) was observed. On the other hand, V was not released from pine sawdust. This might also explain the opposite behaviour of Pb in this study compared to the literature, which generally reports increased removal efficiency with increasing adsorbent doses, being the adsorbents mostly washed with distilled water and dried (Li et al., 2007; Meunier et al., 2002), and submitted to a chemical pretreatment such as acid (H2SO4) (Taty-Costodes et al., 2003) and formaldehyde washing (Taty-Costodes et al., 2003; Vásquez et al., 1994). Fig. 2 shows that by increasing the adsorbent dose from 1.25 up to 75 g L1, the amount of Pb and V ions adsorbed per unit mass (sorption capacity) decreased gradually from 6.4 to 0.01 lg g1 for Pb and from 1.52 to 0.08 lg g1 for V. Sorption capacity decreases with increasing adsorbent dose mainly due to unsaturated sorption sites (Shukla et al., 2002). Those authors point out that particle interaction, such as aggregation, as a consequence of high adsorbent concentration results in decreasing surface area. Still, according to Fig. 2, Pb showed higher affinity with sawdust than V, particularly in the low adsorbent doses of 1.25 and 2.5 g L1. Pb has been reported as a metal with strong sorption by pine sawdust (Taty-Costodes et al., 2003), pine bark (Al-Asheh and Duvnjac, 1997) and hardwood mulches (Jang et al., 2005) compared to other metals. This behaviour has been related to higher electro-negativity (2.33 Pauling scale) and lower atomic radius (154 pm) (Jang et al., 2005) compared to V, for instance. Sorption of metals having a smaller ionic radius has been reported as superior to those with larger ionic radius (Jang et al., 2005). 3.5. Contact time At high adsorbent doses (50 and 75 g L1), the Pb concentration in the aqueous phase increased at contact times below 80 min; after this time, Pb removal is observed (Fig. 3). With the higher adsorbent dose (75 g L1), higher negative reduction (Pb increase) occurred, which suggests that sawdust releases Pb. Although removal became later on positive, by the end of the test (160 min) it was still low, particularly for the adsorbent dose of 75 g L1.

F. Kaczala et al. / Bioresource Technology 100 (2009) 235–243

239

Fig. 2. Effect of contact time on Pb (a,b) and V (c,d) uptake rate by sawdust with initial pH 7.4. Adsorbent doses: (a,c) 1.25 and 2.5 g L1; (b,d) 12.5, 25, 50 and 75 g L1. Initial metal concentrations: (a,b) Pb C0 = 19 lg L1; (c,d) V C0 = 10 lg L1.

The slow increase of Pb removal from 20% (40 min) to 32% (160 min) with the adsorbent dose 25 g L1 (Fig. 3) indicates that

the time to attain equilibrium for Pb was not as fast as shown in the literature (Taty-Costodes et al., 2003). The curve slope suggests

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that higher removal efficiency might be achieved by prolonging the contact time between sawdust and the adsorbate. Nehrenheim and Gustafsson (2008) observed a slow sorption process onto pine bark when investigating a multi-component solution containing Pb. Still according to Nehrenheim and Gustafsson (2008), extended contact times would be necessary in order to gain a better understanding as regards Pb retention by pine bark. Longer time to attain equilibrium in the present study is probably related with the complexity of the industrial wastewater tested where different organic and inorganic compounds compete for sorptive sites, compared to spiked distillate water used in previous studies with pine sawdust (Taty-Costodes et al., 2003), cypress bark, hardwood bark and pine bark (Jang et al., 2005), cone biomass (Ucun et al., 2003). In the present study, contact time of 160 min was enough to reach the equilibrium state using the adsorbent doses of 1.25 and 12.5 g L1 for Pb and 2.5, 12.5 and 75 g L1 for V, respectively (Figs. 3 and 4). Even though the equilibrium for V sorption was reached after 40 min with the adsorbent dose of 2.5 g L1 (10% of removal), the removal efficiency in the first 22 min oscillated between 23% and 8%. Reversible sorption behaviour could be explained by the competition of different cations and anions that are present in the real effluent. Reversible sorption due to competition might also be the main cause of the decrease of V removal from 30% at 5 min to 19% at 160 min when using 1.25 g L1 adsorbent dose. 3.6. Kinetic modelling 3.6.1. Lagergren first-order Pb sorption curve fitted with the Lagergren first-order model when tested with higher adsorbent doses (Table 2), with R2 values of 0.87; 0.91 and 0.96 observed for 25, 50 and 75 g L1 adsorbent doses respectively. On the other hand, V sorption kinetics was best fitted to Lagergren first-order model with lower amounts of sawdust. R2 values of 0.79; 0.70 and 0.71 were observed in the presence of 1.25; 2.5 and 25 g L1 adsorbent dose, respectively. As regards to Pb sorption kinetics with adsorbent doses 1.25, 2.5

Table 2 Lagergren first-order parameters for V and Pb adsorption onto Pinus sylvestris sawdust Adsorbent dose (g L1)

Lagergren first-order k1 (min1)

qexp (lg g1)

qcalc (lg g1)

Deviation (%)

R2

V 1.25 2.5 12.5 25 50 75

0.009 0.022 0.071 0.009 0.009 0.009

2.63 0.44 0.23 0.19 0.12 0.09

0.26 0.19 0.08 0.08 0.04 0.02

90 57 65 58 63 76

0.7859 0.7000 0.4136 0.7123 0.6657 0.6000

Pb 25 50 75

0.008 0.02 0.016

0.24 0.10 0.01

0.19 0.19 0.11

21 49 88

0.8714 0.9120 0.9615

and 12.5 g L1, it was not possible to apply Lagergren, since uptake rates did not present significant variations (Fig. 2a and b). The same occurred with the experiments with initial pH 4.0. 3.6.2. Pseudo-second-order equation(Ho and McKay, 2000) The correlation coefficients are statistically significant both for V (in all cases R2 > 0.98) and Pb (R2 > 0.96, with adsorbent amounts of 1.25, 2.5 and 25 g L1) (Table 3). The experiment with initial pH adjustment resulted in R2 values of 0.80 and 0.97 for Pb and V respectively. Besides high R2 values, theoretical and experimental values of qe presented low percentages of deviation, particularly for V sorption, suggesting that pseudo-second-order kinetics can be considered as the sorption process of Pb and V onto sawdust. Negative values of K2 for V sorption of 0.087 and 0.78 g lg1 min1 with adsorbent doses of 1.25 and 2.5 g L1, respectively are correlated with a rapid initial up take rate of the metal followed by ions desorption, phenomena that is illustrated in Fig. 2c. 3.6.3. Weber–Morris equation – intra particle diffusion model It was observed R2 values ranging between 0.8060–0.8882 and 0.7590–0.9127 for V and Pb (Fig. 5). However, as presented in Table 4, linearity is not applied for all adsorbent doses.

Fig. 3. Effect of contact time and adsorbent doses on Pb removal; initial pH 7.4; Pb C0 = 19 lg L1.

Fig. 4. Effect of contact time and adsorbent doses on V removal; initial pH 7.4; V C0 = 10 lg L1.

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F. Kaczala et al. / Bioresource Technology 100 (2009) 235–243 Table 3 Pseudo-second-order parameters for V and Pb adsorption onto Pinus sylvestris sawdust Adsorbent dose (g L1)

Pseudo-second-order k2 (g lg1 min1)

qexp (lg g1)

qcalc (lg g1)

Deviation (%)

R2

V 1.25 2.5 12.5 25 50 75

0.087 0.78 1.078 0.829 0.935 3.682

2.64 0.44 0.232 0.192 0.12 0.087

1.52 0.44 0.21 0.18 0.12 0.08

42 0 9 6 0 8

0.9800 0.9880 0.9885 0.9913 0.9871 0.9964

Pb 1.25 2.5 12.5 25 50 75

0.038 0.04 0.041 0.116 0.274 1.811

6.4 2.8 0.32 0.24 0.1 0.01

6.41 2.76 0.44 0.28 0.08 0.02

0 1 27 14 20 50

0.9955 0.9617 0.7733 0.9807 0.6847 0.3938

Table 4 Kinetic parameters of Pb and V intra-particle diffusion onto Pinus sylvestris sawdust Adsorbent dose (g L1)

Intra-particle diffusion Ki (lg g1 min0.5)

c (Intercept)

R2

V 1.25 2.5 12.5 25 50 75

0.090 0.022 0.009 0.009 0.004 0.002

2.75 0.63 0.12 0.08 0.06 0.06

0.8060 0.1149 0.5871 0.6825 0.8882 0.5929

Pb 1.25 2.5 12.5 25 50 75

0.132 0.071 0.025 0.019 0.017 0.008

4.69 2.52 0.02 0.03 0.14 0.09

0.7590 0.0165 0.7243 0.8640 0.9127 0.4631

Even though linearity fits to some cases (Table 4), it cannot be stated that intra-particle diffusion process is the unique rate-limiting step taking place in the sorption of V and Pb onto sawdust. The deviation of the linear plots from the origin (y-intercept other than 0) indicates that there are others sorption steps, for instance external diffusion, even though obtained intercept values with a unique intercepting point suggest that intra pore diffusion plays a significant role over the sorption of Pb and V with adsorbent doses of 25 g L1 (intercept = 0.03; R2 = 0.8640) and 50 g L1 (intercept = 0.06; R2 = 0.8882), respectively (Argun et al., 2007). 3.7. Sorption equilibrium It may be observed a statistically significant linearity using the Freundlich model to describe V (R2 = 0.8764) and Pb (R2 = 0.8689) sorption process (Fig. 6).

Fig. 6. Freundlich isotherms for V and Pb adsorption onto sawdust. Initial pH 7.4; Contact time = 160 min; Initial V conc. = 10 lg L1; Initial Pb conc. = 19 lg L1.

Fig. 5. Intra-particle diffusion kinetics and linear correlation for (a) V (adsorbent doses = 1.25, 50 g L1) and (b) Pb (adsorbent doses = 1.25, 25, 50 g L1). Initial pH 7.34.

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Table 5 Freundlich model constants for Pb and V adsorption onto Pinus sylvestris sawdust Metals

V Pb

Freundlich isotherm Kf

1/n

n

R2

0.0105 2E+13

1.6 12

0.625 0.083

0.8764 0.8689

The Freundlich parameters kf and 1/n specify whether the sorption nature is either favourable or unfavourable (Al-Degs et al., 2006; Hamdaoui and Naffrechoux, 2007). The magnitude of the exponent 1/n gives an indication on the sorption intensity, and values of n less than one, as it can be observed for V sorption onto sawdust (Table 5) represents unfavourable sorption intensity. On the other hand, Freundlich model could not be applied to simulate Pb sorption, once an unacceptable negative value for the sorption intensity (1/n) was obtained despite a significant linearity (Table 5). Such result might be reasonable taking into account the complex wastewater indicating that differently than empirically stated by the equilibrium model, the amount of adsorbed Pb per unit mass (qe) in this particular case, is inversely proportional to the equilibrium concentration (Ce) in aqueous phase. 4. Conclusions The present investigation showed that untreated pine sawdust was capable to reduce Pb and V present in real industrial wastewater, regardless its complex composition. Metal sorption and efficient removal is highly pH-dependent and a remarkable increase of the removal efficiency from 32% to 99% for Pb and from 43% to 95% for V was observed when the initial pH was reduced from 7.4 to 4.0. On the other hand, the settling/ sedimentation process previous to sorption phase was proved to be a very important step concerning the treatment of metal-containing wastewater, particularly in the case of treatment without pH adjustment. Whereas V removal was positively correlated with the adsorbent dose, Pb was not. This difference may be explained by the presence of Pb on untreated sawdust surface, which is released when in contact with water. An increase of 140% of Pb concentration in distilled water after 160 min with the adsorbent amount of 50 g L1 was observed. Although the pseudo-second-order kinetics can be considered as the sorption process of Pb and V onto sawdust, at higher adsorbent doses for Pb and lower adsorbent doses for V, sorption curves fitted well to Lagergren first-order. Although it cannot be stated that intra-particle diffusion is the unique rate-limiting step taking place in the sorption of Pb and V onto sawdust, the modelling suggested that this mechanism plays an important role when tested with adsorbent amounts of 25 g L1 and 50 g L1, respectively. Further investigation is needed in order to develop a better understanding over the various processes controlling sorption onto sorptive sites, particularly for real wastewater, since in such medium, sorption mechanisms are expected to be much more complex as the result of the presence of different inorganic and organic compounds. According to the Freundlich isotherm model, sawdust presented unfavourable sorption intensity for V sorption in real wastewater. Freundlich model was not suitable to indicate the capacity of sawdust to sorb Pb, a fact that might be related with the complex composition of the wastewater investigated. Acknowledgements The Swedish Knowledge Foundation (KK-stiftelsen) is acknowledged for the financial support to the project. The CAPES Founda-

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