Environ. Sci. Technol. 2005, 39, 7667-7670
Mercury Ions Removal from Aqueous Solution Using an Activated Composite Membrane M A R IÄ A E L E N A P AÄ E Z - H E R N AÄ N D E Z , * , † , ‡ KARINA AGUILAR-ARTEAGA,† C A R L O S A N D R EÄ S G A L AÄ N - V I D A L , † M A N U E L P A L O M A R - P A R D A V EÄ , ‡ MARIO ROMERO-ROMO,‡ AND M A R IÄ A T E R E S A R A M IÄ R E Z - S I L V A § Centro de Investigaciones Quı´micas, Universidad Auto´noma del Estado de Hidalgo, Pachuca, Hidalgo C.P. 42076, Me´xico, Departamento de Materiales, Universidad Auto´noma MetropolitanasAzcapotzalco, Av. San Pablo No. 180 Col. Reynosa Tamaulipas, C.P. 02200 Me´xico D.F., Me´xico, and Departamento de Quı´mica, Universidad Auto´noma MetropolitanasIztapalapa, Av. Michoaca´n y Purı´sima s/n Col. Vicentina, C.P. 09340 Me´xico D.F., Me´xico
This work presents the results concerning the first use of activated composite membranes (ACMs) for the removal of Hg(II) ions from aqueous solution, using as the ligand di(2-ethylhexyl)dithiophosphoric acid (DTPA). The effects on the removal percentage of Hg(II) of variables such as pH, the nature of the acid, the concentration of mercury (in the feed solutions), and the ligand content (in the membrane) as well as the total surface membrane area exposed to the Hg(II) aqueous solution were studied. During the course of the removal experiments, the membrane was immersed in the Hg(II) aqueous solution in acid media and samples of the solution were taken at different times to enable monitoring of the mercury concentration changes. It was found that when the ACM was prepared with a 1.0 M DTPA casting solution and the feed solution contained 2.49 × 10-4 M Hg(II) in HCl 0.1 M the amount of mercury extracted was higher than 93%. Straightforwardly, additional experiments were carried out with the freeDTPA composite membranes to make up a set of control reference points to verify that removal of the investigated heavy metal was a consequence of the presence of the organic ligand; otherwise there was no Hg(II) concentration variation at all.
Introduction Mercury is generally considered to be one of the most toxic metals found in the environment (1-3). Water contamination due to industrial discharges of this metal is the cause of acute global concerns to environmental authorities. For instance, the present legislation in Mexico limits the range of accepted quantities of the pollutant [Hg(II)] in residual water discharges and national resources from 0.005 mg L-1 (2.49 × 10-8 M) to 0.02 mg L-1 (9.97 × 10-8 M) (4). Moreover, the applicable legislation in the European community establishes that the * Corresponding author: phone: + 52(771)7172000, extensions 6785,6786,and6787;fax: +52(771)7172109;e-mail:
[email protected]. † Universidad Auto ´ noma del Estado de Hidalgo. ‡ Universidad Auto ´ noma MetropolitanasAzcapotzalco. § Universidad Auto ´ noma MetropolitanasIztapalapa. 10.1021/es0505705 CCC: $30.25 Published on Web 09/03/2005
2005 American Chemical Society
monthly average for all mercury discharges in wastewaters from alkaline chloride electrolysis industries using mercury cathodes must not exceed 50 µg L-1 (2.49 × 10-7 M) of mercury, whereas the overall amount of mercury from other industrial activities’ discharge water must not exceed the 50 µg L-1 (2.49 × 10-7 M) (5). Therefore, to meet quality standards, wastewater cleanup is essential prior to discharge. The critical considerations mentioned above directly influenced the design and evaluation of numerous mercury removal processes. Thus, several methods have been used to remove mercury from aqueous media, namely, precipitation (6, 7), ionic exchange (8, 9), solvent extraction (10, 11), and sorption by activated carbon (12-14) and other materials. Particularly, the removal of mercury using polymeric sorbents (15-20) has deserved particular attention, because the materials used have proven to be highly efficient and easy to handle, and in several cases they can be regenerated. Moreover, if these polymers are functionalized with sulfur compounds, then the sorption process becomes facilitated due to mercury’s high affinity for sulfur compounds, which in turn allows formation of very stable complexes, thus enabling their safer long-term storage. The use of sorbent polymers as fibers or beads has resulted in an efficient mercury removal from aqueous media, thus diminishing high mercury contents (sometimes as high as 100 ppm) to levels that comply with international standards. The main goal of this paper is to show the advantages associated with the use of activated composite membranes (ACMs) as polymeric-type sorbents to remove mercury from aqueous media. This new alternative gives the possibility of using this material, particularly in small configurations, having a high sorption capacity, which is based on the interaction with the organic ligand di-(2-ethylhexyl)dithiophosphoric acid (DTPA), included in the solid structure of the membrane. Additionally, the present work proves how the mercury sorbent may be used without further extractionreextraction steps as part of the removal process and that the overall sorption kinetics are good. The experimental parameters selected for the present study were the nature of the acid and the pH in the feed phase solution, the concentration of DTPA in the membrane, and the surface contact area.
Experimental Section Materials. All commercial reagents were Aldrich ACS grade. The supporting paper for the membrane construction was a nonwoven paper Hollytex 3329 from Talas Co. (New York, NY). The organic DTPA ligand (di-(2-ethylhexyl)dithiophosphoric acid) (C16H35O2PS2) was synthesized and purified within the author’s laboratory facilities following the methodology proposed by Bromberg et al. (21). This compound has shown a strong affinity for soft acids, mainly mercury and silver (22). In the same sense, this compound is selective to mercury but not specific for it. ACM Construction Method. The activated composite membranes were prepared as described earlier (23). First, a layer of polysulfone dissolved in N,N-dimethylformamide was deposited onto a supporting paper, and second, the impregnated paper was immersed in a water bath to induce phase inversion polymerization. The thin upper polyamide layer was formed by interfacial polymerization due to the interaction onto the polysulfone layer of two different phases: one of them, the 1,3-phenylendiamine aqueous solution, and the other, a kerosene organic solution containing benzene-1,3,5-tricarbonyl chloride with the organic DTPA ligand. The composite membrane thus prepared was washed VOL. 39, NO. 19, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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with deionized water (18.2 MΩ cm) and dried in a stove for 30 min at 60 °C. For the sake of comparison, an inactivated composite membrane (ICM) was prepared similarly to an ACM but without including the DTPA organic ligand. To evaluate if some DTPA of the ACM had been transferred into the aqueous solution, the phosphorus contents in the mercury-containing aqueous media were evaluated by means of the inductively coupled plasma (ICP) technique (using a Perkin-Elmer Optima 3000 XL). After 4 h of extraction, this element was present in the aqueous mercury-containing media in amounts smaller than the detection limit of the ICP, namely,
