Introduction

Chromium is most commonly found in two oxidation states, Cr(III) and Cr(VI), and all other oxidation states are not stable in aerated aqueous media. Chromium (VI) is a common waste product generated from various industrial processes such as electroplating, wood preservation, metal finishing, chrome plating and leather tanning, and is highly toxic as compared to Cr(III) to biological activities [1]. The threshold value for Cr(VI) has been reported 0.1 mg L−1 [2]. Because of high toxicity of Cr(VI) to living systems it should be removed from wastewaters before it can be discharged into the sea. Cr(III) is a more stable, less toxic, and listed as an essential element (in trace concentration) to provide good health [3]. It is poisonous only at high concentration [4]. while the toxicity and carcinogenic properties of chromium (VI) have been known for a long time [5]. The chemistry of Cr(VI) is greatly dependent on pH of the solution. Cr(VI) normally exists in the anionic form, as Cr2O7 2−, HCrO4 − or CrO4 2− forms depending on pH and concentration (Eqs. 1–3). At pH value below 1, the predominant species is chromic acid (H2CrO4) with dissociation constants of K 1 = 4.1, and K 2 = 1.2 × 10−6 respectively. In acidic media with pH value of 2–4, Cr(VI) exists mostly in the form of dichromate (Cr2O7 2−) ions. At pH between 4 and 6, Cr2O7 2− and HCrO4 − ions exist in equilibrium, and under alkaline conditions (pH > 8) it exists predominantly as chromate anion (CrO4 2−). Chromium (VI) is most commonly encountered in the chromate (CrO4 2−) and dichromate (Cr2O7 2−) anions. The chromate and dichromate anions are in equilibrium:

$$ {\text{Cr}}_{2} {\text{O}}{_{7}}^{2 - } {\text{H}}_{2} {\text{O}} \rightleftharpoons 2{\text{HCrO}}_{4}{^{ - }} \;\;\;\;{\text{K}}_{\text{eq}} = 10^{ - 2.2} $$
(1)
$$ 2{\text{CrO}}{_{4}}^{2 - } \left( {\text{yellow}} \right) + 2{\text{H}}^{ + } \left( {\text{aq}} \right) \rightleftharpoons {\text{Cr}}_{2} {\text{O}}{_{7}}^{2 - } \left( {{\text{red}}\;{\text{orange}}} \right) + {\text{H}}_{2} {\text{O}}\;\;\;\;{\text{K}}_{\text{eq}} = 10^{14} $$
(2)
$$ {\text{Cr}}_{2} {\text{O}}{_{7}}^{2 - } + 2{\text{OH}}^{ - } \rightleftharpoons 2{\text{CrO}}{_{4}}^{2 - } + {\text{H}}_{2} {\text{O}}\;\;\;\;{\text{K}}_{\text{eq}} = 10^{14} $$
(3)

The dominant species is therefore, by the law of mass action, determined by the pH of the solution. The change in equilibrium is visible by a change from yellow (chromate) to orange (dichromate). The various treatment techniques available for the removal and recovery of chromium (VI) from wastewaters are reduction precipitation, ion-exchange, reverse osmosis, lime coagulation and carbon adsorption, solvent extraction, chemical precipitation, evaporation and electrolytic reduction [2, 6–10]. However, most of the methods suffer from drawbacks such as high capital and operational cost of the disposal of the residual metal sludge.

The traditional treatment is reduction to trivalent chrome followed by precipitation. The Cr(III) is precipitated as hydroxide at alkaline conditions using either caustic or lime [11]. The pH of solution strongly affected the degree of adsorption of chromium species. The higher pH, the more removal of Cr(III) and the less removal of Cr(VI) is resulted.

Conducting electroactive polymers such as polypyrrole (PPy), polyaniline (PAni) and polythiophene (PTh) have attracted a great deal of attention in recent years due to their interesting electrical and electrochemical behaviors [11–19]. The electroactive nature or the switching properties of polymers have been utilized as the basis of most proposed applications such as sensors, separation devices, rechargeable batteries and controlled drug-release [14, 20–22].

In our previous studies, we have also shown that PPy and PAni electroactive polymers can be used to remove Cr(VI) ion from aqueous solutions effectively compared to other adsorbents reported so far [23, 24]. Poly (3-alkyl thiophenes) are both soluble and melt processable. They have good chemical and environmental stability with excellent electronic and optical properties suitable for various device applications. Polythiophenes can be prepared both by electrochemical polymerization and chemical oxidative polymerization technique. However, the polymers prepared by electrochemical method are not processable. The mechanism of the oxidative polymerization using ferric chloride has been proposed a radical mechanism (Scheme 1) [13–16].

Scheme 1
scheme 1

Proposed mechanism for oxidative polymerization of poly 3-alkyl thiophene by ferric chloride

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In overall oxidative doping of polythiophene conducting polymers Electron are delocalized along the conjugated backbone of conducting polymers, usually through overlap of π-orbitals resulting in an extended π-system with a filled valence band. By removing electrons from the π-system (“p-doping”), a charged unit called a bipolaron is formed (see Scheme 2). The oxidation process is accompanied by the insertion of anions of acid electrolyte (y−) in order to maintain the charge neutrality of the final polymer [16, 25]. In this study poly 3-methyl thiophene was prepared by chemical oxidative polymerization technique using iron (III) chloride (FeCl3) in chloroform. The polymer was used as coated onto sawdust for removal of Cr(VI) from aqueous solution and is reported for the first time.

Scheme 2
scheme 2

Overall oxidative doping of polythiophene conducting polymers (X = H or alkyl)

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Experimentals

Determination of Cr(VI)

The Cr(VI) determination was carried out spectrophotometrically using a calibration curve (Figs. 1, 2). The calibration curve obtained from standard solutions of potassium dichromate (1–20 mg L−1 vs. Cr) prepared in 0.1 M NaOH. At pH value of 12 the Cr(VI) ions exist as chromate form (λmax = 370 nm, ε = 4900). A Spectrophotometer (Perkin Elmer model 35, USA) was employed for analysis of Cr(VI). A Metrohm pH meter (model 827) with a combined double junction glass electrode, calibrated against two standard buffer solutions at pH 4.0 and 7.0, was used for showing pH values.

Fig. 1
figure 1

The vis spectrum obtained for Cr(VI) in 0.1 M NaOH

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Fig. 2
figure 2

Calibration curve obtained for spectrophotometric determination of Cr(VI)

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Using high concentration of Cr(VI) was avoided, because of oxidative nature of dichromate anions (Eq. 2), which may damage the polymer structure. However, they are only moderately oxidizing at high pH (Eqs. 4, 5).

$$ {\text{Cr}}_{2} {\text{O}}{_{7}}^{2-} + 14{\text{H}}^{ + } + 6{\text{e}}^{-} \to 2{\text{Cr}}^{3 + } + 7{\text{H}}_{2} {\text{O}}\;\;\;\;\left( {{\text{E}}^{\text{o}} = 1.33\;{\text{V}}} \right) $$
(4)
$$ {\text{CrO}}{_{4}}^{2 - } + 4{\text{H}}_{2} {\text{O}} + 3{\text{e}}^{ - } \to {\text{Cr}}\left( {\text{OH}} \right)_{3} \left( {\text{s}} \right) + 5{\text{OH}}^{ - } \left( {{\text{E}}^{\text{o}} = - 0.13\;{\text{V}}} \right) $$
(5)

Coating of Sawdust by Poly 3-Methylthiophene

Polymerization of 3-methyl thiophene was carried out in non aqueous solution [15, 26]. 4 mL freshly distilled 3-methyl thiophene was dissolved in 100 mL chloroform (CHCl3). In order to prepare poly (3-methyl thiophene) (P3MTh/SD), 10.0 g sawdust (35–50 mesh) immersed in 100 mL of 3-methyl thiophene monomer solution (0.4 M) was prepared in chloroform at nitrogen atmosphere for 12 h before polymerization. 100 mL of oxidant solution (0.8 M FeCl3 in chloroform) was added via a dropping funnel while the reaction mixture was stirred vigorously, and then the reaction was allowed to continue for 4 h at room temperature. The sawdust was coated by the polymer and the color of the sawdust changed into black. The coated sawdust with the polymer was then washed with copious amounts of deionized water and methanol and was allowed to dry for 2 days at room temperature. The particles were dried in an oven at 50–60 °C for 2–3 h. The polymer coated sawdust was sieved before use in order to have uniform particles for packing of the column and achievement of better reproducibility.

Sorption Experiments

Cr(VI) solutions for sorption experiments were prepared from K2Cr2O7 salt. In column experiments a glass column with dimensions of 1 cm diameter and 15 cm length was employed. 1.0 g sorbent (P3MTh/SD) was packed in the column (5 cm height), and then the Cr(VI) solution (25 mg L−1) was passed through the column with flow rate of 3 mL min−1 at room temperature. The outlet solution was analyzed for unabsorbed Cr(VI) ion after each 10 mL. The percentage of sorption was calculated using the relationship:

$$ \% {\text{Sorption}} = {\frac{{C_{0} - C_{\text{e}} }}{{C_{0} }}} \times 100 $$
(6)

where C 0 and C e are initial (inlet) and final (outlet or effluent) concentration of Cr(VI) ion (mg L−1). Sorbed Cr(VI) ions was determined using the following equation:

$$ \frac{x}{m} = {\frac{{C_{0} - C_{\text{e}} }}{m}} \times V $$
(7)

x/m stands for mg of Cr(VI) sorbed per gram of sorbent. C 0 and C e are the initial (inlet) and final (outlet or unsorbed) concentrations of Cr(VI), respectively. V is the volume (mL) of Cr(VI) test solution. Alternatively, the following equations were used to calculate desorption percentage of the used adsorbent:

$$ \% {\text{Desorption}} = {\frac{m}{{m_{0} }}} \times 100 $$
(8)
$$ {\text{Subsequently,}}\;m_{0} = {\frac{{\left( {C_{0} - C_{\text{e}} } \right)V}}{W}}\;\;\;\;{\text{and}}\;\;\;\;m = C \times V $$
(9)

where m 0 is mg of the sorbed Cr(VI) onto adsorbent, m is the mg of desorbed Cr(VI) in regenerated solution and V is the volume of regenerant solution (dm3).

Results and Discussions

Sorption of Cr(VI) Ion by P3MTh/SD and SD (Column System)

Effect of pH

In order to investigate the effect of pH on sorption of Cr(VI) ion by 1.0 g of sorbent (SD and P3MTh/SD) packed in glass column. The column was first washed with solutions with the same pH value of Cr(VI) solutions that must pass through the column, separately. The column was finally washed with distilled water. Then 25 mL of Cr(VI) solution (25 mg L−1) at different pH (from 1 to 11 using HCl and NaOH dilute solutions) passed through the column with flow rate of 3 mL min−1. The results obtained are shown in Table 1.

Table 1 Effect of pH on sorption percentage of Cr(VI) by SD and P3MTh/SD
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As our results show, removal percentage of Cr(VI) using both SD and modified SD by poly 3-methylthiophene is strongly affected by pH of the solution. They are effective only at acidic conditions. Sorption of Cr(VI) ion decreases as pH of treated solution increases. As we have already explained the dominant form of Cr(VI) at pH 2.0 is HCrO4 −. So, this anion can be exchanged with the releasable chloride ion in the polymer because of higher affinity and stronger interaction of HCrO4 − ions with the polymer. Based on this result, removal mechanism might be due to ion exchange or redox processes as shown by the following equations:

$$ {\text{P3MTh}}^{ + } {\text{Cl}}^{ - } / {\text{SD}}_{{ ( {\text{solid)}}}} + {\text{HCrO}}{_{4({\text{solution}})}}^{ - } \to {\text{P3MTh}}^{ + } {\text{HCrO}}{_{4}}^{ - } / {\text{SD}}_{{({\text{solid}})}} + {\text{Cl}}{_{({\text{solution}})}}^{ - } $$
(10)
$$ {\text{Cr}}_{2} {\text{O}}{_{7}}^{2 - } \left( {\text{aq}} \right) + 14{\text{H}}^{ + } + 6{\text{P3MTh}}^{0} \to 2{\text{Cr}}^{3 + } \left( {\text{aq}} \right) + 6{\text{P3MTh}}^{ + } \left( {\text{aq}} \right) + 7{\text{H}}_{2} {\text{O}} $$
(11)

Cr3+ ions produced in the course of redox reaction are entrapped in polymer matrix due to chelating properties of polymer. In basic condition, the polymer becomes undoped, so there will be not any anion to be exchanged, so the uptake of Cr(VI) anions is decreased dramatically. With increasing pH of treated solution, Cr(VI) will be in the form of chromate and the surface of sorbent will be covered with OH− group. As a result, the electrostatic interaction (repulsion) between chromate ions and negative sites in the polymer leads to a decrease of Cr(VI) ion sorption.

As we have shown by Eq. 11, due to oxidative nature of Cr(VI) ion under acidic conditions and the electroactive nature of poly 3-methyl thiophene [14] and also presence of oxidizable functional groups in sawdust, the importance of some redox reactions in removal of Cr(VI) can not be ignored.

Effect of Initial Concentration of Cr(VI)

In this experiment, 1.0 g of sorbent (P3MTh/SD and SD) packed in a glass column. The column was first washed with 0.1 M HCl. The column was finally washed with distilled water. Then 25 mL of Cr(VI) solution with different initial concentrations (10–100 mg L−1) passed through the column with flow rate of 3 mL min−1 at room temperature. The results obtained are shown in Table 2 and Fig. 3.

Table 2 Effect of initial concentration on sorption percentage of Cr(VI) by SD and P3MTh/SD
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Fig. 3
figure 3

Effect of initial Cr(VI) concentration on its sorption by SD (filled square), P3MTh/SD (filled triangle)

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As our data show, with increasing of the initial concentration of Cr(VI) the total amount of Cr(VI) uptake using both SD and polymer coated SD is increased. However the Chromium removal by polythiophene coated SD is more than uncoated SD. Removal of Cr(VI) by SD can be due to redox reaction between lignin impurities in sawdust and subsequent complex formation of the Cr(III) ion (the reduced product of chromium hexavalent) and the various surface functional groups (e.g. phenolic, carboxylic, carbonyl and hydroxyl) present in lignin compartment of sawdust [27–29].

Breakthrough Curve

Plotting breakthrough curves is needed to evaluate the performance of an adsorbent using column or flow test. The design of fixed packed bed adsorbent in continuous systems, concentration versus time or volume of solution usually yields as S-shaped curve, at which the solute concentration reaches its maximum allowable value referred to as a breakthrough curve. The point where the effluent solute concentration reaches 95% of its influent value is usually called the point of column exhaustion [30].

For performing this experiment, 1.0 g of SD and P3MTh/SD packed in a glass column, and then Cr(VI) solution with concentration of 25 mg L−1 passed through the column with constant flow rate (3 mL min−1). Each time 10 ml was poured into the column. The outlet solution was analyzed for unsorbed Cr(VI) ion in order to obtain the breakthrough curve. The breakthrough curves obtained for the examined sorbents used for sorption of Cr(VI) in a column system are shown in Fig. 4. As it is evident from our results obtained in this experiment, the break point for P3MTh/SD is observed after treatment of about 300 mL of 25 mg L−1 Cr(VI) polluted solution. Complete exhaustion of the used column is occurred after passing of about 600 mL Cr(VI) solution. With increasing the volume of the sorbent bed in the column, it is quite possible to increase the chromium removal efficiency. As we have previously shown the breakthrough curve obtained for SD toward Cr(VI) adsorption, was very sharp and the break point or column exhaustion occurs very quickly. Therefore, P3MTh/SD seems to be much more effective sorbent compared to SD for removal of Cr(VI) ion in column or flow systems [23].

Fig. 4
figure 4

Breakthrough curves obtained for uptake of Cr(VI) by P3MTh/SD

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Desorption Studies

For performing desorption studies, 1.0 g of adsorbent (P3MTh/SD) was first treated with 25 mL of Cr(VI) with initial concentration of 25 mg L−1 at pH value of 1 and flow rate of 3 mL min−1 at room temperature. Under these conditions it was found that about 95.0% of Cr(VI) was removed under acidic conditions. The exhausted column was then treated with 0.01 M NaOH solution for regeneration. Higher concentration of acid was avoided since SD is not very stable in highly acidic (or basic) solutions. Desorption percentage was calculated using Eqs. 8 and 9.

It was found that about 75% of the sorbed Cr(VI) was recovered from the column. In a separate experiment, when the regenerated column after neutralization and conditioning was employed for adsorption of Cr(VI) under the same conditions (pH = 1, C 0 = 25 mg L−1), sorption percentage observed was 94%. Desorption investigation shows, the introduced column can be used frequently without any significant reduction in sorption capacity performance.

Conclusions

It was found that P3MTh/SD as coated form onto sawdust can be effectively and efficiently employed for removal of Cr(VI) ions from aqueous solutions. Sawdust is a very cheap, economical and environmentally friendly substrate for coating of this polymer. It was also found that sorption/desorption of Cr(VI) processes are strongly pH dependent and are occurred with high efficiency. Sorption of Cr(VI) is occurred under acidic conditions and subsequent desorption is readily achieved upon alkaline treatment of the sorbent. Removal of Cr(VI) ions by the introduced adsorbent in this paper seems to be a complex process. Redox, complex or chelation and ion exchange reactions are possible mechanisms for removal of Cr(VI) ions from aqueous solutions. The finding in this paper seems to be important for pollution control in treatment of water or wastewaters polluted with the highly toxic Cr(VI) ions.