Facile and efficient recovery of mercury based on poly(amine-ester)-bearing metal-complexation and acidic aqueous solution-soluble groups

Introduction

Heavy metal pollution represents an important environmental problem due to the toxic effects of metals. The accumulation of heavy metals throughout the food chain leads to serious ecological and health problems.¹ Mercury is universally recognized as one of the most toxic and dangerous non-biodegradable inorganic pollutants present in aquatic systems.^{2, 3, 4, 5, 6, 7, 8} Mercury is present in many products and processes in common use (for example, chloro-alkali production, pharmaceutical and cosmetic preparations, combustion of fossil fuels, electrical and electronics manufacturing plants, metal processing, metal plating, metal finishing, and pulp and paper industries), resulting in the contamination of aquatic systems.^{2, 3, 4, 5, 7, 8}

Conventional methods for the recovery of metals from water and wastewater include reduction,^{9, 10} oxidation,¹¹ solvent extraction,^{12, 13} precipitation^{14, 15} and adsorption.^{16, 17, 18, 19} Of these, adsorption seems to be the most suitable method for recovery of metals due to its low cost, safety and high efficiency.^{20, 21} Metal adsorption by polymers has been extensively studied;^{16, 17, 18, 19, 20, 21} however, because nearly all polymers are insoluble in aqueous solutions of metal ions, adsorption must proceed heterogeneously, resulting in two important problems (low recovery and low adsorption rate), which represent a hurdle to practical use. Previously, we proposed an efficient recovery and facile process for metal recovery based on a water-soluble polyallylamine with side-chain thiourea groups for metal-complexation groups.²² Since the polymer is soluble in aqueous metal ion solutions, complexation proceeds homogeneously and efficiently. As complexation progresses, cross-linking takes place between the metal-complexation groups and the metal ions, precipitating the polymer complex, which can be easily separated by filtration.

For further development of a metal-recovery process utilizing a water-soluble polymer, we herein report a new facile and efficient recovery system based on a polymer-bearing metal-complexation and acidic aqueous solution-soluble groups. Figure 1 shows a schematic representation of our design: a polymer containing tertiary amine groups that are soluble in acidic aqueous solution on their protonation and that interact with metal ions. The polymer achieves homogeneous complexation with high-recovery efficiency and at a high rate. As complexation progresses, cross-linking takes place between the tertiary amine groups and the metal ions, precipitating the polymer complex, which can be easily separated by filtration. Because metal ion wastes are often produced under acidic conditions, effective recovery of metals in acidic aqueous solution is desirable.²³ Because diamine and diacrylate are available abundantly, the selection of the poly(amine-ester) suitable to a target metal can result in the construction of a number of recovery systems.

Figure 1

Schematic representation of facile and high-recovery system of mercury by a polymer containing metal-complexation groups and soluble groups in acidic aqueous solution. A full color version of this figure is available at Polymer Journal online.

Materials and methods

Materials

1,3-Di-4-piperidylpropane (Tokyo Kasei Kogyo, Tokyo, Japan, >97.0%) was purified by recrystallization from hexane and dried under vacuum. 1,6-hexanediol diacrylate (Alfa Aesar, Lamcashire, UK) was commercially available and used as received. Tetrahydrofuran (Wako Pure Chemical, Osaka, Japan, >99.5%) was distilled and used. Mercury (II) chloride (HgCl₂, Wako Pure Chemical, >99.5%), manganese (II) chloride tetrahydrate (Kanto Chemical, Tokyo, Japan, >99.0%), iron (III) chloride hexahydrate (Wako Pure Chemical, >99.0%), cobalt (II) chloride hexahydrate (Kanto Chemical, >99.0%), nickel (II) chloride hexahydrate (Kanto Chemical, >98.0%), copper (II) chloride (Wako Pure Chemical, >95.0%), ruthenium (III) chloride trihydrate (Kanto Chemical, >98.0%), rhodium (III) chloride hydrate (Aldrich, St Louis, MO, USA, >99.9%), sodium tetrachloropalladate (II) (Na₂PdCl₄, Tokyo Kasei Kogyo, >98.0%), silver (I) nitrate (Kanto Chemical, >99.8%), osmium (III) chloride hydrate (Alfa Aesar, 99.99%), iridium (III) chloride trihydrate, (Aldrich, >99.9%), hydrogen hexachloroplatinate (IV) (Wako Pure Chemical, >98.5%) and sodium tetrachloroaurate (III) dihydrate (Wako Pure Chemical, >95.0%) were commercially available and used as received. Sodium chloride (Wako Pure Chemical, >99.5%), sodium bromide (Wako Pure Chemical, 99.9%), and sodium iodide (Wako Pure Chemical, >99.9%) were used as received.

Methods

¹H nuclear magnetic resonance (NMR) spectra were recorded with JEOL JNM-λ500 (Tokyo, Japan) using tetramethylsilane as an internal standard; the δ values are given in p.p.m. Number-average (M_n) and weight-average (M_w) molecular weights were estimated by size-exclusion chromatography using a system consisting of a Hitachi L-7100 pump (Hitachi Ltd., Tokyo, Japan), Hitachi L-7490 refractive index detector (Hitachi Ltd) and polystyrene gel columns (Tosoh TSK gels α2500 and α3000 (TOSOH CORPORATION, Tokyo, Japan), whose limitations of size-exclusion are 1 × 10⁴ and 1 × 10⁵, respectively). Ultraviolet (UV)/visible (VIS) absorption spectra were recorded with a Hitachi U-3000 UV-Vis spectrometer (Hitachi Ltd).

Synthesis of poly(amine-ester)—typical procedure

Poly(amine-ester) was synthesized according to the reported procedure.²⁴ To a solution of 1,3-di-4-piperidylpropane (1.90 mmol, 400 mg) in tetrahydrofuran (5.00 ml), 1,6-hexanediol diacrylate (1.90 mmol, 430 mg) was added at 50 °C and stirred at the same temperature for 48 h in air. The resulting mixture was poured into hexane (200 ml). The resulting precipitate was collected by filtration with suction and dried in vacuo to obtain poly(amine-ester) (315 mg, 38%).

M_n=8000, M_w/M_n= 3.08. ¹H NMR (500 MHz, CDCl₃, δ, p.p.m., at rt): 1.13–1.77 (2H+2H+2H+4H+2H+4H+4H, br, m, −CH(CHHCH₂)₂NCH₂CH₂−, −CH(CH₂CH₂)₂NCH₂CH₂−, −CH(CHHCH₂)₂NCH₂CH₂−, −CH(CH₂CH₂)₂NCH₂CH₂−, −CH(CH₂CH₂)₂NCH₂CH₂−, −CH(CH₂CH₂)₂NCH₂CH₂OCOCH₂CH₂CH₂−, −CH(CH₂CH₂)₂NCH₂CH₂OCOCH₂CH₂CH₂−), 1.93 (2H, br, −CH(CH₂CHH)₂NCH₂CH₂−), 2.51 (4H, br, −CH(CH₂CH₂)₂NCH₂CH₂OCOCH₂CH₂−), 2.64 (4H, br, −CH(CH₂CH₂)₂NCH₂CH₂OCOCH₂CH₂−), 2.87 (2H, br, −CH(CH₂CHH)₂NCH₂CH₂−), 4.06 (4H, t, J=7.00, −CH(CH₂CH₂)₂NCH₂CH₂OCOCH₂CH₂−).

Metal recovery—typical procedure

An aqueous solution of poly(amine-ester) (pH 1, 5.00 ml, 0.3 wt%) was added into an aqueous solution of HgCl₂ (pH 1, 5.00 ml, 4 mM), and the mixture was stirred at ambient temperature for 2 h. The resulting precipitate was separated by filtration (pore size of filter; 0.45 μm), and aliquot (0.250 ml) of the filtrate was removed for sampling. After appropriate dilution, the metal concentration in the solution was determined by UV/VIS spectrometer. The recovery amount was calculated based on the following equation. pH was adjusted by HCl aq.

Results and Discussion

Poly(amine-ester)s were synthesized by polyaddition of 1,3-di-4-piperidylpropane with 1,6-hexanediol diacrylate in tetrahydrofuran at 50 °C (Scheme 1). The polymer was soluble in water at pH<2 but insoluble at pH>3, because protonation of the nitrogen atoms led to hydrophilicity.

We investigated the recovery of Hg^II using poly(amine-ester). An aqueous solution of poly(amine-ester) (pH 1, 5.00 ml, 0.3 wt%) was added to a 1.0 M aqueous solution of HgCl₂ (pH 1, 5.00 ml, Hg^II concentration: 4.00 mM), resulting in instant precipitation (Figure 2) (dissolution of poly(amine-ester) to an aqueous solution (pH 1, 0.3 wt%) decreased to pH 3. Addition of HgCl₂ to an aqueous solution of pH 3 (4 mM) resulted in the dissolution of HgCl₂. Therefore, HgCl₂ is not spontaneously precipitated by pH change due to polymer addition). The precipitate was separated by filtration, and the concentration of Hg^II in the filtrate was measured by UV/vis spectroscopy, yielding a recovery efficiency of 96%. Thus, poly(amine-ester) was effective for Hg^II recovery, and the polymer complex could easily be separated by filtration.

Figure 2

Photographs of poly(amine-ester) before and after Hg^II recovery. Conditions: aqueous solution of Hg^II: 5 ml (pH 1; Hg^II concentration: 4.0 mM); that of poly(amine-ester): 5 ml (pH 1; 0.3 wt%); ambient temperature.

The effect of pH on the recovery of Hg^II by poly(amine-ester) was examined (Figure 3). Interestingly, the recovery behavior was quite consistent with the solubility of the polymers in Hg^II aqueous solution, i.e., the recovery efficiency increased significantly at pH 2. No Hg^II ions were precipitated at any pH. This demonstrated that homogeneous complexation significantly enhanced recovery.

Figure 3

Effects of pH on the Hg^II recovery by poly(amine-ester). Conditions: aqueous solution of Hg^II: 5 ml (4.0 mM); that of poly(amine-ester): 5 ml (0.3 wt%); ambient temperature for 1 h.

Figure 4 shows ¹H NMR spectra of the polymers with different recovery amounts. As the recovery amount increased, the proton signals adjacent to the nitrogen atom (a, b and h) shifted to lower field, indicating that the nitrogen atoms contributed to the complexation. In 0.1 M Cl⁻ aqueous solution (=pH 1), the species present are HgCl₂, HgCl³⁻ and HgCl₄.²⁵ Three probable interactions were considered (Scheme 2): (a) cross-linking by coordination of the free nitrogen atom to HgCl₂; (b) ion exchange between Cl⁻ and HgCl³⁻; and (c) cross-linking by ion exchange between Cl⁻ and HgCl₄²⁻.

Figure 4

1H NMR spectra (DMSO, rt) of poly(amine-ester)s with different recovery amounts (upper: 126 mg_Hg/g_poly.; lower: 487 mg_Hg/g_poly.). NMR, nuclear magnetic resonance.

Because the homogeneous recovery process using poly(amine-ester) is expected to result in a fast recovery rate, the kinetics was studied. The recovery of Hg^II by poly(amine-ester) was very fast, finishing within 10 min, because the homogeneous adsorption took place (Figure 5). The experimental kinetic data were fitted with a pseudo-first-order kinetic equation:

Figure 5

Changes of recovery of Hg^II by poly(amine-ester). Conditions: aqueous solution of Hg^II: 5 ml (pH 1; Hg^II concentration: 4.0 mM); that of poly(amine-ester): 5 ml (pH 1; 0.3 wt%); ambient temperature.

where Q_e is the equilibrium recovery amount of Hg^II (g_Hg/g_poly.), Q_t is the recovery amount (g_Hg/g_poly.) at time t, and k is the rate constant (min⁻¹). For the recovery of Hg^II by poly(amine-ester), k was estimated to be 0.338 min⁻¹ (correlation coefficient, R²=0.9657). These results demonstrated that improved polymer solubility accelerates the recovery of Hg^II.

The effect of the initial concentration of Hg^II on the amount of mercury recovered by the poly(amine-ester) was examined (Figure 6). The recovery amount increased with increasing Hg^II concentration and tended to approach the plateau region ~45 mM. Based on the plateau region, the maximum recovery by the poly(amine-ester) was evaluated as 487 mg_Hg/_poly. Table 1 compares the maximum recovery amounts of different types of adsorbent. Our polymer had a satisfactory recovery ability, and because a wide variety of diamine and diacrylate are available the proportion of adsorption sites in polymer structure can increase, resulting in a larger recovery amount.

Figure 6

Recovery amounts of poly(amine-ester) as function of initial concentration of Hg^II. Conditions: aqueous solution of Hg^II: 5 ml (pH 1); that of poly(amine-ester): 5 ml (pH 1; 0.3 wt%); ambient temperature for 1 h.

Table 1 Comparison of the maximum recovery amounts of mercury ions on different adsorbents

The recovery of various metals by poly(amine-ester) was examined using a metal ion concentration of 45 mM, which was the optimum concentration for Hg^II recovery. As summarized in Table 2, metals with large atomic radii tended to instantly give rise to cross-linking precipitates. This selectivity is ascribed to the high affinity of the tertiary amine groups for soft metal ions. It is noteworthy that the recovery amounts for all of the metals were very high (123–520 mg_metal/g_poly.).

Table 2 Recovery (g_metal/g_poly.) of various metal ions by poly(amine-ester)^a

Some industrial wastewaters contain, in addition to toxic heavy metal ions, large quantities of other salts such as sodium chloride. Generally, the sole effect of this presence is a high ionic strength that slightly modifies the values of the equilibrium constants, without introducing new reactions in the system. This is not the case for solutions containing Hg^II ions, which are known to form very stable complexes with halide ions.²⁶ The formation of such strong complexes can result in a masking effect that significantly affects the performance of an adsorbent. In this study, sodium halides (NaCl, NaBr and NaI) were chosen as model salts to investigate the effect of halide ions on the recovery of Hg^II ions by the poly(amine-ester). The effect was studied by carrying out a series of recovery experiments in solutions of Hg^II containing various NaX concentrations. Table 3 shows effect of NaX concentrations on the recovery of Hg^II by the poly(amine-ester). It is noteworthy that the recovery efficiency of Hg^II did not decrease in every case, indicating that Hg^II recovery by the poly(amine-ester) was not affected by the presence of halide ions because of its high-recovery ability.

Table 3 The effect of other salts (NaX) on the adsorption of Hg^II by P1^a

In summary, we have successfully developed a facile and efficient recovery process for metals based on a poly(amine-ester) consisting of metal-complexation and acidic aqueous solution-soluble groups. Since the polymer is soluble in acidic aqueous solutions, the metal-complexation proceeds homogeneously and efficiently. As metal-complexation progresses, cross-linking takes place between the metal-complexation groups and the metal ions, precipitating the polymer complex, which can be easily separated by filtration. Hg^II recovery was completed within 10 min, and the maximum amount of mercury recovered by the poly(amine-ester) (487 mg_Hg/g_poly.) was satisfactory. The polymer was also capable of recovering other metals such as Rh^III, Pd^II, Os^III, Ir^III, Pt^IV and Au^III in large amounts. This polymer, bearing metal-complexation and acidic aqueous solution-soluble groups, is expected to be applicable as an efficient recovery material for metals. We are now currently examining the selectivity of Hg^II ion from a mixture of other metal ions.

Synthesis of poly(amine-ester) by polycondensation of 1,3-di-4-piperidylpropane and 1,6-hexanediol diacrylate.

Probable interaction between tertiary amine groups and Hg species.