Removal of Chromium from Electroplating Wastewater by Aminated Polyacrylonitrile Fibers

Materials

Commercial polyacrylonitrile fibers (PAN, Huangshan Brand; SINOPEC Anqing Co.) were used as the matrix for triethylenetetramine (Tianjin Kermel Chemical Reagent Co., Ltd.) grafting following a procedure modified from Deng et al. (2003, 2008) and Deng and Bai (2004). The weight gain calculated from the mass balance of the fibers before and after modification was 33.5%. Further analyses from FTIR and acid–base titration demonstrated that a great amount of amine groups was successfully grafted onto the fiber, attaining an anion exchange capacity of 4.95 mmol/g.

The wastewater used in this study was collected from the electroplating process in a machinery company in Zhengzhou, China. Analyses from atomic absorption spectroscopy (ZEEnit700; Analytik Jena) and visible spectroscopy (722N; Shanghai Precision & Scientific Instrument Co., Ltd.) demonstrated that the total chromium concentration was 53.5 mg/L and the Cr(VI) concentration was 32.7 mg/L [i.e., 61% of chromium existed as Cr(VI)]. Details of the wastewater composition were demonstrated in Supplementary Table S1 (Supplementary Data are available online at www.liebertonline.com/ees).

Sample characterization

FTIR spectra of the TE-PAN samples [i.e., (a) original TE-PAN, (b) TE-PAN after chromium sorption, and (c) TE-PAN after NaOH regeneration] were collected on a FTIR spectrometer (Thermo Nicolet IR 200; Thermo Electron Corp.) working in 4,000–400 cm⁻¹. Morphologies of the original TE-PAN and the TE-PAN after 15 cycles of sorption-regeneration were examined by SEM (JSM-6390LV, Japan).

Batch sorption experiments

Generally, 50 mg of sorbent were mixed with 50 mL of chromium-containing wastewater in a 100 mL conical flask at 25°C. After reaching sorption equilibrium, the supernatant was sampled and filtered, and the filtrate was analyzed by atomic absorption spectroscopy for total chromium concentration. Sorption capacity (Q_e, mg/g) and removal rate (R, %) of total chromium at equilibriums were calculated using equations \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}\begin{align*} Q_e = \frac { ( C_0 - C_e ) \times V } { M } \tag {1}\end{align*}\end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}\begin{align*} R = \frac { C_0 - C_e } { C_0 } \times 100 \% \tag {2}\end{align*} \end{document}

where C₀ and C_e (mg/L) are initial and equilibrium concentrations of total chromium, M (g) is dry mass of sorbent, and V (L) is volume of wastewater. Batch sorption experiments were conducted at different time intervals (1–60 min), various pH values (pH 2–9), and a series of Cr(VI) initial concentrations (10–300 mg/L) to evaluate sorption kinetic, effect of pH on chromium sorption, and sorption isotherms, respectively. For sorption isotherms, synthetic wastewater was prepared with Cr(VI) initial concentrations in the range of 10–300 mg/L.

Column sorption experiments

For the column experiments, a column (inner diameter 13 mm, height 60 mm) packed with 1.5 g of TE-PAN was used, and the empty bed contact time was controlled at 0.5 min for wastewater. Before operation, the bed was rinsed with deioned (DI) water and then pretreated with 0.1 M HNO₃ to convert the sorbent to its protonated form. After being extensively rinsed with DI water, the electroplating wastewater influents were pumped upward through the column at a flow rate of 10 mL/min. The chromium breakthrough curves were obtained by plotting the total chromium concentrations of the samples versus the empty bed volume (BV). The breakthrough point was designated at the BV when total chromium concentration of effluents exceeded 0.5 mg/L.

After breakthrough, 0.1 M NaOH was used for elution of the sorbed chromium. The elution curve was obtained by plotting the total chromium concentration versus the BV of NaOH solution. After regeneration, the column was washed sequentially with DI water, 0.1 M HNO₃ and DI water prior to subsequent sorption. Multiple sorption-regeneration cycles were conducted to demonstrate the sorbent stability in chromium removal from the real wastewater.

Sorption kinetics

As shown by the observed removal rate of Cr(VI) versus sorption time (Fig. 1 and Supplementary Fig. S1), the sorption equilibrium was reached very fast and took only about 10 min for the TE-PAN fiber (removal rate >99%). In contrast, the raw PAN had no significant uptake for Cr(VI), indicating that grafting of amine groups could substantially improve its Cr(VI) sorption capacity. Deng and Bai (2004) also reported that the adsorption equilibrium took about 1 h for Cr(VI) species on fiber material.

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FIG. 1.

Sorption kinetics of total chromium on both raw PAN and as-synthesized TE-PAN fiber; wastewater pH=5.0 (fixed by 0.01 M acetate buffer), sorbent dosage=1 g/L. PAN, polyacrylonitrile.

Sorption isotherms

As illustrated in Fig. 2, sorption capacities (Q_e) of total chromium increased with increasing equilibrium Cr(VI) concentrations, and the two sorption isotherms at 25°C and 40°C had no obvious difference. The estimated maximum uptake (Q_m) of chromium from Langmuir fitting was 133 mg/g, much higher than the value (35 mg/g) reported by Deng and Bai (2004) in their study of chromium removal with animated PAN fibers, demonstrating a greater sorption capacity of the as-synthesized TE-PAN.

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FIG. 2.

Sorption isotherms of Cr(VI) on TE-PAN fiber under 25°C and 40°C; sorbent dosage=1 g/L, pH=5.0 (fixed by 0.01 M acetate buffer). Synthetic wastewater was prepared from Na₂Cr₂O₇ with Cr(VI) initial concentrations 10–300 mg/L. TE-PAN, novel aminated polyacrylonitrile.

Effect of pH

Both batch (Fig. 3) and column (Fig. 4) sorption tests were conducted to elucidate the effect of solution pH on chromium removal by the TE-PAN fiber. As illustrated in Fig. 3, the aqueous pH at sorption equilibrium (pH_e) had a significant impact on chromium removal by TE-PAN. The optimal pH_e was in the range of 2–7 where the chromium removal rate kept above 90%. Increasing the solution pH_e to 8.7 decreased the removal rate to 80.3%. Under the studied conditions, pH_e was always higher than the initial pH (pH₀). This proposed that H⁺ was apt to adsorb on the fiber, with resulting amine groups protonated which was favorable for sorption of anionic Cr(VI) species (such as HCrO₄⁻, CrO₄²⁻). At high pH values, the low solution H⁺ concentration significantly inhibited the amine protonation and consequently decreased the chromium removal rate. Similar pH patterns were reported for Cr(VI) sorption by aminated fibers (Deng and Bai, 2004), anion-exchange resins (Shi et al., 2009), amine-modified celluloses (Anirudhan et al., 2009), etc. The high removal rate (>90%) also suggested that both Cr(VI) and Cr(III) could be efficiently removed by TE-PAN.

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FIG. 3.

Impact of solution pH_e (pH at equilibrium) on chromium removal by TE-APN fiber and the relationship between pH₀ (initial pH) and pH_e; TE-PAN dosage=1 g/L.

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FIG. 4.

Breakthrough curves for chromium sorption by TE-PAN at pH 8.0 (a) and pH 5.0 (b) and their corresponding desorption curves by 0.1 M NaOH (c, d). Sorption conditions: column inner diameter=13 mm, height=60 mm, TE-PAN mass=1.5 g, wastewater flow rate=10 mL/min. Desorption conditions: NaOH flow rate=2 mL/min.

To further evaluate the pH impact, two pH values were studied in the column sorption experiments. For each test, two cycles were conducted. Breakthrough curves from these studies (Fig. 4a, b) demonstrated that at pH 8.0, about 220 BVs of wastewater could be efficiently treated in each cycle. However, when pH of the wastewater was adjusted to 5.0, this volume was increased to about 360–380 BVs. This result was similar with the batch tests, demonstrating that the wastewater pH had a significant effect on chromium removal, and a weak acid condition was always beneficial. Moreover, there was no obvious difference for the volume of wastewater between the two cycles.

Regeneration tests

0.1 M NaOH was used to regenerate the sorbent for its further sorption. Results from the desorption curves (Fig. 4c, d) showed that about 14 BVs of NaOH were needed for nearly complete desorption of the chromium sorbed on TE-PAN at pH 8.0. While the volume increased to about 20 BVs for the TE-PAN exhausted at pH 5.0, due to more chromium to be eluted. In addition, the eluted chromium was highly concentrated within 4–6 BVs (e.g., from 6th to 12th BV for tests at pH 5.0), enabling possible recovery or reuse of chromium.

Further studies demonstrated that after repeating the sorption-regeneration process for 15 cycles, only 5%–6% volume loss of the treated wastewater was observed. This indicated that the synthesized TE-PAN was very efficient and stable in chromium-containing wastewater treatment.

FTIR and SEM

As indicated in Fig. 5 (top), the synthesized TE-PAN had several characteristic peaks. Most of them were derived from the original PAN fiber which could be assigned as follows: broad band ranging from 3,100 to 3,700 cm⁻¹ (stretching vibration of OH and NH groups), 2,937, 2,875, 1,452 cm⁻¹ (C-H stretching and blending), 2,243 cm⁻¹ (C≡N stretching in nitrile group), 1,072 cm⁻¹ (C-O stretching), and 538 cm⁻¹ (C=O twisting). After triethylenetetramine grafting, a new broad band appeared at 1,550–1,680 cm⁻¹ (C=O group in amide and N-H group in amine), demonstrating that amine has been successfully introduced in PAN fiber structure. After chromium sorption, two new bands appeared at 899 and 784 cm⁻¹, which disappeared after NaOH desorption, demonstrating that these bands resulted from the sorbed chromium (Li et al., 2007; Toral et al., 2009; Sun et al., 2010). The complete disappearance of these two bands also suggested that NaOH could desorb chromium completely.

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FIG. 5.

FTIR (top) and SEM photographs (bottom) for (a) the as-synthesized TE-PAN fiber, (b) TE-PAN after chromium sorption (photo not shown), and (c) TE-PAN after 15 sorption-regeneration cycles.

SEM patterns (Fig. 5, bottom) showed that the TE-PAN fiber had a very smooth surface. After 15 cycles of sorption-regeneration, the fiber still kept its good fibrous morphology, indicating that the TE-PAN fiber was stable during the repeated chromium sorption and base regeneration processes.