Application of a Zero-Valente Iron-Per Sulfate System to Treat Petrochemical Wastewater With High-Total Dissolved Solids Containing Para-Chlorophenol

3.1. Materials

Analytical reagent grade or better chemicals as well as deionized water were used in the experiments. Para-chlorophenol (PCP ≥ 99%) was purchased from Sigma-Aldrich. Sodium persulfate (PS ≥ 98%) was obtained from Sigma-Aldrich and its stock solution was prepared in deionized water. Zero Valent Iron (ZVI or Fe° ≥ 99.9% and particle diameter < 10 µm) from Aldrich were used as the PS activator. All the stock solutions used in this study were stored in a refrigerator at 4°C in the dark. All other chemicals were of analytical grade and were used without further purification.

3.2. Wastewater

The petrochemical high-total dissolved solids (TDS) wastewater was collected from effluent of high-TDS wastewater treatment plant of a petrochemical company in Bandar-e-Imam Petrochemical Complex, Iran. The initial characterization showed that the wastewater had a COD of 750 ± 65 mg/L, TDS of 17420 ± 3500 mg/L and a pH of 7.35 ± 0.25. This wastewater was stored was frozen deeply and used for further studies.

3.3. Batch oxidation experiments

Stock solutions of PCP (50 mg/L) and persulfate (10 g/L, or 52.08 mM) were prepared in petrochemical wastewater and deionized water, respectively, prior to each batch experiment. Fifty milliliters of the PCP containing wastewater and 0.5 mL of the persulfate stock solution and then appropriate dosages of ZVI were added simultaneously to a 150 mL bottles, shaken at 150 rpm in an orbital shaker at 25 ± 1°C, to give initial concentrations of PCP and persulfate of 50 mg/L and 10 mM/L, respectively. To examine the effects of ZVI on PCP oxidation by persulfate, a pre-determined amount of ZVI was added to the PCP-persulfate solution. The following ZVI doses were examined at a fixed amount of persulfate: 0, 0.056, 0.14, 0.28, 0.56, 1.4 and 2.8 g/L ZVI (each containing 10 mM/L of PS).

For the pH effect experiment, the oxidation of PCP by ZVI/PS was investigated at the pH range of 3 - 11. The initial pH of the PCP containing wastewater was adjusted by 1.0 M/L H₂SO₄ and 1.0 M/L NaOH and pH was detected using a pH meter (Met Rohm model 713 pH meters). Except for the pH effect experiment and the reuse study of ZVI, the pH of all the original wastewater was 7.35 ± 0.25. At selected time intervals (0, 5, 10, 15, 30, 60, 120, and 240 minutes), a 2 mL sample was collected from each replicate bottle and immediately passed through a 0.22 μm membrane filter (Millipore, MA, USA) for PCP analysis. After filtration, methanol (1.0 mL of methanol for each 2 mL sample), a well-known quenching agent for sulfate radicals, was added and vigorously shaken for five minutes using a vortex shaker to stop the reaction before chemical analysis. For each experiment, controls without ZVI or persulfate were run in parallel under identical conditions.

During the reuse study of ZVI, appropriate doses of ZVI were set into the 150 mL bottles containing PCP solutions (50 mL, 50 mg L-1). Then they were shaken vigorously for approximately two hours. In the next step, the ZVI was separated magnetically and cleansed sequentially via both distilled and deionized water several times and used in three sequential cycles.

All the experiments were prepared in triplicates to ensure reproducibility and to estimate experimental errors. Oxidation efficiency of PCP (R) was calculated according to the following Equation 5:

(5)

Where R (%) represented the PCP removal efficiency, C₀ (mg L-1) was the initial concentration of PCP in the solution, and C_t (mg L-1) stood for the concentration of PCP at t minute.

3.4. Analytical Methods

The concentration of PCP was measured using direct photometric method, according to the procedure of the standard methods to examine water and wastewater (5530 D) by 4-amino anti pyrine and potassium ferric cyanide reagents as well as UV-vis spectro photometer (DR 5000, HACH) by measuring the absorption at a wave length of 500 nm (27).

4.1. Effect of Fe° Dosage and the Molar Ratio of Fe° to PS on PCP Oxidation

To investigate the effect of Fe° dosage on the degradation of PCP, a series of comparative experiments were conducted by adding different amounts of Fe°, while a fixed initial concentration of persulfate (10 mM) was used with the natural pH of petrochemical high-TDS wastewater (7.35 ± 0.25). As shown in Figure 1, the PCP degradation rate was heavily influenced by the Fe° dosage and the molar ratio of Fe° to PS at a fixed PS concentration. The removal percent of PCP was increased from 8.2% to 47.0% when the Fe° dosage went up from 0.056 g/L (molar ratio of [ZVI]: [PS] = 0.1) to 0.56 g/L (molar ratio of [ZVI]: [PS] = 1.0), then it remained unchanged when the Fe° dosage increased from 0.56 g/L (molar ratio of [ZVI]: [PS] = 1.0) to 1.4 g/L (molar ratio of [ZVI]: [PS] = 2.5), but gradually decreased to 37.3% when the Fe° dosage further increased to 2.8 g/L (molar ratio of [ZVI]: [PS] = 5.0). Too high Fe° dosage likely provided too much Fe²⁺ that might scavenge (SO₄^•–) produced in the ZVI/PS system (Equation 6), thereby reducing the overall degradation rate and efficiency.

(6)

Figure 1.

Experimental conditions: [PCP] = 50 mg/L; [PS] = 10.0 mM; room temperature (25 ± 1°C); reaction time 240 minutes, no pH adjustment (7.35 ± 0.25).

It is reported that Fe²⁺ is one of the strongest species that can catalyze PS to generate (SO₄^•–) which can significantly improve the efficiency of the pollutant degradation (22, 28). The previous studies showed that Fe° can be applied as the source of Fe²⁺ in acid media in Fenton reactions (29). In the current study, the combination of Fe° and PS was also highly effective for PCP degradation in petrochemical high-TDS wastewater. With the increase of the Fe° dosage, the amount of (SO₄^•–) in ZVI/PS system increased, consequently leading to the increase of the PCP degradation efficiency. However, too high Fe° dosage could not achieve the high PCP degradation efficiency, since the presence of excessive Fe²⁺ rapidly activated all the PSs and then acted as the scavenger of (SO₄^•–), accounting for the decrease in the oxidation of PCP.

Therefore, the optimum Fe° dosage and Fe° to PS molar ratio was 0.56 - 1.4 g/L and 1.0 - 2.5, respectively, in the current study. The findings were consistent with those of the previous studies that the optimal Fe° to PS molar ratio was 1: 1, and complete oxidation of polyvinyl alcohol (PVA) (30) and 93.2% oxidation of acetaminophen (28) were obtained by Fe° activated PS in 120 minutes. The optimum Fe° dosage of 1.0 g/L would be used in the following experiments.

4.2. Effect of Initial Per-Sulfate Concentration

The effect of initial PS concentration on PCP degradation was investigated with six S₂O₈^2- concentrations (2.5, 5.0, 10.0, 15.0, 20.0 and 25.0 mM) at pH 7.35 ± 0.25 and 25°C with a fixed ZVI dosage (1.0 g/L). The results were displayed in Figure 2. The degradation efficiency of PCP increased from 13.7% to 57.7% when the concentration of PS rose from 2.5 mM to 15.0 mM. However, it was observed that further increase in PS concentration resulted in a decline in PCP degradation. These were mainly due to the PS depletion when the initial PS concentration was insufficient. By increasing the concentration of PS, more PS molecules could reach the ZVI surface and react with Fe°, which resulted in improved PCP degradation. Nevertheless, since the ZVI dosage was fixed, the available amount of (SO₄^•–) was limited. Therefore when excess PS participated in reacting with ZVI to degrade PCP, the PS competed with PCP for sulfate radicals, which resulted in decreased degradation efficiency (Equation 7). On the other hand, the scavenging effect of (SO₄^•–) (Equation 8) reduces degradation efficiency of PCP in ZVI/PS system.

(7)

(8)

This result is similar to the observations of persulfate/Fe²⁺ degrading azo dye orange G at 30 mg/L (31) and p-nitro aniline removal from wastewater by persulfate/Fe₃O₄ nanoparticles process at 0.2 mM/L (16).

Figure 2.

Experimental conditions: [PCP] = 50 mg/L; [ZVI] = 1.0 g/L; room temperature (25 ± 1°C); no pH adjustment (7.35 ± 0.25).

4.3. Effect of Initial pH

The pH value of the reaction solution is usually a crucial parameter affecting the oxidative degradation of organic pollutants. The pH value strongly influences the dissolution of Fe²⁺ from ZVI, which affects the degradation efficiency of PCP. Therefore, the influence of initial pH on the degradation efficiency of PCP was studied through different pH values ranging from 3.0 to 11.0. All of the experiments were conducted at 25°C, 15.0 mM S₂O₈^2­, 1.0 g/L ZVI and 50 mg/L PCP.

The PCP degradation under different initial pH values are shown in Figure 3. It can be observed that PCPoxidation by persulfate activated with ZVI was a pH-dependent process, where the degradation efficiency generally decreased from low to high initial pH. Compared with alkaline solution, rapid degradation of PCP was observed under the acidic condition. After 120 minutes, about 70% of PCP was removed at pH = 5.0; then it remained almost unchanged when the pH decreased from 5.0 to 3.0, whereas only 20.5% was transformed at pH = 11.0. Generally, an acidic condition facilitates the formation of Fe²⁺ from the ZVI surface (Equation 9) that generates more (SO₄^•–), in the ZVI/PS system.

(9)

However, the precipitation of Fe³⁺ occurred when the pH was above 5.0, leading to the adsorption of iron hydroxides onto the surface of ZVI, and consequently formed electronic barriers, which inhibited the corrosion of ZVI and release of Fe²⁺. These results were similar to those of the degradation of acetaminophen in water by Fe°/PS system (28), lindane in aqueous solution by Fe²⁺-activated sodium persulfate (22) and p-nitro aniline in wastewater by persulfate/Fe₃O₄ (16). Hence, the pH to degrade PCP in ZVI/PS system was 5.0.

Figure 3.

Experimental conditions: [PCP] = 50 mg/L; [ZVI] = 1.0 g/L; [PS] = 15.0 mM; room temperature: 25 ± 1°C.

4.4. The Study of Recycling and Reusing of ZVI

It is difficult to collect the excess amounts of ferrous catalyst after its addition to the wastewater, and results in a substantial waste of resources. However, magnetic materials such as iron (Fe°), can be used to address this dilemma. They can be easily removed from the reactor at the end of each repetitive oxidation process, and then used in succeeding runs. To evaluate the reusability of the catalyst, ZVI was repeatedly used to activate the persulfate three times. After each run, the catalysts were collected using a magnet, rinsed with distilled water several times, and then recycled for the next batch. Figure 4 show that when the ZVI cycle time increased, the degradation efficiency of PCP gradually decreased from 72.4% to 51.7%. The ZVI was used until the third cycle to activate the persulfate and degrade PCP; however, the removal efficiency was only 51.7%. This result demonstrated that the activating ability of the ZVI catalyst after being used four times was negligible on the account that the degradation efficiency can reach 51.7% only with persulfate in the reaction. The decrease in the removal efficiency can be explained by the conversion of the Fe²⁺ to Fe ³⁺ on the surface of ZVI.

Figure 4.

Experimental conditions: [PCP] = 50 mg/L; [ZVI] = 1.0 g/L; [PS] = 15.0 mM; initial pH = 5.0; room temperature: 25 ± 1°C.