Removal of Cu (II) from water pollutant with Tunisian activated lignin prepared by phosphoric acid activation

Desalination 250 (2010) 179–187

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Removal of Cu (II) from water pollutant with Tunisian activated lignin prepared by phosphoric acid activation A. Kriaa a,b,⁎, N. Hamdi b,1, E. Srasra b,1 a b

Département de Chimie, Ecole Supérieure des Sciences et Techniques de Tunis, Rue Taha Hussein-Montfleury Tunis, Tunisia Unité matériaux, Technopole Borj Cedria, Tunis, BP 95-2050 Hammam lif, Tunisia

a r t i c l e

i n f o

Article history: Accepted 25 December 2008 Available online 23 October 2009 Keywords: Activated carbon Activation BET surface area Lignin

a b s t r a c t Activated lignin with a relative high BET surface area and a well-developed porosity has been prepared from Tunisian deposit lignin, by H3PO4 activation at various process conditions. Physical and chemical properties of activated carbons produced, implying BET surface area, Boehm titration, Fourier Transform Infrared Spectroscopy (FTIR) and thermogravimetric analysis (TGA), were investigated. It was found that the maximum surface area reached at the carbonization temperature of 500 °C in H3PO4 activation, and that the activated lignin prepared from lignin acidic activation, showed a surface area of 463 m2/g. The potential application of these carbons for the removal of heavy metal contaminant, has been investigated by measuring their adsorption capacities for Cu (II) as representative of main local toxic contaminant found in industrial wastewaters. The results obtained compare well and even favourably with those reported in literature for other unconventional materials. © 2009 Published by Elsevier B.V.

1. Introduction Activated carbon is a high-porosity material, which is useful in adsorption of both gases and solutes from aqueous solution. Therefore, it has been widely used for many industrial applications, particularly in the environment field and wastewater cleaning [1,3]. Nevertheless, the development of methods to re-use waste materials is greatly desired and the production of activated carbons from wastes is an interesting possibility. Lignin is considered as waste, which is generally used only for its fuel value [3]. Therefore, it was of interest to prepare a higher value product such as an activated carbon from natural lignin in order to test its applicability, specifically in wastewater treatments and in agriculture application as fertilisant. On the other hand, it is well known that among activated carbon synthesis process, chemical activation is widely applied because of its lower activation temperature and high product yield as compared to physical process. The most commonly used chemical activation reagent is H3PO4 [3,4] because the activation temperature is relatively low (usually around 400–500 °C) compared to physical activation (N850 °C) and the phosphoric acid can be recovered. We must emphasis that many studies for preparing activated carbon by chemical activation have been carried out [1,5–9]. Some research results have been published about the properties of

activated carbon produced from different natural lignocellulosic materials by H3PO4 activation at a wide range of conditions [10–13]. Commonly, activated carbon, due to its high adsorption capacity was used as adsorbent for the removal of both trace organic contaminants and heavy metals. The main disadvantages of using activated carbon as adsorbent are high adsorbent cost, problems of regeneration and difficulties of separation of powdered activated carbon from wastewater for regeneration. Many studies [1,14,15] have shown that unconventional materials such as olive stone waste, grafted silica, sludge, sludge ash, fly ash, coal, biomass, lignin from kraft black liquors and many others have potential for removal of contaminants from wastewaters. In this study, activated lignin was studied for its potential use as adsorbent for the removal of copper ions from aqueous solutions. Utilization of this material in removal of Cu (II) from wastewater has not been investigated yet. The present paper reports, in addition to physico-chemical characterisation study, the results of equilibrium adsorption studies onto untreated natural lignin and activated lignin at 500 °C. Results of this study will be useful for future scale up using this material as a low-cost adsorbent for the removal of Cu (II) from wastewater.

2. Materials and methods ⁎ Corresponding author. Département de Chimie, Ecole Supérieure des Sciences et Techniques de Tunis, Rue Taha Hussein-Montfleury Tunis, Tunisia. Tel.: +216 7143 00 44; fax: +216 79412825. E-mail addresses: [email protected] (A. Kriaa), [email protected] (N. Hamdi), [email protected] (E. Srasra). 1 Tel.: +216 7143 00 44; fax: +216 79412825. 0011-9164/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2008.12.056

2.1. Precursor The sample is provided from a deposit in Nabeul (situated at the north east of Tunisia) was used as precursor in this study. The geology of this deposit can be simplified into one stratigraphic unit: Oum

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Dhouil formation from Vadabonian age (Middle Miocene). The solid had an average particle size less than 200 µm and was characterised by DRX, FTIR spectroscopy and thermogravimetric analysis (TGA). The results show that the sample is composed by approximately 13% of quartz. 2.2. Preparation of activated lignin 2.2.1. Solidification The procedure resembled to that used by Guo and Rockstraw [4]. Ten grams of natural lignin, as dry basis, finely ground was mixed with 51.3 g of concentrated phosphoric acid solution (Scharlau, AC 1100 ortho-phosphoric acid, 85%, reagent grade, ACS, ISO). The mixture was set on a ceramic boat and placed into muffle furnace at room temperature. The temperature was increased to 170 °C at the rate of 10 °C/min, and maintained at this temperature for 1 h. The temperature of 170 °C was chosen to remove water of phosphoric acid solution. At the end of this step, the mixture becomes a black sticky solid. The impregnation ratio is defined as grams of H3PO4 (100% basis) used per gram of dry precursor. In this study, the impregnation ratios were 1.026 for all samples. 2.2.2. Activation After solidification, the mixture of natural lignin and activated reagent (H3PO4) was heated up to different final carbonization temperatures in the same furnace at the rate of 2 °C/min (the carbonization temperature was varied over the temperature range of 200– 550 °C). The low temperature ramp rate was used to minimize the temperature difference between mixture and furnace, to provide sufficient activation time and to avoid a rapid decomposition of the sample. After reaching the desired activation temperature, and before taking out the sample out of the furnace, a cooling stage in inert atmosphere was necessary. Then, the mixture was removed from the furnace and allowed to cool to room temperature in air. After cooling to room temperature, the activation mixture was washed several times with hot distilled water to remove residual chemical. After the washing process, the sample was dried at 80 °C in a vacuum oven for at least 2 h, to prepare the activated lignin. The final weight of dry sample was recorded to determine production yield. 2.3. Characterisation of pore structure Pore structure analysis was performed using a “QuantachromAutosorb 1 Sorptiometer” through nitrogen adsorption at 77 K in the relative pressure of 10− 6 to 1 atm. Before measuring the isotherm, all the samples were degassed at 120 °C for 4 h in vacuum (10−6 atm). The surface area was calculated by the BET method using the adsorption isotherm of N2. The relative pressure range used for the calculation was 0.05–0.35. The micropore volume was calculated by a t-plots method [16]. 2.4. Boehm titration Boehm titration [17] is one of the most widely used methods to quantify and differentiate surface groups on activated carbons of different acid strengths. For phosphoric acid activated carbons, acidic groups also include phosphorus-containing groups. Therefore, in this study, it was assumed that 0.1 N sodium bicarbonate (pKa = 6.37) neutralizes strong acidic groups (both carboxylic and phosphoruscontaining groups); 0.1 N sodium carbonate (pKa = 10.25) neutralizes both strong and intermediate acidic groups; and 0.1 N NaOH (pKa = 15.74) neutralizes weak acidic groups, as well as intermediate and strong acidic groups. Our method is similar to that used by Guo and Rockstraw, 2006 [4]: 0.2 g of untreated and treated lignin samples were mixed with 50 ml 0.1 N KHCO3, Na2CO3, and NaOH solution respectively in a 250 ml Erlenmeyer flask. Corresponding solutions of 50 ml volume without solid samples were used as blanks. The flasks

were covered with plastic film and were shaken for 48 h at 150 rpm. 8 ml of supernatant from each flask was mixed with 10 ml 0.1 N HCl, then 0.1 N NaOH was used to back titrate the solution while stirring. The difference in NaOH consumed by the samples and the blanks was used to calculate the amount consumed by acidic groups, and the results were expressed as H+ equivalents per gram of sample. In this study, duplicates were performed for each sample and the relative standard error was in the range of 3%. The pH of the untreated natural lignin, in distilled water, has been determined by mixing 1 g of natural lignin with 35 cm3 of water. The suspensions were shaken and thermostated at 298 K for 2 days. 2.5. Thermo gravimetric analysis (TGA) Differential thermal analysis (DTA) and thermal gravimetric (TG) were performed on a SETSYS Evolution-1750 instrument. Approximately 10–20 mg samples were placed in a platinum crucible on the pan of a microbalance and then heated from room temperature up to 800 °C at a heating rate of 10 °C/min while being purged with argon at a flow rate of 100 mL/min and constantly weighted. 2.6. Infrared spectroscopy FTIR absorbance spectra of solid samples were obtained through KBr technique, with the analysis performed on a Nicolet Magua IR 560 in the wave number range of 4000–400 cm− 1. The solid samples were mixed with KBr at a ratio of roughly (1/300), and then the mixture was ground in agate mortar to very fine powder. After drying at 100 °C for 12 h in a vacuum oven, about 300 mg of the fine powder were used to make a pellet. After preparation, the pellet was analysed immediately and the spectra were recorded by 32 scans with 4 cm− 1 resolution. A pellet prepared with an equivalent quantity of pure KBr powder was used as background. 2.7. Equilibrium adsorption experiments The batch method was used to obtain the Cu adsorption as affected by solution pH which was regarded as a principal factor in the analysis of the adsorption process. The experimental procedures were carried out as follows: (1) prepare a series of 125 ml polyethylene bottles containing 30 ml of Cu (II) concentration ranging from 10− 1 to 10− 4 M. (2) Adjust initial pH to cover a range from 3 to 9 by either 0.01 M HCl or NaOH solutions. (3) Add a given amount of solid sample (1000 mg) into the solution. (4) Shake these bottles on a rotating shaker at 50 rpm for 24 h at 25 °C. Based on the results of kinetic adsorption experiments and effect of pH on copper adsorption, a 24 h contact time and pH value around 6.0–6.2, respectively, were found to be adequate for reaching equilibrium adsorption. (5) At the end of shaking, record the final pH of the mixed liquor. (6) Filter the mixed liquor through a 0.1 µm membrane filter paper to collect the supernatant. (7) Analyse the residual Cu concentration in the supernatant with spectrometry of atomic absorption (SAA, Vario 6, Analytic jena). The amount of total Cu (II) removed was taken as the concentration difference between that which was originally added and the remainder. Blank test without adsorbent (natural lignin or activated lignin at 500 °C) in the mixed suspension was also performed to avoid confusion between adsorption and possible copper hydroxide precipitation. The pH value, in distilled water of natural lignin at 25 °C, was 3.68 indicating that it can be considered as an acidic adsorbent. Moreover, the pHZPC of natural lignin was 4, determined instrumentally. The surface charge of this material is pH-dependent due to ionisation of functional groups developed on its surface. Therefore at a solution pH greater than the pHZPC, the lignin material possesses a negatively charged surface which is favourable for cationic Cu adsorption. Noted that Cu2+ is the dominant species involved below pH 7.0 [14], thus other species Cu(OH)02 and Cu (OH)− 3 were not accounted in the

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formation of surface complexes. At low pH values (pH b 3), H+ competes with the Cu ions for the active surface sites and in this region, it is difficult that they form Cu complexes. The isotherm data on Cu adsorption were fitted to Langmuir and Freundlich equations. The duplicate experiments demonstrated the high repeatability of this adsorption method and the experimental error could be controlled within 3–5%.

3. Result and discussion 3.1. Sample characterisation The DRX analysis of sample, provided from deposit site is presented in Fig. 1. It shows that the sample is naturally mixed with a minor amount of quartz. Using the Rietveld analysis method [18,19], the quantification of quartz in the sample is about 16%; the remainder (84%) is constituted by organic compound (lignin). Its surface area is about 10.18 m2/g at 25 °C. Fig. 2 shows The FTIR spectrum of natural lignin at 25 °C, which confirms the presence of quartz. The bands between 793 and 1060 cm− 1 are ascribed to Stretching vibration of Si–O of quartz and silica [20]; the band at around 1120 cm−1 is attributed to stretching vibration of C–H aromatic [21]; the band at 1250 cm− 1 can be caused by C–O stretch [22]; the bands around 1400, 2860 and 2940 cm− 1 are ascribed to aromatic ring stretching vibration [4]; the band around 1710 cm− 1 is usually caused by the stretching vibration of C=O in ketones, aldehydes, lactones, and carboxyl groups; the broad band between 3300–3500 is typically ascribed to hydroxyl groups or adsorbed water. From this data, this indicates the presence of carbonyl-containing groups and the initial aromatization of the natural lignin.

must emphasize that these product yield values show some variability and that the main results are summarized in Table 1. As we can observe, the discrepancies in the product yield values found in the literature are likely due to the varying origins of lignin and in the acidic activation condition process. For natural lignin of this study, weight loss mainly occurs in the stage of temperature increase from ambient to 200 °C. Similar results have been reported by other researchers [4, see references therein] where the activation temperature increase is from ambient to 170 °C. According to these same authors, this observation is explained by the fact that the phosphoric acid accelerates the dehydration of the lignin, promoting the activation reaction and weight loss at lower temperatures.

3.2. Product yield of activated samples

3.3. The development of pore structure

The carbonization temperature was varied over the temperature range of 200–550 °C. After drying process, the final weight of dry sample was recorded to determine production yield. It is well known that yield is usually calculated as a percentage of the weight of final activated product relative to the initial precursor weight. Yield of final activated product as a function of activation temperature (at a weight impregnation ratio between H3PO4 and natural lignin of 1.026) is shown in the Fig. 3. One can observe that the highest product yield (60.89%) is obtained at an activation temperature of 200 °C with a specific surface area of about 69.66 m2/g. Whereas the lowest product yield, obtained at an activation temperature of 500 °C (20.52%), shows the highest specific surface area (463.5 m2/g).Comparatively to other studies, we

The N2-adsorption isotherms for the carbons obtained by H3PO4 activation are displayed in Fig. 4. The analysis of these isotherms provided an approximate assessment of the pore size distributions. As we can observe, all the isotherms had very similar shapes and were almost parallel to each other. According to the IUPAC classification [16], these curves resemble to types I and II isotherms, which represent microporous solids having a relatively small external surface area (e.g. activated carbons, molecular sieve zeolites). All these isotherms had a characteristic H4 hysteresis loops which are often associated with the existence of slit-shaped pores [16]. As shown in Fig. 4, increasing the temperature to 500 °C where the impregnation ratio is constant to 1.026, produces an increase in the adsorptive capacity of activated lignin, except for temperature 550 °C where this parameter decreases.

Fig. 2. FTIR spectra of Tunisian natural lignin at 25 °C.

Fig. 1. Diffractogram of the natural lignin powder at 25 °C.

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Fig. 3. Effect of final activation temperature on the product yield, from natural lignin at impregnation ratio of 1.026.

The textural characterisation results, obtained from nitrogen adsorption isotherms for samples prepared from natural lignin, at temperatures varying from 25 to 550 °C, with an impregnation ratio of 1.026, are presented in Figs. 5 and 6.These figures illustrate that carbon prepared from natural lignin has significant pore volume and surface area even at final activation temperature. The BET surface area increases with final activation temperature at about 500 °C, reaching a maximum (around 463.5 m2/g at 500 °C), and then decreasing rapidly with further increase in the final activation temperature. Clearly, the surface area of this activated lignin is much larger than that of untreated lignin, but it is relatively small compared to those reported in literature [3,4]. We think that the surface area of our activated lignin obtained, could be significantly increased if our natural lignin has undergone pyrolysis and the removal of quartz from the sample. However, these experimental operations necessitate much energy and a lot of time which is not the aim of this work. The influences of chemical reagent H3PO4 and carbonization temperature on the pore volume are shown in Fig. 6. This figure indicates that the micropores are well-developed in the activated lignin prepared in this study. The pore volume (micropore+mesopore+ macropore volume) of the activated lignin prepared by H3PO4 activation, increases with an increase in temperature over the range 250 °C to 500 °C and decreases with an increase in temperature more than 500 °C. At temperature above 500 °C, the carbon structure shrinks and the surface area decreases as shown in Fig. 5. This can be explained by the fact that the removal of carbon atoms at high extent of burn-off results in the elimination of pore walls leading to a decrease in surface area [23]. 3.4. Comparison of the external surfaces of the resulting activated lignin

Fig. 4. Nitrogen adsorption isotherms for carbons obtained by H3PO4 activation, from natural lignin.

ambient temperature. The sample has a very rough surface, an intact external structure where the caking and agglomeration of the carbonaceous aggregates was not observed. On the other hand, Fig. 7(b) shows the structure of a chemically activated lignin at carbonization of 500 °C, for 2 h. As we can observe, the external surface of the chemically activated lignin is full of cavities. We think in agreement with Teng et al. [23] that according to the micrograph, it seems that the cavities resulted from the removal of the phosphoric and polyphosphoric acids during preparation, leaving the space previously occupied by the acids. The carbonization temperature for chemical activation was too low to cause the agglomeration of the char structure. 3.5. Fourier Transform Infrared Spectroscopy (FTIR) results FTIR analysis results of carbons prepared from Tunisian natural lignin, for an impregnation ratio of 1.026 at various final activation temperatures, are shown in Fig. 8.

Scanning Electron Micrographs (SEM) of the surface structures of carbon prepared by H3PO4 activation and natural lignin are compared in Fig. 7. Fig. 7(a) shows the SEM of the untreated natural lignin, at

Table 1 Main results concerning yield product values published in the literature. Chemical Product yield Nature of precursor reagent values (%)

Authors

H3PO4

82

H3PO4

60.89

Guo and Rockstraw, 2006 [4] This study

ZnCl2

55

ZnCl2

67

H3PO4

44–52

Kraft lignin (at impregnation ratio of 1.5) Natural lignin (at impregnation ratio of 1.026) Kraft lignin Kraft lignin (at impregnation ration of 1.0) Lignin from black liquors (at impregnation ratios within the range of 1–3)

Gonzalez-Serrano et al., 1997 [2] Hayashi et al. 2000 [3] Gonzalez-Serrano et al., 2004 [1]

Fig. 5. BET surface area, micropore and external surface areas of natural lignin and of activated lignin between 200 and 500 °C.

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1300–900 cm− 1 could be caused by phosphorus-containing groups [4]. According to Puizy et al. [22] the band around 1220–1180 cm− 1 can be attributed to the stretching of P=O bond in a phosphate ester, O–C bond in P–O–C linkage, or P=OOH bond. This indicates the appearance of phosphorous- containing groups from 200 °C. With an increase of final activation temperature to 300 °C, the relative intensity of band at 1700 cm− 1 shows a decrease compared to that at 200 °C, a sign of the decomposition or removal of carbonylcontaining groups, which are probably carboxylic groups. However, the FTIR spectra of carbons show a number of changes: the relative intensity decrease of band at 1600 cm− 1, the appearance of a new band at 1080–1100 cm− 1 of which the relative intensity increases with final activation temperature. This indicates that at least some of the formed carboxylic groups decomposed at high temperatures accompanying the increase of aromatization degree of the activation mixture. 3.6. Boehm titration results

Fig. 6. Pore and micropore volumes of natural lignin at 25 °C and of activated carbons between 200 and 500 °C.

The spectra show a broad band at 3500–3000 cm− 1and an increase of the relative intensity of band around 1700 cm− 1, a clear symbol of the existence of carboxylic groups. Main absorption bands of activated carbons performed at final activation temperature are summarized in Table 2. For carbons activated by H3PO4, the bands at

In order to compare the formation of surface functional groups on natural lignin at ambient temperature and activated lignin at carbonization temperature of 500 °C (corresponding to highest SSA obtained in this study), Boehm titration of these samples at a fixed impregnation ratio of 1.026 were performed, with results illustrated in Table 3. In our study, it should be noted, that a portion of the natural lignin at 25 °C, as well as for the carbon prepared from natural lignin at 500 °C, was observed to dissolve in 0.1 M NaOH, due to low extent of the sample in the medium. This was displayed by a precipitate, formed after acidulation of the supernatant of 0.1 M NaOH, during the Boehm titration. Therefore, results of weak and intermediate acidic groups obtained for these samples should be higher than the actual measured values. This experimental observation was supported by some authors [4], who found qualitatively similar findings. Table 3 indicates that concentration values of all three types of acidic surface groups on carbon obtained by H3PO4 activation, comparatively to untreated natural lignin, decrease when carbonization temperature reaches 500 °C, in agreement with Guo and Rockstraw [4]. These authors, found similar results from Kraft lignin and cellulose samples, at a fixed activation temperature of 400 °C and at an impregnation ratio of 1.0. Above 400 °C, the concentration of these groups tend to level off with increase of final activation temperature. In our study, we can assume that the formation of acidic groups at an impregnation ratio of 1.026 can be explained as follows: At low temperature (25 °C), untreated natural lignin shows a relatively increased in concentration of surface acidic groups than activated carbon at 500 °C. The pH value in distilled water is 3.68. Our natural lignin sample presents probably mainly several structures of oxygen functional groups that may be found at the edges of graphene layers leading to the high concentration of acidic surface groups (see FTIR results above). Nevertheless, the characterisation of these oxygen functional groups of natural lignin at 25 °C should be investigated in next section as well as a for activated lignin; for example the use of quantitative 1H NMR spectrum. In addition, the knowledge of average molecular, mass and molecular mass distribution of the lignin samples is of interest. With an increase in temperature, the carboxylic acid groups decompose within the range of about 200–300 °C. At 500 °C, mostly carboxylic, phenolic and lactol groups are decomposed and the decrease in concentration of acidic surface groups is probably due to a reaction between the precursor or its acidic hydrolysis products and H3PO4 or other forms of phosphorus-containing acids (pyro- or polyphosphoric acids formed at activation temperature) [4]. 3.7. Thermogravimetric analysis

Fig. 7. (a) Natural lignin at ambient temperature (b) SEM of activated lignin prepared by H3PO4 activation with carbonization at 500 °C for 2 h, corresponding to 20.5% yield.

As stated in the experimental procedure, carbonization was carried out under argon atmosphere. The volatile evolution during

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Fig. 8. FTIR spectra of carbons prepared from Tunisian natural lignin at different final activation temperature with impregnation ratio of 1.026 (gram of H3PO4 per gram of precursor).

carbonization of the H3PO4 treated samples was monitored by thermogravimetric analysis and the results were compared to those of untreated natural lignin. These samples were heated from 25 to 750 °C at a heating rate of 10 °C/min. Fig. 9 shows a comparison of the carbonization behaviour of the lignin samples. One can observe from Fig. 9(a) that carbonization process for the untreated sample (natural lignin) can be approximately described by a strong evolution of moisture and volatile. A sharp peak for volatile evolution is observed below 200 °C (it was found that 15% of weight loss occurred between 50 and 150 °C, corresponding to water loss upon heating). Tar, which mostly occurs between 400 and 600 °C [23], is probably a predominant product of the devolatilisation process; so that two little peaks at 520 and 600 °C are observed (it was found that 24.4% of weight loss occurred between 150 and 700 °C). For the carbonization of the sample treated with concentrated H3PO4 at 500 °C for 2 h, as shown in Fig. 9(b), also a strong evolution of volatiles occurs below 200 °C. The composition of the evolved matter can be water and, possibly, be carbon oxides and various volatile hydrocarbons. It has been reported [23, see references therein] that H3PO4 accelerates the bond cleavage reactions, leading to the early evolution of volatiles. Comparing the results of Fig. 9(b) with those of Fig. 9(a), one can observe that the evolution of tar has been suppressed for the H3PO4 treated samples. This is consistent with other published findings [23] and with the proposition that a more highly cross-linked structure is developed after acid treatment.

Table 2 Main absorption bands of activated carbons performed at final activation temperature. Band position Vibration type (cm− 1)

Reference

1000–1060

Madjeva et Komadel, 2001 [20] Guo et Rockstraw, 2006 [3] Puizy et al, 2002 [22]

1080–1100 1180–1220 1400–1500 1600–1650 1700–1750 3500–3000

Stretching vibration of Si–O of quartz and silica. Stretching vibration of P–O–P in phosphate Stretching vibration of P = O; P–O–C or P = OOH Stretching vibration of –CH2-groups Aromatic ring stretching vibration Stretching vibration of C = O, ketones, aldehyde, lactones and carboxyl groups Stretching vibration of O–H (H2O)

Kubo et Kadla, 2005 [21]

Obviously, the polyphosphoric acids underwent decomposition or evaporation between 600 and 800 °C. 3.8. Copper(II) adsorption isotherm The pH of aqueous solution is an important variable that influences the adsorption of anions and cations at the solid–liquid interfaces. For this reason, a preliminary study was done about the effect of pH on Cu (II) adsorption on the natural lignin and activated carbon at carbonization temperature of 500 °C, for pH between 3 and 9. It can be found that the Cu ions adsorption tends to increase with the increase of pH, reaches a maximum at pH around 6 and then it decreases. Similarly, the Cu adsorption kinetics was carried out on natural lignin and activated carbon (carbonization temperature of 500 °C). The results of the Cu (II) adsorption kinetic experiments at 25 °C and 500 °C show that the majority of Cu (II) adsorption on the two samples was completed in 20– 24 h. Consequently, the equilibrium time was fixed to 24 h in this study. The results of Cu (II) adsorption isotherm experiments are shown in Fig. 10. The Cu (II) adsorption capacity increased with the Cu (II) equilibrium concentration increasing from 0 to 2000 mg/L. The capacity of the natural lignin and activated carbon sample at carbonization temperature of 500 °C, was approximately 72 and 136 mg/g adsorbents, respectively, at pH 6.0 ± 0.2. With a further increase of the Cu2+ equilibrium concentration, the increase of the adsorption capacity was less significant. According to the results of Cu ion adsorption isotherm experiments, the activated carbon had higher adsorption capacities than the natural lignin. It was believed that the surface structural changes of the material play the most important role in the adsorption capacities of the Cu ions. When the natural lignin sample was treated with concentrated H3PO4, the surface structural of the material was

Table 3 Boehm titration results of natural lignin and activated lignin, with an impregnation ratio of 1.026 (grams of H3PO4 per gram of dry lignin). Concentration of surface acidic group (mmol H+/g)

Natural lignin at 25 °C, untreated with H3PO4

Activated lignin, at 500 °C with impregnation ratio of 1.026

Strong acidic groups Intermediate acidic groups Weak acidic groups

2.14 1.34 2.06

1.40 1.12 1.73

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Fig. 9. Weight loss and evolution rate during carbonization of the different lignin samples, using a heating rate of 10 °C/min (a) untreated natural lignin; (b) activated lignin at 500 °C for 2 h, having an impregnation ratio of 1.026.

changed. This is confirmed by an increased in SSA, from 10 to 463.5 m2/g, which resulted in creation of microporosity. It is generally possible to express the results of experimental adsorption measurements in the form of one or more equilibrium adsorption isotherm theories. The Langmuir and the Freundlich isotherm theories were tested in this study since they have been widely applied and found effective in contaminant sorbent investigations. Two typical isotherms, as described below in Eqs. (1) and (2) were used for fitting the experimental data: Freundlich equation: 1=n

qe = KF Ce

ð1Þ

Langmuir equation: qe = qm KL Ce = 1 + KL Ce

Fig. 10. Copper adsorption isotherms at 25 °C and pH 6–6.2.

ð2Þ

Where qe is the amount adsorbed at equilibrium (mg/g) and Ce is the equilibrium concentration (mg/L). The other parameters are different

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Table 4 Estimated isotherms parameters for Cu (II) adsorption.

Table 5 Adsorption capacity of Cu (II) by various adsorbents.

Isotherm type Langmuir

Natural lignin Activated carbon at 500 °C

Adsorbents

pH

Temperature qm (°C) (mg/g)

KL (L/mg)

References

Activated carbon

–

20

0.189

Machida et al., 2005 [24]

Freundlich

qm (mg/g)

KL (l/mg)

r2

KF

1/n

r2

72.46 136.98

0.0031 0.0061

0.991 0.994

0.778 2.38

0.574 0.572

0.988 0.816

isotherm constants, which can be determined by regression of the experimental data; n is a constant related to energy and intensity of adsorption. The estimated model parameters with the correlation coefficient (r2) are shown in Table 4. It is demonstrated that the experimental data of Cu (II) adsorption on these samples could be well fitted by these isotherms. Clearly, the Langmuir equation provided better fitting in terms of r2 (0.99). The value of KL and qm are determined from the slopes and intercepts of the straight-line plots (Fig. 11b). One can observe that Langmuir parameters KL and qm of activated lignin are higher (almost the double) than those for untreated lignin. On the other hand, the values of KF and 1/n are determined from the slope and intercepts of Log Ce vs Log Ce plots (Fig. 11a). The values of 1/n between 0 and 1 represent good adsorption of Cu (II) adsorption on natural and activated lignin. The Cu ion's adsorption on different unconventional materials has been widely studied during recent years by some authors [14,24–31]. A comparison of the adsorption affinity KL and the maximum Cu (II) adsorption capacity, qm of natural and activated lignin at carbonization temperature of 500 °C, with those of other low- cost adsorbents

Clay minerals Spent activated clay Stevensite (clay)

5–6 27

3.56

10.9–3.2 0.85–2.97 Weng et al., 2007 [14] 27.6 17 Benhammou et al., 2005 [26] 8.97 1.16 Erdem et al., 2004 [25]

6

25

Natural zeolites

6

30

Waste materials Fly ash Mustard oil cake

5.0 4.0

30 20

34.9 454

– 0.139

Gupta, 1998 [27] Ajmal et al., 2005 [28]

7.2

30

90.9

1.46

Jr et al., 2006 [29]

5.5

25

95.3

0.283

4.0

20

294

5.26

6 6

25 25

72.46 136.98

0.031 0.0061

Artola et al., 2000 [30] Gulnaz et al., 2005 [31] This study

Biomass Cassava tuber bark waste anaerobically digested sludge Dried activated sludge Natural lignin Acidic activated lignin

reported in the literature, is given in Table 5. As we can observe, the adsorption capacity of both untreated and H3PO4 treated lignin compare well with other unconventional adsorbents such as biomass (cassava tuber bark waste, anaerobically digested sludge). However, it is higher than those of spent activated clay (SAC), stevensite and fly ash materials. As shown, some other waste materials such as mustard oil cake dried activated sludge exhibit a higher adsorption capacity than that of both lignin samples. Differences in Cu adsorption capacities are due in part to variation in properties of the adsorbents such as SSA, structure, functional groups etc. Other than the adsorbent properties affecting the adsorption, the solution matrixes such as pH, temperature, organic ligands, and the presence of competing cations would also influence on the adsorption to various degrees. Although, the adsorption capacity of activated lignin is relatively high, its adsorption affinity (KL) for Cu is quite low as compared with other adsorbents. 4. Conclusions

Fig. 11. (a) Freundlich and (b) Langmuir plots for Cu (II) ions adsorbed on natural lignin at 25 °C and H3PO4 -activated lignin at 500 °C.

This study has demonstrated that H3PO4 is a suitable activating agent for the preparation of relatively high-porosity carbons from natural lignin found in the north east of Tunisia. Chemical activation through thermal treatment of H3PO4-impregnated lignin, provided from a deposit, allows obtaining activated carbons with a relative high surface areas and a well-developed microporosity that make them good candidates as aqueous phase adsorbents. In our study, concerning the potential use of activated natural lignin as adsorbent for water pollutant removal, a good combination of operating conditions leading to best activated carbons, involves an impregnation ratio and an activation temperature around 1.026 g H3PO4/g lignin and 500 °C, respectively. In this context, the SSA of natural lignin is increased from 10 to 463.5 m2/g after acidic treatment with H3PO4 solution. Similarly, the pore volume follows the same trend, from 0.049 to 0.285 cm3/g and is confirmed by the scanning electron microscopic study which shows that the external surface of a H3PO4 activated lignin is full of cavities. The adsorption capacity of the copper by these carbons (activated lignin), of order 136 mg Cu/g for the important target compound used as a water toxic pollutant (Cu (II)) compares well and even favourably for other activated carbons and adsorbent materials.

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