Introduction

Chromium is usually encountered in both hexavalent and trivalent forms in aqueous solution as a consequence of different industrial processes, i.e. dyes and pigments production, film and photography, galvanometry, metal cleaning, plating and electroplating, leather treatment and mining (Patterson 1987).

Hexavalent chromium is of particular concern because of its great toxicity. Some of the available conventional processes used to remove chromium in this oxidation form are: (a) reduction to trivalent chromium followed by precipitation as chromium hydroxide, (b) removal by ion exchange and (c) removal by adsorption. Most of these methods are costly due to operational, treatment and sludge disposal costs.

The use of dead biosorbents for removal of toxic heavy metals from waste streams has emerged as an alternative to the existing methods as a result of the search for low-cost, biodegradable and innovative methods (Volesky 2003).

The potential of grape stalks wastes, produced as a result of wine production, to remove Cr(VI) from aqueous solutions has been recently reported (Fiol et al. 2003). Metal sorption was found to be pH-dependent and maximum sorption (59.8 mg g−1 dry grape stalks) was found at initial pH 3.0. The results obtained showed that these wastes can be efficient sorbents of hexavalent chromium. Nevertheless, the fragility of the residues due to their low mechanical strength can pose problems of losses of sorbent in their industrial application for large-scale processes of liquid waste streams metal decontamination (Bai and Abraham 2003). Therefore, in order to solve in part these problems we have recently entrapped the grape stalks wastes in calcium alginate beads (Fiol et al. 2005).

Calcium alginate was chosen as entrapping polymeric matrix because this material is biocompatible, low-cost and the technique for encapsulating is very simple (Scott et al. 1989). However, different biomasses have been entrapped in this matrix during the previous years, i.e. bacteria (Sag et al. 1995), algae (Aksu et al. 1998) and fungi (Bai and Abraham 2003). In addition, the use of grape stalks has advantages over the use of live biomass as dead biomasses do not need nutrients and are resistant to the physical–chemical properties of heavy metals solutions.

In one above-mentioned studies we investigated the encapsulation procedure and the influence of contact time, and pH on Cr(VI) sorption process (Fiol et al. 2005). A composition of 2% (w/v) of grape stalks encapsulated in calcium alginate beads was found to be the most efficient, equilibrium was achieved in 24 h and maximum sorption 71.98×10−3 mg bead−1 (225 mg Cr(VI) g−1 grape stalks) occurred at initial pH 3.0.

As a suitable treatment process for the removal of chromium from wastewater has to ensure successful removal of both hexavalent and trivalent chromium, the objective of this paper is to investigate if grape stalks encapsulated in calcium alginate can be used for the removal of Cr(VI) when Cr(III) is present in the solution.

Experimental

Materials and solutions

Grape stalks were kindly supplied by a wine manufacturer from Cuenca region (Spain). The wastes were first cut into small pieces, then rinsed three times with distilled water and finally dried in an oven at 110°C until constant weight was achieved. Once dried the waste particles were sieved and the smallest ones were ground to get a fine powder (<500 μm). Alginic acid, sodium salt from brown algae purchased from Fluka (Switzerland) was used as hydrocolloid gelling material. As fixing solution a calcium chloride (CaCl2·2H2O) solution was used. Cr(III) and Cr(VI) solutions were prepared by dissolving appropriate amounts of the respective chromium salt, Cr(NO3)3·9H2O and K2Cr2O7, in distilled water. These reagents were analytical grade and were purchased from Panreac (Barcelona, Spain). Chromium standard of 1000 mg l−1 solution, purchased from Carlo Erba (Milano, Italy), was used for atomic absorption calibrations.

Preparation of the beads

A 1% (w/v) Na-alginate solution was prepared by solving 1 g of sodium alginate into 100 ml distilled water at a temperature of about 65°C. Then, the gel was allowed to cool down at room temperature and 2 g of powdered grape stalks wastes was added to the gel with continuous stirring. Once the mixture was homogeneous it was forced through a micropipette tip by a peristaltic pump. The resulting gel droplets were collected in a stirred reservoir containing 200 ml of a chemical “fixing” solution of 0.1 M CaCl2. The beads were allowed to harden in this solution for 24 h. After this time hard spherical beads of 2% (w/v) of grape stalks (2% GS-CA beads) were obtained. The beads were filtered and rinsed several times with distilled water to remove calcium chloride from the bead surface. Then they were stored in distilled water until use. The experimental set-up used to get the beads has been previously shown elsewhere (Fiol et al. 2004).

Metal uptake procedure

For the different experiments, 40 beads of 2% GS-CA were put into contact with 15 ml of Cr(VI) and Cr(III) solutions in single and binary mixtures and shaken in a rotary mixer Cenco Instrument at 25 rpm until equilibrium was reached. After agitation the beads were filtered through a 0.45 mm cellulose filter paper (Millipore Corporation). After filtration, the total concentration of chromium, i.e. Cr(VI) + Cr(III), in the remaining solution was determined by Flame Atomic Absorption Spectroscopy (FAAS) (Varian SpectrAA220FS). Hexavalent chromium was analysed by the standard colorimetric 1,5-diphenylcarbazide method in a spectrophotometer (Cecil, CE2021). The concentration of trivalent chromium was determined as the difference between total chromium and hexavalent chromium concentrations.

The amount of metal removed per bead was calculated by a mass balance. When preparing chromium solutions initial pH was adjusted at pH 3.0 with 0.1 M HCl. At the beginning of the experiments the pH was checked and readjusted if necessary but any further readjustment was made during the experiments. The pH of the remaining solution after sorption was measured using a Crison Model Digilab 517 pHmeter. Each test was carried out in duplicate. The average results are presented in this paper.

Sorption experiments

Sorption experiments consisted of contacting beads with binary mixtures solutions containing different Cr(VI) concentrations (50–150 mg l−1) and 100 mg l−1 Cr(III) at pH 3.0. For comparison sake, the sorption of 100 mg l−1 Cr(III) and the sorption of the same initial concentrations of Cr(VI) used in binary mixtures was studied in single metal solutions.

Results and discussion

Effect of contact time

Before undertaking the study of binary mixtures of Cr(VI) and Cr(III) on 2% GS-CA beads, preliminary experiments were carried out in order to ascertain the contact time that was necessary to achieve the equilibrium state characterized by unchanging sorbate concentration in the solution. Experiments were performed stirring in different tubes 40 (2% GS-CA) beads with 15 ml of either trivalent and hexavalent chromium in single metal solutions. The tubes were drawn at pre-determined intervals of time for chromium analysis.

For an initial concentration of 100 mg l−1, the results showed that trivalent chromium uptake was faster than the one for the hexavalent chromium (Fig. 1). As seen, in the case of Cr(III) the 80% of the total metal sorbed is achieved during the first 15 min while it takes 24 h to attain the same yield in the case of Cr(VI). Moreover, as seen in the figure, 10 h was enough for Cr(III) to attain equilibrium while Cr(VI) sorption process was not completed until 48 h. Based on these results a shaking time of 48 h was assumed suitable for the subsequent sorption experiments.

Fig. 1
figure 1

Chromium uptake using 2% GS-CA as a function of time. Initial Cr(VI) and Cr(III) concentration: 100 mg l−1. Initial pH: 3

Full size image

The different sorption profiles showed may be attributed to different mechanisms governing the two different chromium forms uptake. At pH 3.0 the major species of Cr(VI) present in solution is the anionic species (HCrO4 −) while in the same conditions, the cationic species Cr3+ and Cr(OH)2+ are the major species of Cr(III). The fast kinetics of Cr(III) suggest that ion exchange could be one of the mechanisms responsible for trivalent chromium uptake. Indeed, Araújo and Teixeira (1997) who studied Cr(III) uptake by calcium alginate beads postulated that at low and medium Cr(III) concentrations, ion exchange was the predominant mechanism.

In the case of Cr(VI), we already reported in a previous work that in this form, chromium is mainly sorbed on the grape stalks via metal diffusion into the particle of the sorbent with a consequent slow kinetics (Fiol et al. 2004). Taking into account the reported results in the above mentioned studies, Cr(III) is expected to be mainly sorbed on the calcium alginate and Cr(VI) on the grape stalks.

Sorption experiments

In order to study the effect of the presence of one of the chromium forms on the sorption of the other form, binary mixtures containing different concentrations of hexavalent chromium and 100 mg l−1 of Cr(III) were prepared. The concentration of both chromium forms in the remaining solution after the sorption process was accounted separately. The results corresponding to the uptake of Cr(VI) in binary mixtures are presented in Fig. 2 together with the results obtained for Cr(VI) sorption in single solutions.

Fig. 2
figure 2

Cr(VI) uptake using 2% GS-CA at different initial metal concentration in single and binary mixtures. Initial pH: 3.0

Full size image

As can be seen in the figure, the presence of 100 mg l−1 of Cr(III) exerts, in all the cases, a positive effect on hexavalent chromium removal compared to the results obtained in single Cr(VI) solutions.

If now we compare the results obtained when the beads were contacted to 100 mg l−1 Cr(III) in single solution and the same metal concentration in binary mixtures containing different Cr(VI) concentrations, a similar trend can be observed (Fig. 3). In binary mixtures, Cr(III) uptake increases with increase in the concentration of hexavalent chromium in solution. Again the positive effect of one chromium form on the other is observed.

Fig. 3
figure 3

Cr(III) uptake using 2% GS-CA in single and binary mixtures. Initial pH: 3.0

Full size image

In order to get an explanation to the observed increase of Cr(III) sorption in the presence of increasing concentrations of Cr(VI), we determined the concentration of calcium ion released and measured the final pH in both Cr(III) single and binary mixtures. After the process of sorption of 100 mg l−1 Cr(III) in single solution the concentration of calcium was 56 mg l−1 and the final pH 4.1 and the results were 54 mg l−1 Ca2+ and final pH 5.4 in the 100 mg l−1 Cr(III) and 100 mg l−1 Cr(VI) binary mixture. The results corresponding to the blank solution (Milli-Q water adjusted at pH 3.0) were 30 mg l−1 Ca2+ and final pH 6.2. In the blank solution, the calcium released can be attributed to the exchange of these ions and protons and the increase of pH from 3.0 to 6.2 to the protonation of the calcium alginate carboxylic groups. In Cr(III) single solutions, the extra release of calcium is presumably due to the exchange of these ions with Cr(III) ions. The decrease of pH, compared to the blank, suggests the competence between protons and metal ions for the active sites. In the binary mixture, the similar concentration of calcium ions released after sorption, indicates that the extra uptake of Cr(III) must be due to another mechanism different from ion exchange. A possible explanation is that the increase of final pH due presumably to Cr(VI) uptake, would make Cr(III) sorption more favourable resulting in a major adsorption. Indeed, Ibáñez and Umetsu (2002), who studied Cr(III) uptake by calcium alginate beads, reported that the maximum metal uptake was found at a pH within the range 4.5–5.5.

In the case of Cr(VI), the release of Ca2+ ions was 13 mg L−1 and the final pH 7.0 in single metal solution (100 mg l−1 Cr(VI)) and 53 mg l−1 of Ca2+ and final pH 5.4 in binary mixture solution (100 mg l−1 Cr(VI) and 100 mg l−1 Cr(III)). In the single solution, the decrease of calcium released and the increase of final pH, compared to the blank, indicate that protons are involved in Cr(VI) uptake, thus, less protons are available for exchange with Ca2+ ions of the polymer which results in lower concentration of these ions in the remaining solution and in a decrease of pH. In the binary mixture, the higher release of calcium ions is due to ion exchange between calcium and Cr(III) ions and the competence between protons and Cr(III) ions implies the lowest protonation of the active sites with the consequent decrease of pH. Moreover, the decrease of the pH is presumably responsible for the major uptake of Cr(VI) in the binary mixture. In a previous work, it was found that HCrO4 − is the major Cr(VI) species sorbed on grape stalks and this species prevails in the pH range 1.5–5.0 and up to this pH value this species percentage decreases dramatically (Fiol et al. 2005).

Further work will be needed to investigate accurately the mechanisms governing the chromium sorption process and the influence of final pH solution on the amount of total chromium removed in binary mixtures.

Conclusion

The novel sorbent based on grape stalks encapsulated in calcium alginate was found to be efficient for Cr(VI) and Cr(III) removal from waters containing chromium in both forms. The presence of both Cr(VI) and Cr(III) in binary mixtures results in an increase of the amount removed of both chromium forms compared to the results obtained in single metal solutions. In both sorption cases, the variation of final pH in binary mixtures compared to the one in single solutions seems to be responsible for the extra sorption observed in both chromium forms. Further work will be devoted (1) to investigate the observed synergistic effect on sorption of the two chromium forms using grape stalks encapsulated in calcium alginate and (2) to deeply investigate the mechanism governing the overall chromium removal process. Finally, these preliminary studies underscore that the proposed sorbent (2% GS-CA beads) can be used for the removal of chromium from wastewaters through a cost-effective and environmentally friendly process.