Adsorption of Cr(VI) and Cd(II) on charcoal from Alep pine: effects of simple chemical and thermal post treatments.

Abstract

In the present work we report the adsorption of heavy metals Cr (VI) and Cd (II) on an adsorbent made up from rough charcoal of Pin of Alep. The charcoal prepared traditionally (dehydrated at 100[degrees]C and carbonised at 280[degrees]C) is crushed and sieved to 0 < 0.25 mm in diameter. The raw charcoal shows a specific area of 410 [m.sup.2]/g. The characterisation of surface functional groups of charcoal after chemical post treatments by distilled water, hydrochloric acid, sodium hydroxide or by thermal treatment at 600[degrees]C shows the presence of carboxylic acid, phenolic, lactone and carbonyl functional groups. The adsorption capacities of the different charcoals were determinate and the modelling of the isotherms was done according to Langmuir and Freundlich models. The samples CHRS (treated by NaOH solution) and CHRT (heat treatment till 600[degrees]C) present promising adsorptive properties. On the basis of the obtained results, it is possible to conclude that the charcoal of pine of Alep can be used for the elimination of the mineral pollutants such as chromium and cadmium in water with moderate capital costs.

Introduction

Chromium in a natural state is mainly in the form of chromite (Cr[O.sub.3]), of crosses (PbCr[O.sub.4]) or related to iron oxides (FeO.Cr[O.sub.3])[1] It is a toxic compound, in particular in its form h6xavalente it constitutes a major pollutant of various industrial effluents. It results from its various uses a harmful significant polluting load for both human and environment. The over-exposure with chromium dust leads to irritations with the deterioration of the skin and possibly to a wide breathing and probably with the cancer of the fabric epithelium of the lungs. The European Community (C.E.E 1980) and the World Health Organization (O.M.S 1984) fixed the acceptable maximum content of total chromium in the water intended for the human consumption with 0,05 mg/l. Beyond this concentration chromium becomes toxic and its ingestion can cause epigastric pains, severe vomiting and diarrhoeas [2]. Parallel to these toxic effects on the man hexavalent chromium inhibits the food of certain fish of sea (Citharichtys stigmateus) with 2.2 mg/l [3]. The elimination of the chromium of industrial worn water was studied by many researchers [4], [5], [6]. One of the techniques used for the elimination of Cr (VI) is its reduction to a trivalent state Cr (III), then its precipitation in the form of Cr[(OH).sup.3] in alkaline medium. [7] showed that Cr (VI) is eliminated to 95% by filtration on activated carbon. [8] having studied the elimination of the same pollutant on carbons activated differently and showed a significant adsorption of Cr (VI) is obtained with carbons activated thermally.[9]showed that Cr(VI) in water solution ([C.sub.0] = 1,872 mg/l) is eliminated at 100% from a load of 1 g/1 of a mixture (1:1) of ash and Wollastonite. In addition, [4] showed that this pollutant is eliminated at 100%, by sawdusts treated with phosphate, for Cr (VI) concentrations ranging between 8 and 50 mg/l.

Cadmium is a heavy metal present in nature mainly in the zinc ores but seldom in the form of pure mineral, pollution by cadmium does not cease worsening because of metallurgical industry and of the factories of incineration of household refuse, the value of drinking water directive recommended by the World Health Organization (WHO) and the American association of water work (AWWA) are 0,005 Mg. The high toxicity of cadmium was observed for the first time in Japan in 1955 (disease of Itai-Itai). A mine poured its used water, polluted by cadmium, in a river being used for the irrigation The polluted rice consumers are reflected to suffer from immunizing deficiencies, renal attacks, apathy, pains in the members and deformations of the skeleton. Many deaths occurred. It presents also a risk on health: by oral way, the symptoms observed are: an episode of gastro-enteritis with epigastric cramps, the vomiting, the diarrhoeas and myalgias, or by ingestion way, an intoxication mortal was however observed following the voluntary ingestion of 5 gr of cadmium iodide. [10].

At the present time, there are a lot of physical chemical or biological processes for the water treatment (coagulation, flocculation, decantation, centrifugation, purification by membranes, adsorption, ions exchange, advanced oxidation methods etc.) [11]. The choice of the type of treatment is done according on one side to the nature of the pollutants and to the other side to economical aspect of the process. Nevertheless, whatever the adopted solution, in one step at least, adsorption is used. The most common material for adsorption is active carbon (AC) of vegetal origins like coconut shells or wood. This material is rather expensive and this strongly limits its use in several countries. A lot of researches were consequently directed towards the production of low cost but efficient adsorbents (clay, sawdust, lignite or bark of wood) [4], [5], [6]. [8],. AC adsorptive properties depend on its physical and chemical properties such as porosity, specific surface area, surface functional groups and surface additives. It is well known that the origin of the precursors and production process are of paramount importance on the final properties of the solid adsorbent. Recently, the adsorptive properties of some industrial or agricultural low cost by-products have been described. For example, the adsorption of Cr(VI) on bituminous coal [12], sphagnum peat moss, coconut husks and palm pressed fibres [13], sawdust, sugarcane bagasse, sugarbeet pulp and maizecob [14] and thujas oriantalis [15] has been reported. The validity of the phenomenon of metal biosorption by a variety of biological materials is sufficiently established. Attention now needs to be focused on the search of cheaper and more efficient biosorbents capable of binding toxic metals from multimetal solutions in a continuous flow system. For example, husk of black gram (Cicer arientinum) has been recently reported [16], [18] to remove 100% Cd (11) from 10 mg/l solution within 30 min and regenerating completely by desorbing 99.9% of the adsorbed metal. In a recent review [17] gives an exhaustive picture of the polysaccharide materials used as adsorbents for the water treatment. In the present work, we report values of well-known thermodynamic functions and isotherm studies performed to elucidate the equilibrium adsorption behaviour of Cr(VI) and Cd(II) solutions on charcoal from Alep pine (area of Tiaret--Algeria). The effect of charcoal simple post-treatments and pH on the adsorption have been investigated and discussed.

Experimental

Preparation of Samples

The material used is a charcoal (CHRB) produced from sawdust of Alep Pine (area of Tiaret Algeria) by heating the sample at 100[degrees]C during 4 hours and at 280[degrees]C during 2 hours. After crushing and sieving to [empty set] < 0.25 mm in diameter CHRB was treated with distilled water (CHRE), with acid HCl 1N (CHRA) and with base NaOH 1N (CHRS) for 24 hours (10g/1) then filtered and dried at 105[degrees]C. CHRB as charcoal was also heated for 2 hours at T=600[degrees]C (CHRT) in order to follow the influence of thermal treatment on the adsorptive properties temperature lower than 110[degrees]C, since resol can be cured by heating to about 160-200[degrees]C.

Characterisation of Charcoal

Specific Surface Area

The specific surface area of starting charcoal CHRB was determined by sorption measurements of benzene, which is frequently taken as standard adsorptive [12] these analyses were done in LCA Laboratory Sanit Avold France. As a result the starting charcoal CHRB presents a BET specific area of 410 [m.sup.2]/g.

pH and conductivity

0.1 g of charcoal is added to 100 ml of distilled water under agitation (600 rpm) at T = 20[degrees]C, and then the pH and the conductivity of the suspension are recorded directly from the apparatus. These analyses were done with a multiparameter analyser (pH, conductivity, temperature and oxygen) Type Inolab multilevel 1.

Dosage of functional groups of charcoal surface.

Many methods are used to quantify the surface functional groups of a solid surface; in our case we adopt the method of Boehm [19]. Briefly, samples of 0.5 g. of washed and dried charcoal are left for 72 hours with 50 ml of solutions (0.1 N) of the following bases: NaHC[O.sub.3]; [Na.sub.2]C[O.sub.3]; NaOH and [C.sub.2][H.sub.5]ONa. Then 25 ml of each mixture is dosed with HCl solution (0.1N) in presence of helianthine by NaHC[O.sub.3] and [Na.sub.2]C[O.sub.3] or phenolphthalein by NaOH and [C.sub.2][H.sub.5]ONa.

Study of Adsorption

1g.of the studied sample is suspended in 500 ml of chromium (VI) (obtained from [K.sub.2]Cr[O.sub.4] salt) or cadmium (II) (obtained from CdS04,7H20 salt) solutions with various concentrations. The pH of the medium is controlled by adding HCl O,1N or NaOH O,1N at T=20[degrees]C. The suspension is stirred continuously at 600 rpm, each 50 minutes, 10 ml of solution is removed and filtered. The filtrate is analyzed by spectrophotometers (filter photometer WTW photolab S12) by putting two drops of the appropriate reagent (1-(4-nitrophenyl)-3-(4-phenylazophenyl)triazene for Cadmium) and (diphenylcarbazide for chromium) in 5 ml of the filtrate solution and the all is put in the spectrophotometer which gives directly the ions concentration.

Results and Discussion

It is well known that the surface chemistry of solid has a great influence on their exchange properties. On the other hand, bio solids have non negligible amounts of ashes. A simple washing or acidic treatment could change significantly their chemical composition and consequently their adsorption behavior, especially in our case where wood was heat treated at low pressure. It should be added, that our post treatments could affect significantly the charcoal constitution and for example leading to some residual lignin or hemicellulose dissolution. The evolutions of pH and electric conductivity of the post treated charcoal samples are reported in figures 1 and 2. We can note that the pH of the suspensions of the samples (CHRS) and (CHRE), increases with time and become constant after 210 min. comparatively, the starting charcoal (CHRB) reaches a higher pH. The pH change is not significant for the CHRA sample (5.3 to 6.02), and after one hour becomes approximately constant. In the same way, for (CHRB), (CHRS) and (CHRE) clear increases in the electric conductivity of the medium are observed. The increases are less significant for (CHRS) and (CHRE) compared with those of (CHRB). Indeed, the washing of charcoal with NaOH or water involves an elimination of a portion of the metals present in the charcoal. When (CHRB) is washed with HCl 1N (CHRA), the elimination of metal ions is almost total, since the contact of this sample with distilled water does not involve any modification of the solution conductivity. To sum up, it seems demonstrated that a washing by HCl 1N leads in our conditions to a quasi total removal of ionic species from the surface of our samples. The results obtained for the analysis of the surface functional groups are summarised in table (1). According to the Boehm method the surface functions are classified into four groups: Group I acidic functions such as carboxylic acid neutralised by NaHC03; Group II is constituted by cyclic esters (lactones) which are measured by neutralisation difference between [Na.sub.2]C[O.sub.3] and NaHC[O.sub.3]; Group III formed of phenol compounds measured by neutralisation difference between NaOH and [Na.sub.2]C[O.sub.3] and finally Group IV containing the. carbonyl functions which are measured by neutralisation difference between [C.sub.2][H.sub.5]ONa and NaOH. We can note for (CRHS) a general decrease of the functional groups in comparison with those of (CHRA) and (CHRE). This can be attributed simply to the basic post treatment allowing a pre neutralisation of the groups. It should also be noted that if the total amount of functional groups remains constant between (CHRE) and (CHRA), their distributions are different. Some chemical reactions, like ester or lactone hydrolysis have certainly to be invoked to explain this evolution [8].In the case of the thermally treated sample (CHRT) a logical decrease of thermally labile carboxylic groups is observed connected with the apparition of more stable functional groups (Group IV) The phenomena of adsorption of heavy metal on charcoal can described as follow.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

According to Frumkin[20] the surface groups CxO, Cx[O.sub.2] formed during the activation of the raw material of coal undergo in acidic medium to the following transformations equation (3,4) In acid medium hexavalent chromium exists in the shape of anions HCr[O.sup.4-] [21] The process of adsorption can then proceed according to the equations (5,6,7,8): These equations show that for a mole of chromium adsorbed there are two moles of ion hydroxide (OH-), the addition of an acid solution to the reaction medium makes it possible to neutralize the ions hydroxides and consequently to lead to the displacement of balance of the equations (7,8) in the direction supporting the adsorption of chromium (VI). Those confirm the results obtained previously with regard to the influence of the pH.

In the case of cadmium the mechanism of this adsorption can be explained in the way equation (9a,b,c)), The cadmium ions are in the form of [Cd.sup.+2] or Cd[(OH).sup.+] can take part in a physical adsorption according to reactions equation (l0a,b). During an adsorption of copper (II) on activated carbon F400 [22], showed that the hydroxylic groups of the surface of coal can influence the adsorption of this metal on this material. The sites existing on the coal surface in form -C[(OH).sub.2] and -COH contribute to reactions of adsorption of [Cu.sup.+2] with these functions. The same phenomenon can be transposed to the case of the [Cd.sup.+2], consequently the functional groups of the raw coal surface (GI, GII and GIII) can take part in the reactions of adsorption with this element according to the reactions equation (11a,b,c,d)The functions of surface can then take part in a chemical adsorption of the ions of cadmium. [23] Highlighted the aptitude of coal to distribute the protons of the carboxylic groups of surface by cations in aqueous medium. Such a phenomenon of ionic exchange will be responsible for a certain selectivity of adsorption of the surface of coal for the metal ions in solution.

In our case the adsorption measurements were done in static mode. The kinetic of the adsorption shows that equilibrium was reached after 600 min for Cr (VI) and 250 minutes for Cd (II) in all cases. It should be noted that in the case of Cd (II) a rapid adsorption is observed initially (within 60 min) for all the samples meaning at least two different sites of adsorption. The value of the fixed pH is an important parameter. For the adsorption of Cr (VI) the optimal pH is 2 [24] and for Cd (II) this value is 6 [25]

The adsorption isotherms are given in the following figure (3) for Cr (VI) and figure (4) for Cd (II) respectively (sample: CHRB 1g/1). Logically, the Langmuir isotherme equation (1) represented by Stum and Morgan [26] and the empirical relation of Freundlich equation (2) is very much used by the water delicatessens, these two models could be used for Cr (VI) adsorption. For example, the results for CHRB are: For Langmuir model Figure (5) the maximum capacity (T=20[degrees]C and pH= 2) is 8.3 mg/g and [R.sup.2] = 0.9777; For Freundlich model Figure (6) n = 3.31, K' = 5.29 and [R.sup.2] = 0.9823. But as observed previously for the kinetic of adsorption and due certainly to the multimodal sites adsorption, both Langmuir Figure (7) and Freundlich Figure (8) models can not be used for the representation of Cd (II) adsorption isotherms. The comparative adsorption capacities in mg/g for 10 mg/l of Cd(II) and Cr (VI) solutions at pH = 2 and 6 respectively (at T=20[degrees]C) are CHRA (2.6 , 1.8) CHRT (8.9, 6.4) CHRS (7.6, 9.4) CHRB (6.5, 6.4) CHRE (6.4, 5.0). The samples CHRT and CHRS present the higher adsorption capacities. The samples CHRB and CHRE have logically very close properties and CHRA due to the acidic treatment has the lowest capacity. The beneficial effect of a thermal post treatment on the adsorptive properties of charcoal is once more time observed.

1/[GAMMA] = 1/[[GAMMA].sup.[infinity]] + 1/K.[[GAMMA].sup.[infinity].[C.sub.r] (1)

Ln ([C.sub.ad]/m) = Ln K' + (1/n) Ln [C.sub.r] (2)

[C.sub.x]O + [H.sub.2]O [left and right arrow] [C.sub.x.sup.2+] + 2O[H.sup.-] (3)

or [C.sub.x][O.sub.2] + [H.sub.2]O [left and right arrow] [C.sub.x][O.sup.2+] + 2OH (4)

[C.sub.x.sup.2+] + HCr[O.sub.4.sup.-] [left and right arrow] [C.sub.x]OH[O.sub.3][Cr.sup.+] (5)

or [C.sub.x][O.sup.2+] + HCr[O.sub.4-] [left and right arrow] [C.sub.x][O.sub.2]H[O.sub.3]Cr+ (6)

Or while combining (1) and (2) according to:

[C.sub.x]O + [H.sub.2]O + HCr[O.sub.4.sup.-] [left and right arrow] [C.sub.x]OH[O.sub.3][Cr.sup.+] + 2O[H.sup.-] (7)

or [C.sub.x][O.sub.2] + [H.sub.2]O + HCr[O.sub.4.sup.-] [left and right arrow] [C.sub.x][O.sub.2]H[O.sub.3]Cr.sup.+] + 2OH (8)

[C.sup.-] + [Cd.sup.2+] [??] C[Cd.sup.+] (9a)

[C.sup.-] + C[Cd.sup.+] [??] [C.sub.2]Cd (9b)

[C.sup.-] + CdO[H.sup.+] [??] CcdOH (9c)

2[C.sup.-] + [Cd.sup.2+] [??] [C.sub.2]Cd (10a)

[C.sup.-] + CdO[H.sup.+] [??] CcdOH (10b)

[C.sup.-] : being an active site of negative charge.

2CO[H.sup.2+] + [M.sup.2+] [right arrow] [(C[O.sup.-]).sub.2][M.sup.2+] + 4[H.sup.+] (11a)

2COH + MO[H.sup.+] [right arrow] [(C[O.sup.-]).sub.2]MO[H.sup.+] + 2[H.sup.+] (11b)

2COH + M [(OH).sub.2] [right arrow] [(C[O.sup.-]).sub.2]M[(OH).sub.2] + 2[H.sup.+] (11c)

2COH + M [(OH).sub.3.sup.-] [right arrow] [(C[O.sup.-]).sub.2]M[(OH).sub.3.sup.-] + 2[H.sup.+] (11d) M = Cd

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Conclusion

We demonstrate in this paper that low cost adsorbents can be produced from local biomass. In our case, we used pine of Alep. Very moderate thermal treatment (till 280[degrees]C) leads to the production of adsorbent with promising properties considering chromium or cadmium pollutants adsorption. The preparation of studied adsorbents is far to be optimized as it is observed. A very simple thermal post treatment till 600[degrees]C increases significantly the adsorption of both metals. It should be noted that the adsorption kinetic of cadmium presents several steps and certainly involves different sites. The failure to use both Langmuir and Freundlich models in the representation of the cadmium isotherms supports our hypothesis. Logically, the adsorption capacities for both metals are pH dependant. The optimum pH values are respectively 2 for Cr (VI) and 6 for Cd (II), and at in these conditions the adsorption capacities reach, in the best case, 8.9 mg/g (for CHRT--Cd (II)) and 9.4 mg/g (for CHRS--Cr (VI)).

References

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Messaoud Chaib * and Fatima Hattab LCE, BP 78 Universite Ibn-Khaldoun Tiaret 14000 Algeria

* Corresponding author E-mail: [e-mail]

Table 1: Distribution of the functional surface groups according
to the method of Boehm in meq/g of solid.

                   G I      G II    G III    G IV     [summation]
function sample                                       meq/g

CHRE               0.19     0.61    1.32       0        2.12
CHRS               0        0.08    0.52       0        0.6
CHRA               0.4      0.24    1.44       0        2.08
CHRT               0        0.28    0          0.2      0.48