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Dynamic study of lead removal from aqueous solution using Posidonia Oceanica fixed bed column.

Introduction

Lead, one of the most toxic heavy metals, is considered to be particularly dangerous for both human health [1] and environment [2, 3]. Several technologies exist to eliminate heavy metals from wastewater including chemical coagulation and precipitation [4], ion exchange [5], ultrafiltration [6], etc. However, the most effective and economical technique for heavy metal removal, even at low concentration, is biosorption. This technique utilizes the ability of certain natural materials that are available in vast quantities to eliminate this metal [7]. Among these biomaterials, the use of marine algae [8, 9, 10, 11, 12, 13] is quite attractive because of their abundance as a result of their rapid growth which often creates environmental and aesthetic problems to coasts. The carboxylic, hydroxyl and amino groups in algal cell-wall polysaccharides act as binding sites for metals [9, 12].

Frequently, the sorption capacity of biosorbents is evaluated through batch experiments [8, 14,15] where the contact time is long enough to reach the equilibrium state. However, from an industrial perspective, practical application of biosorption processes should make use of continuous stirred adsorber or packed bed column configurations where large quantities of effluent be treated despite the fact that no sufficient contact time is provided [16-19].

Thomas model, used by a large number of researchers, is a powerful analytical tool for experimental data description and prediction [20-25]. This model gives the opportunity to scale up the dynamic biosorption process in view of industrial application.

The main objective of this work was to analyse sorption characteristics of Posidonia oceanica, a Tunisian waste aquatic plant, for the removal of lead ions (Pb II) from aqueous solutions in continuous flow operations. The performance of the biosorbent was evaluated in terms of fixed and released amount of metal during sorption and regeneration processes, respectively. Initial lead concentration and flow rate were the main parameters used to evaluate the efficiency of Thomas analytical model for describing lead removal dynamics in fixed bed column using Posidonia oceanica.

Material

Experimental method

Sorbent preparation. Balls of Posidonia Oceanica (P.O.) were collected from Tunisian coasts. The collected material was initially washed with tap water to remove any attached dirt particle and then dried in sunlight for 1 week. Then, they were cut in small pieces and ground (45-1000 um) using a grinder (AM80 Nx2). Finally, the obtained powdered material was washed with distilled water and dried for 24h at 60[degrees]C. The final material was sieved and the experiments were carried out, without further treatment, for sorbents with particle sizes ranging from 200 to 300um.

Lead aqueous solution. Lead solutions were prepared with Pb[(N[O.sub.3]).sub.2] (Prolabo) and adjusted with 0.1M HN[O.sub.3] to pH 3-3.5 to avoid metal precipitation. Lead concentrations were determined with a Shimazu atomic absorption spectrometer ([[lambda].sub.Pb] = 283.3 nm).

Column experiments. Experiments were performed in a stainless steel column with an inner diameter, [d.sub.i], of 1cm, and a length, L, of 10cm, packed with Posidonia oceanica ([approximately equal to]2g). The biosorbent in the column was first pre-washed firstly with distilled water and then with 0.04M HN[O.sub.3] solution before starting an elution experiment. The lead solution (10, 20, 30 or 50 mg [L.sup.-1]) was pumped down flow (peristaltic pump ISMATEC ISM828) into the column at different flow rates (8.45, 10.45, 12.74 and 14.77 mL [min.sup.-1]). Column effluent samples were collected at predefined times by a programmable fraction collector (Kholer minicollector MC 30/60). The pH of the effluent was also recorded (pHmeter Jenway). After the column reached saturation, the spent sorbent with metal ions was regenerated using 0.04M HN[O.sub.3] solution (pH 3-3.5). This regeneration was done at the same flow rate as that of the elution step. Two consecutive sorption-desorption cycles were carried out for the different inlet concentrations.

Breakthrough curve analysis

A breakthrough curve represents the solute outlet concentration, C (mg [L.sup.-1]), versus time, t (h), or effluent volume, V(L). In this work, the outlet concentration is normalized with inlet concentration, C/[C.sub.0] while the effluent volume (bed volume) is normalized with porous volume of the biosorbent, V/[V.sub.p]. The analysis of breakthrough curve allows the calculation of some useful parameters such as:

The amount of fixed lead during the sorption step:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

The released amount of lead during the desorption step:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

The regeneration efficiency:

E = [q.sub.r] / [q.sub.f] x 100 (%) (3)

where: M (g) is the mass of biosorbent in the column, [V.sub.tot] (L) is the total volume of lead solution or 0.04M HN[O.sub.3] solution pumped through the column during the sorption or the desorption step, V (L) is the effluent volume, C and [C.sub.0] (mg [L.sup.-1]) are the effluent and the initial lead concentration, respectively.

Thomas model, which is widely used to describe the column sorption, assumes Langmuir type isotherms and no axial dispersion. It is derived with the assumption that the rate driving force in sorption obeys second order reversible reaction kinetics. The Thomas model has the following form:

C = [C.sub.0] / 1 + exp([[k.sub.th]/Q ([q.sub.0]M - [C.sub.0] V)) (4)

where: Q (L [h.sup.-1]) is the volumetric flow rate through column, [k.sub.th] (L [mg.sup.-1] [h.sup.-1]) is the

Thomas model constant and [q.sub.0] (mg [g.sup.-1]) is the biosorption capacity.

The non-linear method described by Han and co-workers [23] is trial-and-error procedure, which is suitable to computer operation, was used to determine the Thomas model constant, kth, and the biosorption capacity, [q.sub.0]. An optimization routine to maximize the coefficient of determination, [r.sup.2], between experimental data and Thomas model was adopted using the solver add-in with Microsoft's spreadsheet, Microsoft Excel:

[r.sup.2] = [SIGMA][([C.sub.mod] - [[bar.C].sub.exp]).sup.2] / [SIGMA][([C.sub.mod] - [[bar.C].sub.exp]).sup.2] + [([C.sub.mod] - [C.sub.exp]).sup.2] (5)

where: [C.sub.mod] and [C.sub.exp] are the lead effluent concentration (mg [L.sup.-1]) at any time obtained from the Thomas model and from experiment, respectively, [[bar.C].sub.exp] (mg [L.sup.-1]) is the average of [C.sub.exp].

Results and Discussion

Effect of flow rate

Lead sorption and desorption curves obtained for initial concentration of 50 mg [L.sup.-1] and at different rate flows are plotted in Figure 1.

[FIGURE 1 OMITTED]

The experimental data were evaluated and the sorbed/desorbed quantities with respect to flow rate are presented in Table 1. It is clear from these results that there is no significant effect of the the flow rate, for the proposed study range, onto lead sorption or desorption behaviour. In all cases, the breakthrough point, [(V/[V.sub.p]).sub.b], (the bed volume at which lead concentration reaches 5% of the inlet value) is around 330 and the exhaustion point, [(V/[V.sub.p]).sub.e], (the bed volume at which lead concentration reaches 95% of the inlet value) is close to 560 (Table 1).

The sorption process, when the uptake is independent on the flow rate, is considered to be controlled by intraparticle and external mass transfer (13). For intraparticle mass transfer step, a slower flow rate will provide a longer time for sorption to take place and the bed capacity will be higher. On the other side, if the process is subjected to external mass transfer control, a higher flow rate will decrease the film resistance and result in an increase in the bed capacity is observed. Since these two effects counteract each other, the response of the rate parameter will be intermediate of these two extremes and both effects will occur at the same time. However, the low value of bed capacity (around 0.38 mg [g.sup.-1]) compared to batch experiments [15] (around 40 mg [g.sup.-1]) seems to indicate that the metal ions do not have enough time to sorb and leave the column before equilibrium occurred [18].

Figure 2 shows that the sorption and desorption (transformed into [(1-C/[C.sub.0]).sub.des]) curves are superimposable indicating a reversible sorption, which is favourable to the regeneration process.

[FIGURE 2 OMITTED]

Thomas model gives a good agreement with the experimental breakthrough curves (Figure 2) and the model parameters obtained by a non linear regression analysis (5) are listed in Table 1.

Effect of lead initial concentration

The effect of the lead concentration in the inlet effluent on the sorption and desorption curves is shown in Figure 3.

[FIGURE 3 OMITTED]

The comparison of these breakthrough curves reveals that when the pollutant concentration increases, the biosorbent is saturated earlier since the exhaustion point, [(V/[V.sub.p]).sub.e], decreases from 2600 to 520 when lead concentration increases from 10 to 50 mg [L.sup.-1] (Table 1). The extend of the breakthrough curves, estimated by the value of the difference between the exhaustion point, [(V/[V.sub.p]).sub.e] and the breakthrough point, [(V/[V.sub.p]).sub.b], varies from 2465 to 190 when lead concentration increases from 10 to 50 mg [L.sup.-1]. This behavior is due to the sorption driving force which is larger for higher concentration causing an increase in sorption capacity (Table 1).

Regeneration process

For industrial wastewater treatment by means of sorption process, the regeneration of the sorbent is an important step. Figure 4 shows the sorption and desorption curves for two cycles at different lead inlet concentration and at 8.45 mL [min.sup.-1].

[FIGURE 4 OMITTED]

The fixed amount of lead and the regeneration efficiency after each cycle are given in Table 2 as well as the total fixed amount of lead after the two cycles.

It results from Figure 4 that, for a given initial lead concentration, the two cycle sorption/desorption curves are almost superimposable, particularly for the highest inlet concentration. Table 2 shows that the regeneration efficiency falls from 100 (first cycle) to 85 (second cycle) and from 99 (first cycle) to 81 (second cycle) for [C.sub.0]=10 and 20 mg [L.sup.-1], respectively while no significant change in the regeneration efficiency is noticed for higher concentrations. For lower concentration, the decrease in regeneration efficiency may be due to the prolonged sorption as the exhaustion point decreases from 2600 to 520 (Table 1) when the concentration increases from 10 to 50 mg [L.sup.-1].

The overall performance of the packed bed can be estimated by adding the fixed amount of lead after two service cycles (Table 2). It can be seen that the bed capacity increases from 0.208, 0.550, 0.269 and 0.377 mg [g.sup.-1] (first service cycle) to 0.409, 0.455, 506 and 0.748 mg [g.sup.-1] (first and second service cycles) for an initial concentration of 10, 20, 30 and 50 mg [L.sup.-1], respectively.

pH profile during sorption/desorption process

Figure 5 shows the shapes of pH and lead breakthrough curves during sorption/desorption process at two different inlet lead concentrations (30 and 50 mg [L.sup.-1]) and at 8.45 mL [min.sup.-1].

[FIGURE 5 OMITTED]

During sorption process and for both inlet concentrations, lead ions exchanged with [H.sup.+] ions involving a decrease in pH value. The pH profiles during desorption process shows the opposite behavior since the pH increases when lead concentration decreases because of the exchange between lead ions previously sorbed and proton ions introduced with the regeneration solution.

However, a competition occurs between the two positive ions ([Pb.sup.2+] and [H.sup.+]) involved in the sorption process [26, 27]. This competition can be observed through the pH values at the breakthrough point [(V/[V.sub.p]).sub.b] for both curves ([C.sub.0] = 30 and 50 mg [L.sup.-1]). Indeed, from the beginning of the curves to the breakthrough points, the inlet lead ions are totally sorbed instead of the proton ions previously fixed. These latter, added to the inlet protons, must give an outlet pH higher that the inlet one (3-3.5). Yet, this is not the case since the pH values at the exhaustion points are 3.6 and 3.4 for [C.sub.0] = 30 and 50 mg [L.sup.-1], respectively. These pH values are nearly the same that the inlet one indicating that the proton ions present previously into the fixed bed are not totally desorbed when lead ions are sorbed because of both ions compete for the biomaterial positive binding sites.

Conclusion

This work shows the ability of Posidonia oceanica, a marine biomass widely spread in Mediterranean coasts, to remove lead ions from aqueous solution using a fixed bed column. The main conclusions of this study are the following:

* The sorption process of Pb(II) ions is strongly dependent from initial lead concentration but independent from flow rate suggesting both intraparticle and external mass transfer mechanisms in sorption process.

* The Thomas model is a good tool for describing the breakthrough data from column experiments and for predicting concentration-time profile for any other experimental condition.

* The reduced sorption capacity (0.208-0.377 mg [g.sup.-1]) seems to indicate that, in one hand the pollutant did not have enough time to sorb and left the column before equilibrium occurred. On the other hand, competition between proton and lead ions onto the same sorption sites reduces lead retention.

* The biomass sorption capacity can be enhanced with consecutive sorption/desorption cycles, which allow a significant increase of the fixed pollutant (i.e. from 0.208 to 0.409 mg [g.sup.-1] and from 0.377 to 0.748 mg [g.sup.-1] after two cycles of sorption).

References

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* S. Dridi-Dhaouadi and M.F. M'Henni

Departement of Chemistry, Faculty of Sciences of Monasir, 5000 Monastir, Tunisia

E-mail: sonia.dridi@fsm.rnu.tn
Table 1: Column sorption-desorption data at
various flow rates and lead inlet concentrations.

 Amount fixed (mg [g.sup.-1])
Flow rate: Inlet concentration:
Q (mL [C.sub.0](mg [q.sub.f] [q.sub.f]
[min.sup.-1]) [L.sup.-1]) (exp.) (mod.)

8.45 10 0.208 0.251
8.45 20 0.255 0.255
8.45 30 0.269 0.262
8.45 50 0.377 0.402
10.45 50 0.378 0.415
12.74 50 0.394 0.423
14.77 50 0.387 0.421

Flow rate: Thomas constant [r.sup.2] Regeneration
Q (mL [k.sub.th] (Eq.2) efficiency E (%)
[min.sup.-1]) ([h.sup.-1]
 L[ mg.sup.-1])

8.45 3.016 0.98 100
8.45 3.326 0.97 99
8.45 7.046 0.98 98
8.45 7.708 0.99 100
10.45 8.472 0.99 96
12.74 10.925 0.99 100
14.77 13.964 0.99 100

Flow rate: Breakthrough Exhaustion
Q (mL point point
[min.sup.-1]) [(V/[V.sub.p]).sub.b] [(V/[V.sub.p]).sub.e]

8.45 135 2600
8.45 140 1655
8.45 330 900
8.45 330 520
10.45 310 560
12.74 335 565
14.77 350 615

Table 2: Column sorption-desorption data for
two cycles at various lead inlet concentrations.

 Inlet Amount Regeneration
 concentration: fixed efficiency
 [C.sub.0] [q.sub.f] E (%)
 (mg [L.sub.-1]) (mg [g.sup.-1])

First 10 0.208 100
cycle 20 0.255 99
 30 0.269 98
 50 0.377 100

Second 10 0.201 85
cycle 20 0.200 81
 30 0.237 99
 50 0.371 100

 Amount fixed after two
 cycles of sorption

 [C.sub.0] [q.sub.f]
 (mg [L.sup.-1]) (mg [g.sup.-1])

First 10 0.409
cycle 20 0.455
 30 0.506
 50 0.748

Second
cycle
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Author:Dridi-Dhaouadi, S.; M'Henni, M.F.
Publication:International Journal of Applied Chemistry
Date:Sep 1, 2011
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