1. Introduction

Wastewater discharged from electroplating,pesticides,herbicides and tannery industries usually contain Ni²⁺,which can cause environmental problems and serious toxicological concerns. The current technologies used to remove Ni²⁺ from water/ wastewater include chemical precipitation ^[1],adsorption and membrane processes ^{[2, 3]}. Among these technologies,adsorption is themost commonly used approach to remove heavy metal ions from water/wastewater with high efficiency and easy operation. In the present work,a novel chelating resin (poly divinylbenzeneacrylate- hydroxamic acid: DVB-MA-HDA) is prepared for the removal of Ni²⁺ from water. Different from traditional chelating resin,DVB-MA-HDA can chelate Ni²⁺ by both surface modified function groups and the polymer skeleton,which might increase the number of adsorption sites effectively. SEM,BET surface area, pore structure,elemental analysis,FT-IR spectroscopy and XPS were used to characterize DVB-MA-HDA. Adsorption isotherm and kinetic curve are also performed to test DVB-MA-HDA adsorption capacity for Ni²⁺. 2. Experimental

All reagents were of analytical grade and all solutions were prepared with distilled water. The synthesis needed two steps as shown in Scheme 1. (1) The copolymerization of methyl acrylate (MA) and divinylbenzene (DVB) was carried out using the free radical polymerization method. 40 mL of MA,5 mL of DVB (80%) and 200 mL of distilled water were mixed in a three-necked flask. Then the combined initiator system (BPO,n-heptane and toluene) was added and the mixture was vigorously shaken for 10 min. The reaction was conducted for 10 h with stirring at 368 K. The products (DVB-MA) were filtered and washed with methanol and distilled water sequentially. (2) 10.0 g of DVB-MA and 5 mL of emulsifier were added into a 100 mL,3% (v/v) hydroxylamine solution,then the pH value was adjusted up to 12.0 using 10% (m/ m) sodium hydroxide solution. The mixture was stirred for 12 h at 323 K and the final product named as DVB-MA-HDA was filtered, washed with distilled water and dried at 313 K to constant weight. Then DVB-MA-HDA was characterized by SEM,BET,element analysis,FT-IR and XPS.

Adsorption isotherm and kinetic curves of Ni²⁺ on DVB-MAHDA were conducted according to the method reported in literature ^[4]. The concentration of Ni²⁺ (c_e) was determined using atomic adsorption spectrophotometer. Thus,q_e (mg/g),the adsorption capacity was calculated by Eq. (1):

where V is the volume of solution (L),Wis the weight of dry sponge (g),c₀ and c_e are the initial concentration and equilibrium concentration of Ni²⁺ in the solution,respectively.

After adsorption,DVB-MA-HDA was collected and gently washed with distilled water,then agitated with 25.0 mL of 0.1 mol/L EDTA solution. The desorption ratio was calculated from the amount of adsorbed Ni²⁺ and the final concentration of Ni²⁺ in desorption medium. This adsorption-desorption cycle was repeated five times using the same adsorbent. 3. Results and discussion

The scanning electron microscopy (SEM) photo of DVB-MAHDA is shown in Fig. 1,which indicates that the diameter of DVB-MA-HDA ranged from 0.1 mm to 0.3 mm. BET surface area of DVB-MA-HDA was 18.4 m²/g. Fig. 2 suggests that the pore distribution of DVB-MA-HDA is mainly meso-pore structure. Traditionally,the meso-pore of resin was considered beneficial for the adsorbates to enter the resin’s pore channels,which might improve the adsorption dynamics capacity.

As shown in Table 1,nitrogen content of DVB-MA-HDA increased after chemical modification,which indicated the existence of hydroxamic acid structure in DVB-MA-HDA. In Fig. 3a,DVB-MA-HDA had distinctive adsorption peaks compared with DVB-MA: the peak near 1572.8 cm^-1 is related to the ‘-N-H’ bond,and the peak near 3432.5 cm^-1 is caused by the ‘-O-H’ bond ^[5].

It was reported that hydroxy groups could chelate heavy metal ions in water ^[5]. As shown in Fig. 3b,the resonance of the ‘-O-H’ bond in DVB-MA-HDA shifts from 3432.5 cm^-1 to 3429.4 cm^-1 after the adsorption of Ni²⁺,which suggests that chelating interactions between the ‘-O-H’ and the Ni²⁺ occurred in the sorption process. Thus,the adsorption mechanism of the Ni²⁺ by DVB-MA-HDA could be inferred by the XPS spectrogram and shown in Fig. 4 ^{[5, 6]}. The peak at 855.8 eV indicates that the ‘Ni’ is chelated by an ‘O’ ^[7].

The isothermal equilibrium adsorption data are fitted by the Langmuir and Freundlich equations:

where q_e is the equilibrium adsorption capacity (mg/g),c_e is the equilibrium concentration (mg/L). q_m represents the monolayer saturation adsorption capacity of adsorbate and b is the relative constant. K_F and n are characteristic constants. The constants evaluated by applying the isotherm equation are shown in Fig. 5. Langmuir model is reliable because the correlation factor (R²) is greater than 0.99,and the monolayer saturation adsorption capacity for Ni²⁺ was 139.4 mg/g.

As shown in Fig. 6,adsorption kinetic curve indicated that DVBMA- HDA can quickly remove Ni²⁺ from water. More than 60% of the adsorption amount of Ni²⁺ at equilibrium occurred within the initial 20 min. The pseudo-first-order and pseudo-second-order equations were used to fit the kinetic adsorption data ^[4],which were expressed as:

where F = qt/qe,qt was the amount of Ni²⁺ removed (mg/g) at time t, qe was the equilibrium adsorption capacity (mg/g),K₁ (min^-1) and K₂ (g/mg min) were the rate constants of first-order and pseudosecond- order adsorption. The kinetic parameters for adsorption of Ni²⁺ on DVB-MA-HDA were shown in Fig. 6. Based on the correlation coefficients (R²),the pseudo-second-order equation could fit the experimental kinetic data better,suggesting an intraparticle diffusion process as the rate-limiting step of the adsorption in solution ^[8].

0.1 mol/L EDTA solution could desorb Ni²⁺ from DVB-MA-HDA effectively. Even after five cycles,the desorption rate of Ni²⁺ could still maintain at more than 95% level without any obvious decrease, which indicated a good sorption-desorption capacity of DVB-MAHDA for Ni²⁺. 4. Conclusion

A novel chelating resin (DVB-MA-HDA) for the removal of Ni²⁺ from water was successfully prepared by copolymerization of methyl acrylate and divinylbenzene,and modified with hydroxylamine groups. Langmuir equation could describe the adsorption isotherm well,which suggested a mono-layer adsorption of Ni²⁺ on DVB-MA-HDA. The monolayer saturation adsorption capacity for Ni²⁺ was 139.4 mg/g. The adsorption kinetic curves of Ni²⁺ could be fitted by the pseudo-second-order equation well, suggesting an intra-particle diffusion process as the rate-limiting step of the adsorption.More than 60% of the adsorption amount of Ni²⁺ at equilibrium occurred within the initial 20 min,which demonstrated DVB-MA-HDA had a good dynamics performance. 0.1 mol/L EDTA solution could desorb Ni²⁺ from DVB-MA-HDA effectively,which indicated a good sorption-desorption capacity of DVB-MA-HDA for Ni²⁺. Acknowledgments

We acknowledge generous support provided by the National Science Funds for Young Scientists (No. 51008152),Jiangsu Province Science Funds for the Integration of Production-Study- Research (No. BY2012154) and Jiangsu Province Environmental Production Research Funds (No. 2012006).