The removal of caffeine (CAF) in aqueous solution by peroxymonosulfate oxidant activated with cobalt ion was investigated under a variety of operating conditions. The effects of various operating parameters, such as oxone and Co2+ concentrations, pH value, and the coexistence of dissolved organic matter and inorganic anions on the removal of CAF have been investigated. The removal efficiency increased with the increase in the concentrations of oxone and Co2+ ion added. The additions of chloride, bicarbonate, and sodium humate have negative effects on the removal of CAF. Near-neutral condition (5.0 < pH < 7.0) is favorable for the removal of CAF. Based on our experiments, 100% degradation of 50 mg/L CAF can be achieved within 4 minutes under the conditions of 1.00 mM oxone and 0.10 mM Co2+ ion at pH 5.0–7.0.

INTRODUCTION

Pharmaceuticals and personal care products (PPCPs) have gained a great deal of attention in the past decade because of their extensive use in veterinary medicine, agricultural practices, human health, and cosmetic care. When freely discharged into water, PPCPs could cause negative impacts on aquatic organisms and the ecological environment because they cannot be easily degraded and are potentially toxic to the aquatic environment and human life.

Among the PPCPs, caffeine (CAF) is one of the most commonly used drugs in the world, being detected frequently in foods, beverages, and medicines. CAF is regularly metabolized by humans. The disposal of unconsumed coffee and soft drinks is the main source of CAF in wastewater treatment systems. Monitoring of influent and effluent in sewage treatment plants (STPs) and in the aquatic environment has shown that CAF is present extensively in higher concentration. For example, CAF has been detected at concentrations from 5.1 to 76.8 ng/L in Lake Simcoe watershed (Kurissery et al. 2012). Meanwhile, the CAF concentrations in raw water and drinking water are 1.1 μg/L to 106 μg/L and 0.22 μg/L in Brazil, respectively (Sodré et al. 2010). In wastewater treatment plants in Beijing, CAF has been observed at concentrations of 3.4–6.6 μg/L in influent (Sui et al. 2010). Furthermore, CAF possibility persists in water because of its high solubility (21.7 g/L) and negligible volatility (Zio et al. 2005). Therefore, developing an efficient water treatment method for CAF removal from aquatic environments is necessary.

Among the emerging treatment approaches, advanced oxidation processes (AOPs), which could generate hydroxyl radicals with powerful oxidizing ability, are the most effective for the degradation of hazardous, refractory and non-biodegradable organic contaminants in various types of water. Typical AOPs, such as ozonation (Rosal et al. 2009), Fenton process (Klamerth et al. 2009; Trovó et al. 2013), and photocatalysis (Marques et al. 2013), have been used for CAF degradation in water.

Several researchers reported that the Fenton process is an effective process to degrade CAF in various types of water. Klamerth et al. (2009) used the photo-Fenton process to degrade CAF in STP effluent, and a higher degradation rate was observed. Trovó et al. (2013) optimized the main experimental conditions for degrading CAF in different water using the photo-Fenton process in a solar pilot plant. They found that under optimized conditions (52.0 mg/L CAF, 10.0 mg/L Fe2+ and 42.0 mg/L H2O2), the CAF concentration could reach the quantitation limit (0.76 mg/L) after 20 minutes in ultrapure water, and 40 minutes in STP effluent. Although Fenton's reagent has been proved to be practically feasible for removal of organic pollutants, it faces several non-ignorable restrictions and requirements such as acidic pH, high ferrous iron concentration and some resistant species.

Recently, peroxymonosulfate has been the focus of considerable scientific and technological interest as an alternative oxidant in the oxidation of organic pollutants under mild conditions. Oxone (2KHSO5·KHSO4·K2SO4) provides strong peroxymonosulfate () oxidant and can generate sulfate radicals () with higher standard reduction potential (2.5–3.1 eV) than hydroxyl radicals (1.8–2.7 eV) (Anipsitakis & Dionysiou 2003; Qi et al. 2014). The peroxymonosulfate anion can be activated by ultrasound, heat, base, transition metals, and quinone activation. Among the various methods, transition metals, particularly Co2+, are the best activator (Anipsitakis & Dionysiou 2003; Ji et al. 2015). Generation of sulfate radical by Co2+ is as follows: 
formula
1
 
formula
2
A series of experimental studies have reported that oxone/Co2+ can degrade organic pollutants effectively, such as antibiotics (Su et al. 2012), herbicides (Ji et al. 2015) and phenolic compounds (Anipsitakis & Dionysiou 2003; Anipsitakis et al. 2006). Qi et al. (2013, 2014) reported that CAF can be effectively degraded by cobalt-incorporated MCM41/PMS. Compared with the Fenton process, its advantages of wide applied pH range and rapid degradation rate provide a suitable alternative for pollutant oxidation.

In this study, degradation of CAF was examined using cobalt(II) activation of oxone. The factors influencing degradation performance, such as pH, oxone and cobalt ion concentrations, and initial CAF concentration, were optimized. The effect of common inorganic ions and organic compounds in water, such as Cl, , , and sodium humate, was investigated to evaluate the stability of this process.

METHODS

Materials and apparatus

CAF (USP/BP grade), sodium humate VEOLIA (NaH, AR), oxone(2KHSO5·KHSO4·K2SO4, AR), and cobalt sulfate (CoSO4·7H2O, AR) were obtained from Shanghai Jingchun Biochemical Technology Co., Ltd (Shanghai, China). Sodium chloride (AR), sodium nitrate (AR), and sodium bicarbonate (AR) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd (Tianjin, China). All other chemicals were of AR-grade quality and used without further purification. Ultrapure water with a resistivity of 18.2 MΩ·cm generated from a Veolia pure water treatment system was used to prepare the solutions. Methanol obtained from J. T. Baker was degassed before being used in high-performance liquid chromatography (HPLC).

Procedure

Experiments were conducted in a 100 mL conical flask at ambient temperature (25 ± 2 °C). A known volume of CAF solution and desired amounts of oxone solution were transferred into the reactor. Solution pH was adjusted using sulfuric acid (0.1 mM) or sodium hydroxide (0.1 mM) and determined using a PHS-3C pH meter (Shanghai, China). With the appropriate concentration, the desired amounts of cobalt sulfate were added to the solutions. A magnetic stirrer provided a continuous and homogeneous mixing. At given time intervals, 2 mL of the reaction solution was withdrawn from the reactor and mixed immediately with excess sodium thiosulfate solution (1.0 M), a well-known quenching agent for sulfate and hydroxyl radicals and used to prevent further reaction. The sample was then filtered using a filter needle (Shimadzu-GL NY 0.45 μm) to determine the concentration of the remaining CAF.

Analytical methods

The concentration of the remaining CAF was analyzed using HPLC (LC-10AT, Shimadzu) conducted on a C18 (150 mm length × 4.6 mm i.d., 5 μm particle size) reverse-phase column. A mixture of methanol and ultrapure water (65/35, v/v) was used as the mobile phase at a flow rate of 1.0 mL/min. The detector was set at 273 nm. Total organic carbon (TOC) was determined using the Shimadzu CPHCN-200 TOC analysis system. CAF removal was calculated as follows: 
formula
3
where R% is the removal ratio of CAF, and C0 and Ct are the initial and remaining concentrations of CAF, respectively.

RESULTS AND DISCUSSION

Removal of CAF at different medium pH values

The medium pH is a critical factor for the degradation of organic pollutants in water by oxone oxidation. The removal efficiency of CAF in aqueous solution at different initial pH values was investigated at pH values ranging from 2.0 to 10.0 with an initial CAF concentration of 50 mg/L. The oxone and Co2+ ion concentrations were 1.0 mM and 0.1 mM, respectively. The results are shown in Figure 1. CAF can be removed easily in near-neutral solutions. At pH 5.0–7.0, 100% of CAF was degraded within 4 minutes. The removal ratios decreased in acidic and basic solutions. These results corresponded with the results of Chan & Chu (2009). The removal of CAF was inhibited significantly at extremely acidic conditions (pH 2.0). This finding can be attributed to the decrease in concentration because oxone could not be activated effectively by the cobalt ion in acidic solution (Ji et al. 2015). When the initial pH increased to 10.0, the removal of CAF decreased remarkably because precipitation of the Co(OH)2 complex occurred in basic aqueous solution.

Figure 1

Removal of CAF in aqueous solution at different pH values. The inset is the molecular structure of CAF.

Figure 1

Removal of CAF in aqueous solution at different pH values. The inset is the molecular structure of CAF.

Notably, the optimum pH for this process is near neutral. However, under weak basic conditions (such as pH 9.0), the degradation ratio of CAF can still reach 84% within 4 minutes. This finding indicates that solution acidity could not be adjusted before treating most wastewaters with this process, which could significantly reduce the cost of wastewater treatment.

Effect of oxone and Co2+

A series of experiments were conducted with different oxone concentrations from 0 to 2.00 mM ([Co2+] = 0.10 mM, pH 7.0) to investigate the effect of oxone concentration on the removal of CAF. The experimental results are shown in Figure 2(a). The removal efficiency of CAF increased with the increase in oxone concentration because oxone was the driving force for the formation of radicals. CAF can be totally degraded in 4 minutes with 1.00 mM oxone. The degradation efficiency did not improve significantly when the initial oxone concentration was higher than 1.00 mM. Considering the practical application and economic cost, 1.00 mM oxone was selected for the subsequent experiment.

Figure 2

Removal of CAF in aqueous solution with different oxone and Co2+ concentrations: (a) with different oxone concentrations ([Co2+] = 0.10 mM); (b) with different Co2+ concentrations ([oxone] = 1.00 mM).

Figure 2

Removal of CAF in aqueous solution with different oxone and Co2+ concentrations: (a) with different oxone concentrations ([Co2+] = 0.10 mM); (b) with different Co2+ concentrations ([oxone] = 1.00 mM).

The removal of CAF with 1.00 mM oxone at different Co2+ ion concentrations from 0 to 0.15 mM was also investigated. As shown in Figure 2(b), the removal efficiency increased with the increase in the amount of Co2+ ions added. CAF cannot be degraded with oxone without the addition of Co2+. The addition of 0.01 mM Co2+ ion corresponded to 59% removal in 4 minutes. However, when the concentration of Co2+ increased to 0.10 mM, the removal of CAF improved substantially and reached 100% within 4 minutes. For the catalytic reaction of oxone, Co2+ is the best catalyst. The higher the concentration of Co2+, the greater the number of radicals. Thus, a higher removal efficiency was observed. Co2+ ion has been identified as a microbial growth inhibitor in water treatment processes at relatively high concentrations (Gikas 2008). Therefore, 0.10 mM Co2+ ion was adopted for further study.

Effect of the initial CAF concentration

The effect of the initial CAF concentration on the degradation ratio was determined by varying the concentration of CAF from 25 to 100 mg/L. The operating conditions were kept constant ([oxone] = 1.00 mM, [Co2+] = 0.10 mM, pH 7.0) at ambient temperature. The results are shown in Figure 3. The removal efficiency was inversely proportional to the initial CAF concentration. Within 4 minutes, the removal ratio decreased from 100 to 45% with the increase in CAF concentration from 25 to 100 mg/L. This decrease in degradation ratio is a common result for AOPs (Wang et al. 2014). When the Co2+ and oxone concentrations are constant, the amount of would be constant. However, the total amount of CAF in the aqueous solution increased with the increase in the initial concentration. As such, the removal ratio of CAF is decreased. Based on the aforementioned experiments, 100% degradation of less than 50 mg/L CAF can be achieved within 4 minutes under the optimum conditions.

Figure 3

Effect of initial CAF concentration on the removal of CAF.

Figure 3

Effect of initial CAF concentration on the removal of CAF.

Effects of the coexistence of dissolved organic matter and inorganic anions

Natural waters contain various inorganic anions and natural dissolved organic matter that influence the rates of free radical-induced reactions. The effects of humic acid (HA) and most common anions in surface waters on the removal efficiency were investigated. The experiments were conducted in the presence of 20 mg/L nitrate, chloride, and bicarbonate and 10 mg/L NaH. The initial concentration of CAF was 50 mg/L. The oxone and Co2+ concentrations were 1.00 mM and 0.10 mM, respectively. The initial pH value of the solution was 7.0. The results shown in Figure 4 indicated that the degradation of CAF was hardly inhibited by nitrate, but was evidently suppressed by HA, chloride, and bicarbonate.

Figure 4

Effects of coexistences of chloride, nitrate, bicarbonate and sodium humate on the removal of CAF.

Figure 4

Effects of coexistences of chloride, nitrate, bicarbonate and sodium humate on the removal of CAF.

HA, a macromolecular organic compound, can compete with CAF in consuming , thus showing a negative effect on CAF degradation. The removal ratio of CAF decreased from 100 to 91% with the addition of 10 mg/L NaH for 4 minutes.

The presence of the Cl ion reduced the removal of CAF. The removal efficiency decreased from 100 to 83% with the addition of 20 mg/L Cl ion for 4 minutes. The effect of the Cl ion on different AOPs has been studied previously (Ji et al. 2015). Previous studies have shown that the presence of Cl could significantly affect the radical-related reaction performance of different AOPs. The chemical mechanism and rate constants of the sulfate radical with chloride in the aqueous phase can be calculated using the following reactions (Ji et al. 2015): 
formula
4
 
formula
5
 
formula
6
 
formula
7
 
formula
8
 
formula
9
Thus, Cl can consume the radical in the process, which leads to the formation of less-reactive inorganic radicals, such as Cl and Cl2•−. As such, the inhibitory effect was observed.

The ion is a well-known •OH radical scavenger. can react with •OH to form , which is a more selective oxidant. In this study, the removal efficiency of CAF was only 81% with the addition of 20 mg/L ion. The ion showed a stronger inhibitory effect than the Cl ion. The reason may be that ion could react with radicals to form like it does with •OH radicals. The ion cannot be oxidized with or •OH radicals. So the degradation of CAF was hardly inhibited by nitrate.

Mineralization of CAF

The ability of this process to mineralize the CAF molecules was evaluated by determining TOC removal. Temporal changes in the TOC and concentration are shown in Figure 5. The results showed that, with the increase in the oxone and Co2+ concentrations, TOC removal increased slowly. Although 100% of CAF was degraded after a reaction time of 4 minutes, only 36% of TOC removal was observed. A comparison of the concentration and TOC curves clearly showed that, although CAF was almost fully consumed, its mineralization, leading to CO2, H2O, and NH3, was rather slow. Therefore, persistent organic intermediates, which are inefficiently oxidized as CAF, are likely generated during the process of CAF degradation.

Figure 5

CAF concentration (Ct/C0) and total organic carbon (TOCt/TOC0) for oxone/Co2+ aqueous degradation.

Figure 5

CAF concentration (Ct/C0) and total organic carbon (TOCt/TOC0) for oxone/Co2+ aqueous degradation.

CONCLUSIONS

  1. CAF in aqueous solution can be removed rapidly by peroxymonosulfate oxidant activated with cobalt ion.

  2. The removal efficiency increased with the increase in the concentrations of oxone and Co2+ ion added.

  3. Near-neutral condition (5.0 < pH < 7.0) is favorable for the removal of CAF.

  4. The additions of chloride, bicarbonate, and NaH have negative effects on the removal of CAF.

  5. Based on our experiments, 100% degradation of 50 mg/L CAF can be achieved within 4 minutes under the conditions of 1.00 mM oxone and 0.10 mM Co2+ ion at pH 5.0–7.0.

  6. Although CAF was almost fully consumed, its mineralization, leading to CO2, H2O, and NH3, was rather slow.

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