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——题记Chemical modi?cation of silica-gel with diethylenetriamine via an end-group protection approach for adsorption to Hg(II)

ABSTRACT

Four kinds of silica-gel supported diethylenetriamine adsorbents with different structures, were prepared by so-called ‘heterogeneous-direct-amination’’(hetero-DA), ‘homogeneous-

direct-amina tion’’ (homo-DA), ‘‘heterogeneous end-group protection’’ (hetero-EGP), and ‘‘homogeneous end-group pr otection’’ (homo-EGP) methods, respectively. The obtained products were named SG-HE-dD, SG-HO-dD, SG-HE-pD and SG-HO-pD, respectively (where SG means silica-gel; HE means heterogeneous, HO means homogeneous, d means direct, p means protected and D means diethylenetriamine). Their structures were characterized by FT-IR, elemental analysis, porous structure analysis and thermogravimetric analysis. The adsorption capabilities of such adsorbents towards Hg(II) were studied and evaluatedby static method.SG-HE-pD and SG-HO-pD showed higher performance towards Hg(II) adsorption than corresponding counterparts SG-HE-dD and SG-HO-dD, even though the former two possessed lower contents of diethylenetriamine (DETA). The kinetics data indicated that the adsorption proce ss was governed by the ?lm diffusion and followed pseudo-?rst-order rate model for SG-HE-dD,SG-HO-dD and SG-HE-pD and pseudo-second-order model for SG-HO-pD.The Langmuir model was applied to ?t the experimental equilibrium data for all adsorbents.The end-group protection method exhibited its advantage in preparation of effective adsorbent for metal ions uptake compared to the direct-amination method.

Keywords:

Chemical modi?cation of silica-gel

Diethylenetriamine

End-group protection

Adsorption

Hg(II)

1. Introduction

As a result of heavy metal ion release from industrial wastewater, water pollution has become a serious problem. Waste streams contain solutions of metal ions, such as mercury, nickel,copper, lead, and so on. In particular, mercury compounds which are harmful to human health and aquatic life must be removed from sewage down to extremely low concentrations. Many literatures have discussed the removal of metal ions from aqueous solutions using approaches based upon adsorption, solvent extraction and polymeric membranes [1–4]. Recently, researches have reported the application of organic molecule-functionalized mesoporous silica-gel for the removal of metal ions from aqueous solution [5–7]. In this method, the organic molecule anchored on the surface of silica-gel relied on coordination bonding to the metals. Thus the approach featured advantages over those used in other methods because of its good selectivity for metal ions and free operation. Moreover, silica-gel was used as an matrix, not only due to the facility of immobilization process, but also to be relatively simple in comparison to organic polymers, with fastermetal ion-exchange kinetics, good swelling resistance in different solvents, and chemical stability [8]. It followed that a similar polyamine approach could be applied to metal ion-containing aqueous solutions via chelation on amine groups using silica-gel modi?ed with polyamines [9–10].

Anchoring an organic polyamine on the silica-gel surface via covalent bond is the most reliable method for preparing silica-gel-based adsorbents [11,12]. The direct heterogeneous synthesis and homogeneous synthesis have often been used for these purposes[13–15].

2. Experimental

2.1. Materials and methods

Silica-gel obtained from Tenghai Chemical Institute, Qingdao,Shandong, China, with particle size in the range 60–100 mesh was treated and activated as before [17]. The silylant agent 3-glycidoxypropyltrimethoxysilane (GPTS, Qufu Wanda Chemicals Factory, China) and salicylaldehyde (Sal, Shanghai Chemical Factory, China) were used without further puri?cation; diethyle-netriamine (DETA) and toluene were distilled under reduced pressure just before use. Solvents used were of analytical reagent grade. Stock solutions of Hg(II) (0.1 mol dmà3) were prepared as follows: analytical reagent grade Hg(NO3)2áH2O was dissolving in 20 cm3 (0.5 mol dmà3) of HNO3 to avoid hydrolysis, then diluted to 1000 cm3. The buffer solution of pH 1.0–2.0 was adjusted to a desired value with HNO3/H2O and pH 3.0–6.0 was adjusted with sodium acetate/acetic acid solutions. 2.2. Preparation of SG-HE-dD, SG-HO-dD, SG-HE-pD and SG-HO-pD

The preparation of SG-HE-dD, SG-HO-dD, SG-HE-pD and SG-HO-pD (where SG means silica-gel; HE means heterogeneous, HO means homogeneous, d means direct, p means protected and D means diethylenetriamine) was according to the approach described in Ref. [21]. The typical procedures as follows.

Hetero-DA method: 20.0 g of silica-gel reactivated with 20 cm3(92.0 mmol) of GPTS in toluene, under re?ux of the solvent in dry nitrogen. After ?ltering and washing, the solid (SG-G) was dried.Then SG-G (10.0 g) was reacted with DETA (26.1 cm3, 240.1 mmol) and the solid was ?ltered, washed and dried, to obtain the product,SG-HE-dD.

Homo-DA method: GPTS (10 cm3, 46.0 mmol) reacted with DETA(10 cm3, 92.0 mmol) in methanol (100 cm3). The mixture was re?uxed under a nitrogen atmosphere for 12 h and the solution was concentrated to approximately 60 cm3. The resulting compound GD reacted with 10.0 g of silica-gel suspended in 100 cm3 of dry toluene. The ?nal solid was ?ltered, extracted and dried, to obtain the product, SG-HO-dD. Hetero-pD method: The Schiff base, diethylenetriamine bissalicylaldehyde (DETA-bS), was ?rst produced by the method described in the literature [22]. DETA-bS (30.0 g) reacted with 10.0 g of SG in methanol. After re?uxing, the solid was ?ltered,extracted and dried, to obtain the compound, SG-HE-DS. The salicylaldehyde was liberated during hydrolysis and the product SG-HE-pD was obtained as reported [19]. The product SG-HE-DS(6.3 g) was stirred with 100 cm3 of HCl (6 mol dmà3) at 333 K for 24 h and salicylaldehyde was liberated as yellow oil during the hydrolysis. Yellow product SG-HE-pD in the hydrochloride form was ?ltered off, washed with ethanol and dried under vacuum. The resulting product in the free amine form was obtained by stirring with distilled water by adding sodium hydroxide (pH 8.0–9.0) till pH of solution remained at 8.0–9.0 for 2 h. Then it was washed by distilled water until the washings were neutral.

Homo-pD method: 24.0 g of DETA-bS reacted with 8 cm3(37.5 mmol) of GPTS in 100 cm3 of methanol. The mixture was re?uxed under dry nitrogen atmosphere and the solvent was evaporated subsequently. This condensed solution reacted with silica-gel of 8.0 g in dry toluene. After re?uxing, the solid was ?ltered, washed and dried to give the product named SG-HO-DS. The resulting product SG-HO-pD was obtained during hydrolysis as previously described [19].

2.3. Effect of pH on adsorption

The effect of pH on the adsorption of Hg(II) was studied by adding 50.0 mg of silica-gel adsorbents to 1 cm3 of 0.1 mol dmà3 Hg(II) and 19 cm3 of buffer solution at different pH values in 100 cm3 Erlenmeyer ?ask. This mixture was mechanically shaken for 24 h at 303 K to attain equilibrium. Then a certain volume of the solutions was separated from the adsorbents and the residual concentration of Hg(II) was detected by means of AAS. The adsorption amount was calculated according to the equation.

q=(C0-C)V/W

q, the adsorption amount (mmol gà1), C0 and C, the initial and the ?nal concentrations of metal ions in solution, respectively(mmol cmà3), V, the volume (cm3), W, the weight of SG-HE-dD, SG -HO-dD,SG-HE-pD and SG-HE-pD (g).

2.4. Adsorption kinetics

Adsorption kinetics were performed by mixing a 30 mg of silica-gel adsorbents with 20 cm3 of Hg(II) (5.0 ? 10à3 mol dmà3,pH 4.0) solution in a 100 cm3 Erlenmeyer ?ask at 293 K. One milliliter of the solution was taken at different time intervals,where the residual concentration of Hg(II) was determined via AAS.

2.5. Adsorption isotherms

Complete adsorption isotherms were obtained by soaking 30 mg of dry adsorbents in a series of ?asks containing 20 cm3 of different initial metal ion concentrations varying from 0.625 to 10 mmol dmà3 for 3 h. The initial pH was adjusted to 4.0 for Hg(II) while keeping the temperature at 293 K. Later onthe residual concentration was determined where the Hg(II) uptake was estimated.

3. Results and discussion

3.1. Preparation

A direct reaction of the synthesized silylant agent with activated silica-gel in non-aqueous solvent yields the immobilized compound. When the objective was to expand the organic chain, in order to include a series of desirable functions, two main routes are followed: (i) heterogeneous—the previous anchored silica-gel reacts to another molecule or (ii) homogeneous—the silylant agent is ?rstly prepared before immobilizing [14]. In present paper,we reported the similar incorporation of DETA through heterogeneous and homogeneous methods and chose GPTS as coupling agent to link silica-gel and functional molecular DETA. The procedures were represented in Scheme 1(A, B). A uniform distribution of ligands on the surface of silica-gel prepared by nhomogeneous route was considered better than by the heterogeneous route [13]. However, homogeneous route still cannot avoid intra- and inter-strand bridging reactions due to N-alkylation at the primary amine groups and secondary amine in DETA [16]. These cross-linking reactions should decrease the amount of immobilized ligand (DETA). Moreover, complicated cross-linkings should bring about steric restrictions on the formation of metal chelate.

In attempting to eliminate undesirable cross-linkings and increase the adsorption capacity, we protected the primary amine groups of the DETA by Schiff base formation prior to immobilization, and then the original amine could be regenerated by acid hydrolysis. The objectives, SG-HE-pD and SG-HO-pD, were presumed to have a structure as depicted in Scheme 1, in which GPTS had the unique chance to react with the secondary amine of DETA.

4. Conclusion

The following conclusions can be deduced from t he above results:

(1) FT-IR and elemental analysis con?rmed the four kinds of ef?cient modi?cation of diethylenetriamine bridged by 3-glycidoxypropyltrimethoxysilane on silica-gel.

(2) The BET surface area, BJH desorption average pore diameter and BJH desorption cumulative volume of pores of silica-gel decreased after functionalization. On condition that the N content was low for SG-HE-pD and SG-HO-pD, their values of BET surface area, BJH desorption average pore diameter and BJH desorption cumulative volume of pores were respectively larger than those for SG-HE-dD and SG-HO-dD for the larger pores were stuffed because of the serious cross-linking during the direct-amination reaction and the hydrogen bonding interactions between basic amino groups and residual silanols.

(3) Thermogravimetric analysis revealed that the adsorbents prepared by heterogeneous reaction had higher stability(almost 673 K) although a low adsorption capacity. And those four kinds of adsorbents can be applied in the case of no more than 523 K.

(4) The maximum adsorption capacity was observed on SG-HO-pD, although a lower N content than SG-HO-dD was observed.The maximum adsorption of all adsorbent occurred at pH 4.0 for Hg(II). An adsorption kinetics study indicated that pseudo-?rst-order rate model provided an excellent ?tting of SG-HE-dD, SG-HO-dD and SG-HE-pD and pseudo-second-order model provides an excellent ?tting of SG-HO-pD. The isotherm adsorption data of Hg(II) on all adsorbents was well ?tted by the Langmuir isotherm model.

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