Simultaneous removal of atrazine and copper using polyacrylic acid-functionalized magnetic ordered mesoporous carbon from water: adsorption mechanism.

Highly efficient simultaneous removal of atrazine and Cu(II) was accomplished using synthesized polyacrylic acid-functionalized magnetic ordered mesoporous carbon (P-MMC) as compared to magnetic ordered mesoporous carbon (MMC) and ordered mesoporous carbon (OMC). The mutual effects and interactive mechanism of their adsorption onto P-MMC were investigated systematically by binary, preloading and thermodynamic adsorption procedures. In both binary and preloading systems, the adsorption of atrazine was inhibited to some extent by the presence of Cu(II) because of selective recognition and direct competition, but the presence of atrazine had negligible effect on Cu(II) desorption. With the coexistence of humic acid (0-20 mg L-1), both atrazine and Cu(II) sorption increased slightly in sole and binary systems. With the concentration of coexisting NaCl increasing from 0 to 100 mM, the adsorption capacity for Cu(II) slightly decreased, but as for atrazine adsorption, it decreased at first, and then increased slightly in sole and binary systems. P-MMC was applied to treat real environmental samples, and the sorption capacities for atrazine and Cu(II) in real samples were all more than 91.47% and 96.43% of those in lab ultrapure water, respectively. Finally, comprehensively considering the relatively good renewability and the superior behavior in the application to real water samples, P-MMC has potential in removal of atrazine, Cu(II) and possibly other persistent organic pollutants from wastewater.

Recalcitrant organic lass="Chemical">comlass="Chemical">pounds and heavy lass="Chemical">pan class="Chemical">metals are common in the environment, and give rise to a serious toxicological threat to the ecosystem and human health. For example, atrazine (2-chloro-4-ethylamine-6-isopropylamino-1,3,5-triazine), a model selective triazine herbicide used in agriculture industries, leading to great ecological and human health concern because of its widespread application and persistence123. Moreover, due to the interference of atrazine endocrine hormone metabolism, some studies suggested that atrazine was considered as a potential carcinogen45. Heavy metals (e.g., copper, lead and cadmium) have been mainly used in agricultural and metal plating/coating industries67. Copper is reported as widespread heavy metals in wastewater and natural environment. Although copper is an essential micronutrient, it can cause copper poisoning at high concentration in humans such as severe headaches, hair loss, hypoglycemia, kidney damage, gastrointestinal problems and even death89.

Additionally, recalcitrant organic lass="Chemical">comlass="Chemical">pounds and heavy lass="Chemical">pan class="Chemical">metals commonly coexist in real environments such as agriculture production, bio-treated effluent and dye wastewater, giving rise to more serious damage to the ecosystem and humans due to their combined toxicity and relative mobility1011. Consequently, finding effective ways to control these combined pollutants has aroused continuous concern recently.

The traditional methods such as advanced oxidation, chemical precipitation, biological treatment and membrane treatment have been extensively applied to remove recalcitrant organic lass="Chemical">comlass="Chemical">pounds and/or heavy lass="Chemical">pan class="Chemical">metals with a positive effect12. However, these methods suffer from disadvantages such as high cost, low efficiency, and secondary pollution1314. Adsorption, as a green technology, has been widely utilized for the successful removal of recalcitrant organic compounds and/or heavy metals in recent years1516171819. Various adsorbents such as polymeric resins, soil, biosorbents and chitosan have been applied2021. For examples, Tao et al.22 used the natural sediment to remove atrazine and dye compounds simultaneously, and investigated the competitive adsorption between atrazine and dye compounds. Krajnc et al.23 and Xu et al.21 applied different polymeric resins to adsorb atrazine with high efficiency, and clarified its removal mechanism; Zhang and co-workers found soil, river and marine sediments could be used to remove tetracycline, and the presence of Cu(II) facilitated tetracycline adsorption on soils and sediments at low pH24.

Moreover, searching for new simple, regenerated recycled and inexpensive methods to lass="Chemical">control recalcitrant organic lass="Chemical">pan class="Chemical">compounds and heavy metals is also of considerable interest. Alternatively, carbon-based materials (e.g., carbon nanotubes, graphene, biochar and mesoporous carbon) with their controlled pore size distribution, high surface area to volume ratio and manipulatable surface chemistry, overcome many of these intrinsic limitations. Sorption studies using carbon-based nanomaterials was reported as high adsorption capacity, effectiveness and rapid equilibrium rates for the removal of recalcitrant organic compounds and heavy metals25. For example, Chen and co-workers illuminated the effect of copper on the sorption of 2,4,6-trichlorophenol and vice versa on multiwalled carbon nanotubes (MWCNTs)26. Cao and co-workers evaluated the ability of dairy-manure derived biochar to sorb lead and atrazine. In addition, ordered mesoporous carbons (OMCs) one kind of carbon-based materials, with large surface area and pore volume, unique pore size, and excellent physicochemical and thermal stability are promising candidates in pollutant removal27.

In our recent report, we synthesized lass="Chemical">cobalt nanolass="Chemical">particles-embedded magnetic ordered mesolass="Chemical">porous lass="Chemical">pan class="Chemical">carbon282930 and nitrogen/phosphorus-functionalized magnetic ordered mesoporous carbon with large specific surface area, and sufficient pore regions for the effective removal of recalcitrant organic compounds and heavy metals from wastewater3132333435. These works demonstrated that transition metal elements such as Co and Fe282930 or nonmetal atoms of P and N3134 can be used as adsorbent to accomplish primary amorphous ordered mesoporous carbon (OMC) graphitization, strengthening their excellent acid-base stability and hydrophilic properties. For example, the introduction of magnetic nanoparticle of Fe can greatly improve their practical application in separation and reutilization28, in addition, the introduction of nonmetal heteroatom of N element, and functionalized polyethylenimine (PEI) or polyacrylic acid (PAA) to equip ordered mesoporous carbon with amino (-NH and -NH2) groups, and oxygen containing groups (-OH and -COOH)353637, boosting the chelation with recalcitrant organic compounds and heavy metals.

Based on the above analysis, the purpose of this research was 1) to investigate the removal efficiency of lass="Chemical">atrazine and/or lass="Chemical">pan class="Chemical">Cu(II) by polyacrylic acid (PAA)-functionalized magnetic ordered mesoporous carbon (P-MMC), 2) to investigate the mutual effects and inner mechanisms of their adsorption onto P-MMC by sole, preloaded and binary systems, 3) to investigate the influences of water quality parameters on the removal performance of P-MMC including pH value, ionic strength and humic acid.

Results and Discussion

Comparisons of adsorption properties for Cu(II) and atrazine onto different mesoporous carbon material. Sole systems for atrazine

In order to investigate the eflass="Chemical">fect of magnetic and grafting lass="Chemical">pan class="Chemical">polyacrylic acid for the removal of atrazine and Cu(II), three mesoporous carbon materials including OMC, MMC and P-MMC were used to compare their adsorption capacities. Freundlich and Langmuir models were both used to describe isotherm data of atrazine. The equations and related parameters of Freundlich and Langmuir models are listed in Text S-2, and the results was presented in Fig. 1 and Table S-1. It was observed that the adsorption capacities for atrazine were in the following order: P-MMC > MMC > OMC. P-MMC exhibited the highest adsorption capacity, because of a larger number of carboxyl and hydroxyl groups in P-MMC may offer additional affinity and more available binding sites for atrazine through hydrogen bonding. The Freundlich model with higher correlation coefficients (R2) and lower values of the sum of the absolute errors (EABS) could better describe the adsorption isotherms for atrazine than the Langmuir model (Fig. 1), suggesting that pores and some heterogeneity on the surfaces of P-MMC played an important role in atrazine adsorption. The results were consistent with the previous works where the Freundlich isotherm was more suitable than the Langmuir isotherm for the adsorption of atrazine on various adsorbents (Table S-2), such as humic acid and silica gel mixtures38, humic acids coated nanoparticles39 and magnetic multi-walled carbon nanotube40.

Figure 1

The adsorption isotherms of atrazine by P-MMC, MMC and OMC (initial pH of 5.0, temperature at 303 K).

In addition, value of 1/n < 1.0 represents an advantageous adsorption lass="Chemical">condition. Therefore, the Freundlich exlass="Chemical">ponent 1/n gave an indication of the favorability of lass="Chemical">pan class="Chemical">atrazine adsorption by P-MMC, MMC and OMC. However, according to the KF values, P-MMC had the highest adsorption capacity toward atrazine among all tested mesoporous carbon materials.

Sole systems for Cu(II)

The adsorption isotherms for lass="Chemical">Cu(II) of these materials are shown in Fig. 2 and Table S-1. It was observed that the adsorlass="Chemical">ption calass="Chemical">pacity was in the following order: lass="Chemical">pan class="Chemical">P-MMC > MMC > OMC, P-MMC exhibited the highest adsorption capacity. As shown in Fig. 2, the Langmuir model with higher correlation coefficients and lower values of the average relative error (ARE), gave a better fit to the equilibrium data than the Freundlich model, suggesting that the existence of homogeneous active sites within the adsorbent and the monolayer adsorption of Cu(II) on P-MMC313741. Moreover, P-MMC showed the highest maximum adsorption capacity mainly due to the highest chelating affinity of oxygen containing groups to Cu(II), as reported previously3142434445. In addition, it is noted that P-MMC showed a relatively high sorption capacities for Cu(II) and/or atrazine in comparison with other adsorbents (Table S-3).

Figure 2

The adsorption isotherms of Cu(II) by P-MMC, MMC and OMC (initial pH of 5.0, temperature at 303 K).

Binary systems

Figure 3 presented the adsorption amounts for lass="Chemical">atrazine and lass="Chemical">pan class="Chemical">Cu(II) onto the adsorbents of mesoporous carbon materials from both single and double systems. The distribution coefficients (Kd) are mass weighted partition coefficients of targeted pollutant between the solid and solution phases35, the equation and related parameters are listed in Text S-2, Supporting Information. It could be clearly seen that the sorption distribution coefficients (log Kd) of P-MMC was larger than others whether in a single or double systems. Consequently, the superior properties of P-MMC for adsorption of both atrazine and Cu(II) demonstrate the potential for successful simultaneous removal of typical Cu(II) and atrazine, which should be explored further to develop an efficient method.

Figure 3

Adsorption amounts for atrazine and Cu(II) onto P-MMC, MMC, and OMC from both single and double systems (initial concentrations of 15 mg·L−1 and 30 mg·L−1 for atrazine and Cu(II), initial pH of 5.0, temperature at 303 K).

Mutual effects upon the adsorption of both atrazine and Cu(II) onto P-MMC. Adsorption isotherms

Figure 4A and Fig. S-4A presented the adsorption isotherms for lass="Chemical">atrazine with or without lass="Chemical">pan class="Chemical">Cu(II). The amounts of adsorbed atrazine onto P-MMC decreased with the increasing of Cu(II) initial concentration from 0 to 70 mg·L−1. As shown in Table S-4, the Freundlich equation fits the experimental data well, indicating that some heterogeneity on the pores or surfaces of P-MMC played an important role in the adsorption of atrazine, and different sites with several adsorption energies were involved4046. The Freundlich constant (KF) of atrazine declined obviously in the presence of Cu(II) with 30 mg·L−1 and 70 mg·L−1, suggested that Cu(II) had a suppression effect on the atrazine sorption. The suppression impact might be attributed to the direct competition of both adsorbates for similar sites such as hydroxyl and hydroxyl group.

Figure 4

Mutual effect of atrazine and Cu(II) adsorption onto P-MMC; (A) Freundlich isotherms for atrazine with interaction to Cu(II) of 0, 30 and 70 mg·L−1 tested at 303 K, at an initial pH of 5.0; (B) Langmuir isotherms for Cu(II) with interaction to atrazine of 0, 15 and 30 mg·L−1 tested at 303 K, at an initial pH of 5.0.

In addition, the adsorption isotherms for lass="Chemical">Cu(II) from sole and binary systems by lass="Chemical">pan class="Chemical">P-MMC are compared in Fig. 4B and Fig. S-4B, and the obtained characteristic parameters are tabulated in Table S-4, Supporting Information. The Langmuir model gave a better fit than the Freundlich model. And the impact of atrazine on Cu(II) desorption was almost negligible, suggested that Cu(II) showed a higher affinity toward P-MMC than atrazine.

Thermodynamic investigations

The thermodynamic equation and parameters are listed in Text-2, Supporting Information. The vant’s Hoff plot (Fig. S-5) is plotted as ln Kd versus 1/T to callass="Chemical">culate ΔH° and ΔS°. The thermodynamic lass="Chemical">parameters for lass="Chemical">pan class="Chemical">atrazine and Cu(II) are summarized in Table S-5, Supporting Information. The values of ΔH° for atrazine were all negative, confirming that the adsorption of atrazine onto P-MMC was exothermic47. All the ΔH° values were less than 40 kJ mol−1 thereby implying that the sorption of atrazine onto P-MMC is mainly a physisorption process4849. The positive values of ΔS° implied an increased randomness at the solid−solution interface50. The value of ΔG° is negative, indicating the spontaneous nature of the adsorption of atrazine onto P-MMC.

On the lass="Chemical">contrary, all the values of ΔH° for lass="Chemical">pan class="Chemical">Cu(II) were positive, indicating that the adsorption of Cu(II) onto P-MMC is endothermic and driven by chemical reaction10. And the positive ΔS° suggested that increased randomness at the solid solution interface occurred in the internal structure of the uptake of Cu(II) onto P-MMC. Similarly, the negative value of ΔG° could suggest the feasibility and spontaneity toward Cu(II) adsorption.

Preloading identification

To further identify the interaction mechanisms behind the lass="Chemical">co-adsorlass="Chemical">ption of lass="Chemical">pan class="Chemical">Cu(II) and atrazine onto P-MMC, a series of preloading equilibrium experiments were also conducted. The sequential adsorption procedures were performed, and the results are shown in Fig. 5. As seen in Fig. 5A, in the simultaneous adsorption studies, the higher Cu(II) concentration, resulting in the smaller values of the sorption distribution coefficients (log K) and adsorbed atrazine, which suggested that Cu(II) had a suppression effect on the atrazine sorption. In addition, when atrazine was preloaded on P-MMC, the values of the sorption distribution coefficients (log K) and adsorbed atrazine also decreased with the increase of the initial concentration of Cu(II), suggesting that Cu(II) could compete with atrazine for the same adsorption sites (Fig. 6), and atrazine could be readily displaced by Cu(II), which possessed a stronger binding affinity and accordingly suppressed atrazine adsorption.

Figure 5

(A) The effect of the initial concentration of Cu(II) on atrazine adsorption in preloading and simultaneous adsorption studies. (B) The effect of the initial concentration of atrazine on Cu(II) adsorption in preloading and simultaneous adsorption studies (The fixed concentration of atrazine and Cu(II) was 15 mg·L−1 and mg·L−1, respectively).

Figure 6

The proposed mechanisms for atrazine and Cu(II) sorption.

Besides, the eflass="Chemical">fects of lass="Chemical">pan class="Chemical">co-adsorption experiments by fixing Cu(II) concentration but with different concentrations of atrazine, and by preloading P-MMC with Cu(II) followed by exposure to different concentrations of atrazine solution were also investigated, and the results are presented in Fig. 5B. In the co-adsorption studies, varying atrazine concentrations had a relatively smaller suppression effect on the sorption of Cu(II) due to the (log K) and adsorbed Cu(II) show little change. And in the Cu(II)-preloading experiments, the changes of (log K) and adsorbed Cu(II) are small, suggesting little Cu(II) were desorbed and a stronger binding affinity of Cu(II) on to P-MMC. Actually, the inhibited sorption of recalcitrant organic compounds by heavy metal was previously reported, which was generally attributed to the competition for binding sites. A typical example is wheat ash reported by Wang and co-workers51, which was used to investigate effects of heavy metal on the sorption of 2,4,6-trichlorophenol (TCP), and an obvious suppression of the adsorption of 2,4,6-trichlorophenol was observed by copper and lead, due primarily to competition for adsorption sites [the complexation of Cu(II) and Pb(II) was likely via carboxylic, hydroxylic and phenolic groups of ash, and these same functional groups also reacted with TCP during sorption]. Similar results involving Cu(II) and p-Nitrophenol were also obtained due primarily to competition for adsorption sites52. In view of the foregoing, site competition possibly played a primary role in the co-adsorption and preloading adsorption.

Proposed mechanisms of direct competition on P-MMC

As mentioned above, a possible reaction mechanism for lass="Chemical">Cu(II) and lass="Chemical">pan class="Chemical">atrazine adsorption onto P-MMC is proposed in Fig. 6. In single system, Cu(II) may be removed in two ways: complexation with carboxyl and hydroxyl groups of P-MMC as well as direct adsorption onto the pores of ordered mesoporous carbon and iron nanoparticles. And atrazine is also removed by adsorption on the surface of ordered mesoporous carbons as well as the hydrophilic sites of oxygen containing groups (-OH and -COOH) (Hydrogen bond). Therefore, site recognition and competition could occur for the simultaneous removal of atrazine and copper using P-MMC. As seen in binary system, a portion of atrazine on carboxyl and hydroxyl groups could be displaced by Cu(II) in the sequential experiment, because Cu(II) exerts stronger binding affinity than atrazine4053. In addition, there existed neither electrostatic repulsion nor electrostatic interaction between atrazine and P-MMC at pH 5.0, indicating that the suppression of Cu(II) for atrazine sorption might be attributed to the direct competition of both adsorbates for similar sites such as hydroxyl and hydroxyl group, not the change of surface charges. Thus, the adsorption of atrazine was suppressed in the binary systems due to the occupation of hydrophilic sites by Cu(II).

In addition, it was initially expected that chemi-lass="Chemical">comlass="Chemical">plexation of lass="Chemical">pan class="Chemical">Cu(II) would neutralize the P-MMC surface charge, enhancing the hydrophobic effect, thus increasing the sorption of hydrophobic atrazine. However, in the preloading system, the sorption of atrazine was actually decreased in the presence of Cu(II). This was probably because chem-complexation did not decrease the competitive sorption from water molecules a bit53. Two mechanism may therefore be responsible for the diminishing sorption of atrazine in the presence of Cu(II). On the one hand, inner-sphere and outer-sphere complexes are formed through carboxyl and hydroxyl groups and hydration, which occupy part of the surface of P-MMC. On the other hand, the complexed Cu(II) are likely to host one or more hydration shells of dense water4054, and these large metal cation hydration shells may intrude or shield the P-MMC hydrophobic and hydrophilic sites (carboxyl and hydroxyl groups), and thus compete with atrazine for surface sites indirectly, resulting in the suppression of atrazine adsorption around the Cu(II)-complexed moieties.

Influence of ionic strength and humic acid

The presence of lass="Chemical">salts and lass="Chemical">pan class="Chemical">humic acid in atrazine and Cu(II) wastewater may affect the atrazine and Cu(II) sorption. Therefore, the influence of salt ionic and humic acid strength on the removal of atrazine and Cu(II) by P-MMC was studied. As seen in Fig. 7A, the adsorption capacities for Cu(II) slightly decreased with the increase of NaCl concentration in sole and binary systems, which can be well explained that the Cu(II) would shape electrical double-layer complexes with P-MMC, and thus the adsorption would be hindered as the strong-electrolyte NaCl concentration increased. Similar results involving Pb(II) and mesoporous carbon material was reported in our previous work31. Moreover, the chemical surface complexation between Cu(II) and carboxyl, hydroxyl groups occurred and also conduced to the removal of Cu(II). Therefore, the small negative effects of ionic strength on Cu(II) sorption were not significant.

Figure 7

Effects of ionic strength (A) and humic acid (B) on sorption distribution coefficient (Log Kd) for duplicate points sorption of atrazine and Cu(II) on P-MMC. (initial concentrations of 15 mg·L and 30 mg·L−1 for atrazine and Cu(II), initial pH of 5.0, temperature at 303 K).

However, it is diflass="Chemical">ferent for lass="Chemical">pan class="Chemical">atrazine adsorption. Results depicted in Fig. 7A showed an increase in NaCl concentration decreased atrazine uptake firstly, and then increased slightly in sole and binary systems. The possible reason was that the water cluster was formed on the surface of P-MMC by hydrogen bonding with H2O molecule with lower NaCl concentration, which could interface and decreased the hydrophobic atrazine uptake. The water cluster would be destroyed at higher NaCl concentration, and it consequently increased atrazine adsorption slightly. Thus, a smaller negative effect occurred on the adsorption amount of atrazine with the lower NaCl concentration.

Relatively strong sorption of lass="Chemical">humic acid on lass="Chemical">pan class="Chemical">P-MMC was reported, and the effect of humic acid on the Cu(II) and atrazine sorption on P-MMC was displayed in Fig. 7B. Changing the humic acid concentration from 1 to 20 mg L−1, sorption of Cu(II) increased slightly in sole and binary systems. The small positive effect on the adsorption amount of Cu(II) could be explained by the hydrophilic fractions of humic acid with various functional groups, such as carboxyl, phenol, hydroxyl, amine and quinine groups which can bind heavy metals55565758.

Similar phenomenon was found for lass="Chemical">atrazine ulass="Chemical">ptake. Changing the lass="Chemical">pan class="Chemical">humic acid concentration from 1 to 20 mg L−1, sorption of atrazine increased slightly. The positive effect caused by humic acid on atrazine may be explained by that the common functional groups such as carboxyl, phenol, hydroxyl, amine and quinine groups from humic acid have strong bond ability with atrazine by hydrogen bond. Similar results involving Cu(II) and p-Nitrophenol were also obtained due primarily to competition for adsorption sites10.

Regeneration and leaching experiments

The regeneration of lass="Chemical">P-MMC was lass="Chemical">pan class="Chemical">conducted by adding Cu(II)/atrazine-loaded P-MMC solids to 10 mL 20% acidic ethanol solution, and the mixture was stirred at 25 °C and 150 rpm for 24 h. The results were showed in Fig. S-6, Supporting Information, after five sorption/desorption cycles, the adsorption efficiency remained at high level above 81% for Cu(II) and 89% for atrazine, indicating the relatively excellent adsorption stability of P-MMC. Hence, in consideration of its regeneration abilities and magnetic separation, P-MMC improved the potential for the removal of organics and heavy metals in more complex environments by in-situ regeneration and reuse. In addition, as shown in Fig. S-7, the leaching of Fe from P-MMC was slightly less than MMC, and the leaching of Fe from P-MMC in regeneration process was not serious (Fig. S-8), which may be ascribed to the surface modification of polyacrylic acid.

Application in real water samples

lass="Chemical">P-MMC was alass="Chemical">plass="Chemical">plied to treat real lass="Chemical">pan class="Species">environmental samples including tap water, river water and landfill leachate to investigate its practical application, and the results were shown in Fig. 8. It was observed that the sorption capacity for Cu(II) in tap water and landfill leachate were less than that in lab ultrapure water. It may be that there existed many common cations [e.g., Na+, Ca2+, Mg2+, and K+] in these water samples. These cations not only occupied the adsorption sites of P-MMC, but also impacted ionic strength, aggrandizing the contact difficulty between Cu(II) and P-MMC. In addition, the Cu(II) uptake amount in landfill leachate was a little higher than tap water, due to plentiful organic matters in landfill leachate, such as humic acid and fulvic acid, and thus improved Cu(II) adsorption slightly. Moreover, the Cu(II) uptake amount in river water was the highest, due to higher pH was favorable to Cu(II) adsorption by precipitation and electrostatic attraction265960.

Figure 8

The practical application of P-MMC to real samples (initial concentrations of 15 mg·L−1 and 30 mg·L−1 for atrazine and Cu(II), initial pH of 5.0, temperature at 303 K).

As for lass="Chemical">atrazine, it was observed that the sorlass="Chemical">ption calass="Chemical">pacity in talass="Chemical">p lass="Chemical">pan class="Chemical">water, river water and landfill leachate were less than that in lab ultrapure water. It might be attributed to the coexisting cations [e.g., Pb2+ Mg2+, and Cd2+] in real water samples, which may compete with atrazine for the same adsorption sites. The atrazine uptake amount in landfill leachate was a little higher than tap water and river water, due to plentiful organic matters in landfill leachate, such as humic acid and fulvic acid, and thus improved atrazine adsorption slightly. The sorption capacities for atrazine and Cu(II) in real samples were all more than 91.47% and 96.43% of those in lab ultrapure water, respectively (Fig. 8). These results indicated that P-MMC exhibited relatively superior behavior in removing atrazine and Cu(II) in real samples.

Conclusions

Ordered mesoporous lass="Chemical">carbon lass="Chemical">pan class="Chemical">combining properties of magnetism and polyacrylic acid were synthesized for the simultaneous removal of Cu(II) and atrazine. P-MMC contained both hydrophilic and hydrophobic sites and exhibited relative excellent adsorption of Cu(II) and atrazine. The results showed that site recognition and site competition were the predominant adsorption mechanisms. Humic acid and ionic strength exerted some positive influence on the removal of atrazine and Cu(II). Besides, the regeneration studies revealed that P-MMC could be effectively recovered with acidic ethanol. The superior properties demonstrated by P-MMC indicated that it could be applied to wastewaters containing both Cu(II) and atrazine.

Methods

Preparation of P-MMC and characterization of materials

The mesostructured lass="Chemical">SBA-15 lass="Chemical">pan class="Chemical">silica template was synthesized as described previously in our laboratory316162. Magnetic ordered mesoporous nanocomposite (MMC) was synthesized by following a co-impregnation method with slight alterations63. Details about the preparation of MMC can be seen in Text S-1, Supporting Information (SI). Besides, P-MMC was synthesized as described previously in our laboratory37. The structural information for P-MMC, including transmission electron microscopy (TEM) images are shown in Fig. S-1, and the detail description can be seen in Supporting Information. Furthermore, our previous work showed that the P-MMC was enriched in carboxyl and hydroxyl group37. Thus, these groups and numerous pores on the outside of the OMC implying that P-MMC could contain both hydrophilic and hydrophobic sites3137.

Batch adsorption studies

The equilibrium experiments were performed in a 50 mL stoppered lass="Chemical">conical flasks in a lass="Chemical">pan class="Chemical">water bath shaker by mixing 5 mg of adsorbent and 10 mL of solution containing various concentrations of atrazine and Cu(II). The concentration range of atrazine and Cu(II) was 1–30 mg·L−1 and 10–150 mg·L−1, respectively. The pH of the solution was adjusted to 5.0 with HCl or NaOH, which was determined in the preliminary experiments (Fig. S-2 and Fig. S-3). The flasks were completely sealed and agitated in an incubator shaker at 303 K and 150 rpm for 24 h. Then, the concentrations of atrazine and Cu(II) in the residual aqueous phase were determined by HPLC (Agilent 1100, USA) equipped with an UV-vis photodiode array detector and a Perkin-Elmer Analyst 700 atomic absorption spectrophotometer (AAS, Perkin-Elmer, USA), respectively. Experiments were carried out in duplicate, and the arithmetic mean values were calculated, with the standard deviations less than 5%.

In the preloading adsorption experiments, fresh lass="Chemical">P-MMC was first equilibrated with 1–30 mg·L−1 of lass="Chemical">pan class="Chemical">atrazine or 10–150 mg·L−1 of Cu(II), and then the preloaded P-MMC was conversely fed by Cu(II) or atrazine at various concentrations to obtain a second quilibrium.

Stability and regeneration studies

To carry out the leaching experiment, 5 mg lass="Chemical">MMC or lass="Chemical">pan class="Chemical">P-MMC was dispersed in 10 mL of Cu(II) solutions with different pH. At the specific contact time, the suspension was separated and the leached iron concentration in the supernatant was determined by a flame atomic absorption spectrometer (FAAS, PerkinElmer AA700, USA). The feasibility of regenerating P-MMC for repeated use was investigated by using acidic ethanol. Specifically, after accomplishment of the adsorption experiments, the Cu(II)-loaded (or atrazine-loaded) adsorbent was magnetically separated and subsequently added into 10 mL 20% acidic ethanol for desorption at 150 rpm at 303 K. After washing thoroughly with ultrapure water to neutral pH, the regenerated adsorbent was recycled and reused, and the adsorption performance of P-MMC was investigated again, meanwhile, leached iron concentration in the supernatant was also determined.

Application of P-MMC to real water samples

Tap lass="Chemical">water (from Changsha No. 2 Drinking lass="Chemical">pan class="Chemical">Water Company), river water (from Xiangjiang, Hunan), and landfill leachate (from a municipal solid waste landfill in Changsha, China) samples were used as real environmental sample in this study to test the efficiency of P-MMC. Specifically, 5 mg P-MMC was added into different initial concentrations of Cu(II) or atrazine solution for adsorption test at 303 K and 150 rpm for 24 h. Then, the concentrations of atrazine and Cu(II) in the residual aqueous phase were determined.

Goodness-of-fit measure (GoFM)

The model parameters for Freundlich and Langmuir isotherm have been determined by minimizing the difpan class="Chemical">ference lass="Chemical">pan class="Gene">between the experimental and modeled Qe values (through the sum of the absolute errors (EABS) and the average relative error (ARE)) using the iterative method64.

The sum of the absolute errors (EABS) model:

The average relative error (ARE)

“exp” and “calc” show the experimental and calpan class="Chemical">culated values.

Additional Information

How to cite this article: Zhou, Y. et al. Simultaneous removal of lass="Chemical">atrazine and lass="Chemical">pan class="Chemical">copper using polyacrylic acid-functionalized magnetic ordered mesoporous carbon from water: adsorption mechanism. Sci. Rep. 7, 43831; doi: 10.1038/srep43831 (2017).

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