April 15, 2007
Removal of Humic Substances From Water By Means of Calcium-Ion- Enriched Natural Zeolites
By Capasso, S; Colella, C; Coppola, E; Iovino, P; Salvestrini, S
The ability of the natural zeolited Neapolitan Yellow Tuff (NYT) enriched with calcium ions to remove humic acids from water was evaluated by batch adsorption equilibrium tests and dynamic experiments carried out by percolating humic acid solutions through a small NYT column (breakthrough curves). Under the experimental condition explored, the sorption capacity increases with the ionic strength and has the highest value at pH 7.4. The partition coefficient for a low concentration of humic acid ([humic acid] [arrow right] 0), at pH 7.4 in 0.01 M sodium chloride, was approximately 1000 L/kg, versus the value of approximately 100 L/kg in the absence of the alkaline metal salt. Therefore, after humic acids have been adsorbed in a column filled with the calcium-ion- enriched tuff, a reduction of the salt concentration in the ongoing solution enhances the release of the adsorbed material. These findings show that NYT can be used for the removal of humic acids from water. Water Environ. Res., 79, 305 (2007).
KEYWORDS: humic acids, zeolite, Neapolitan Yellow Tuff, adsorption, water purification.
Humic substances represent approximately 50 to 80% of the organic matter in water coming from terrestrial sources, lakes, and rivers. In many cases, their amount is particularly high and not acceptable for drinking water. Normal concentrations in ground, surface, and estuarine waters are in the range 0 to 30 mg/L (Thurman and Malcolm, 1981). A concentration higher than 15 mg/L has been recorded in the drinking water of some European regions (Hdi et al., 1995). Humic substances are natural polymers of aromatic blocks, characterized by a broad molecular weight distribution and high chemical heterogeneity with acidic character. Depending on the solubility, humic substances are divided into fulvic and humic acids. In water, the latter are soluble only at neutral and basic pH values. The interaction of hydrophobic pollutants with the core of humic substance molecules is responsible for their higher concentration in surface water than their solubility in pure water (Chiou et al., 1987). Many techniques are commonly used in waterworks for the abatement of humic substances, with coagulation and interaction with insoluble materials by anion exchange being the most widespread. Removal of humic acid by modified granular activated carbon (Cheng et al., 2005), soluble cationic polyelectrolytes (Kam and Gregory, 2001), and by metalmodified silica gels (Moriguchi et al., 2005) has also been reported.
In a previous study, we have shown that the enrichment of zeolitic tuffs by divalent cations markedly increases its capacity to bind humic acids (Capasso et al., 2005). Zeolites are naturally occurring aluminosilicates widely used as catalysts and as ion exchangers and molecular sieves in the chemical industry and waterworks (Colella et al., 1994; Sarioglu, 2005; Schmidt, 2001). From a structural point-of-view, they are characterized by high surface areas, high cation exchange capacities, and three- dimensional cagelike structures, with channel apertures on the order of a few Angstrom units (Newsam, 1986). Removal of ammonium and some heavy metal cations from drinking water by zeolites and removal of cationic radioactive species (i.e., ^sup 127^Cs and ^sup 90^Sr) from nuclear plant wastewater and contaminated groundwater have been analyzed in many works and used in practical applications (Adabbo et al., 1999; Bosch et al., 2004). Moreover, there is increasing interest in the uptake of ions and neutral molecules by surfacemodified zeolites (Bowman et al., 1995; Benkli et al., 2005). Charged molecules, too large to fit into the zeolite channels, are excluded from the internal surface. They can replace the charge- balancing cations present on the surface, thus modifying the zeolite surface and enriching the material of new interesting ion exchange and sorption properties. A recent paper, for example, reported the applications of modified zeolites by long-chain quaternary amines to control the chemical pollution of groundwater, removal of organic compounds from oilfield waters, and removal of pathogens from wastewater effluent (Bowman, 2003).
Starting with the above considerations, an investigation has been carried out on the use of Neapolitan Yellow Tuff (NYT), a widespread zeolitic-rich material in the Neapolitan area (South Italy) for water purification from humic substances.
Materials. Tuff. The examined tuff sample, belonging to the huge formation of the NYT, came from extended quarries in Marano (Naples, Italy). This material represents the more recent (12 000 years before present) tuffaceous formations of the Phlegraean Fields (Napoli, Italy), covering an area of approximately 13 km^sup 2^. The chemical composition of the sample used was as follows (in g/kg): silicon dioxide = 529.1, aluminium oxide = 147.3, iron oxide = 40.3, magnesium oxide = 10.8, calcium oxide = 20.7, potassium oxide = 75.7, sodium oxide = 27.6, phosphorus pentoxide = 1.1, and water = 141.0. The cation exchange capacity, measured by the cross-exchange method, was 1.90 meq/g. The percentage of exchangeable cations in equivalents, obtained by back-exchanging the samples with ammonium and analyzing the displaced cations by atomic absorption spectroscopy (Perkin Elmer AAnalyst 100 instrument, Perkin Elmer, Wellesley, Massachusetts), was as follows: 47.1% sodium, 25.7% potassium, 1.9% magnesium, and 25.4% calcium. The total zeolite content, estimated by the water vapor desorption procedure, was 54%, with 37% phillipsite and 17% chabazite. Ancillary phases present in the tuff include analcime, potassium feldspar, pyroxene, and a trace of mica. Note that, although analcime is a zeolite, it does not take part in cation exchange to a great extent because of its low exchange kinetics at room temperature. Details on the above experimental procedures can be found in previous publications (de'Gennaro and Colella, 1989; Pansini et al., 1996).
Enriching Neapolitan Yellow Tuff Sample Using Calcium Ion. Approximately 1 kg of NYT, ground to a fineness
Humic Acid Purification. Humic acids were obtained from Fluka (Sigma-Aldrich, Switzerland). Approximately 10 g of humic acid was suspended in 1 L water containing 10 mL of concentrated fluoridric acid and 10 mL of concentrated hydrochloric acid. After vigorous stirring for 1 day, the mixture was filtered, the precipitate washed with 1-M hydrochloric acid and water, and finally suspended in 0.8 L water. A suitable volume of 1-M potassium hydroxide was added to the suspension to bring the pH to 9.0. After 1 day, the mixture was filtered and stored in a refrigerator for 1 week after bringing the pH to 1.5 by adding concentrated hydrochloric acid. Afterwards, the purified humic acid was collected by centrifugation and washed with water. Humic acid was introduced to a dialysis tube (molar mass cutoff = 3500 Da) and dialyzed against distilled water, until no significant change was observed in the conductance of water external to the dialysis bag (
Humic Acid Equilibrium Sorption Experiments. Batch sorption tests were performed at room temperature (25C) in 50-mL polyethylene test tubes at pH 8.5, 7.4 (0.01 TrisH^sup +^/Tris buffer), and 6.3. Regarding the latter, humic acid was dissolved in 0.01-M sodium hydroxide and the pH adjusted using hydrochloric acid to the decided value. Different amounts of Ca^sup 2+^-enriched tuff samples, ranging from 0.25 to 2.0 g, were mixed with 30 mL of 800 mg/L humic acid solution containing the selected buffer and a neutral salt (potassium chloride or sodium chloride). The samples were mildly shaken for 48 hours, and the pH was periodically checked. If variations in the pH value were higher than 0.1 unit, a few drops of concentrated Tris base or hydrochloric acid were added. Afterwards, the supernatants were centrifuged at 10 000 rpm for 15 minutes and analyzed by visible spectrometry at 450 nm.Analyses carried out after a longer time (4 days) did not give significantly different results. The absorbance was detected at 450 nm because, at this wavelength, the acquired signal allowed analysis of the relative concentrated samples without too much diluition. Moreover, at this wavelength, there was a good linear relationship between absorbance and humic acid concentration (De Nisi, 2004). Tris buffer was selected in this study because it had been shown that chloride and TrisH^sup +^ ions do not affect the humic acid sorption isotherm on the Ca^sup 2+^-enriched NYT (De Nisi, 2004).
Small-Column Studies. A plastic column, 1.5 cm in diameter and 7 cm high, was packed with 13.0 g of NYT, and the empty space on top of the column was filled with glass spheres, 2 mm in diameter. The solution to be tested, 35 mg/L of humic acid in 0.01-M sodium chloride, pH 7.4, and 0.01 M TrisH^sup +^/Tris buffer, was pumped upward through the packed columns to minimize channelling of the eluant solution using a peristaltic pump at 12.0 mL/h. Fractions of 14 mL were collected and analyzed by visible spectroscopy. Before the humic acid sorption run, the column filled with tuff was eluted with 200 mL of water and 0.01-M Tris buffer until reaching a pH value in the eluate coincident with that of the ongoing solution. Approximately 500 mL of the Tris buffer were used. The pore volume, defined as the volume of influent solution required to fill a packed NYT column's pore space, was determined by adding 20 mL of water to 13.00 g of NYT. After 1 week, the water on the solid phase was sucked off by a syringe, and the remaining water in the NYT pore space was determined by weighing. The pore water was equal to 0.432 g per 1 g NYT.
Results and Discussion
The Role of Calcium Ion on Humic Acid Sorption. The Ca^sup 2+^- enriched tuff undoubtedly exhibits an affinity for humic acid greater than the natural tuff, as can be deduced from the difference between their sorption isotherms shown in Figure 1. These sorption isotherms are of type "L mx", according to the Giles classification (Giles et al., 1960).
The curves in the figure have been modeled using the following empirical equation:
X/m = m^sub 1^ . (C)^sup 0.5^ + m^sub 2^ . (C)^sup m^sub 3^^ (1)
C = solution concentration (mg/L),
X/m = solid-phase equilibrium concentration (mg/kg), and
m^sub 1^, m^sub 2^, and m^sub 3^ = constants whose values have been computed by least square fit.
The curves aim only to make the observation of the experimental data easier for the readers. Because, at the pH values of the experiments (6.3 to 8.5), humic acids are in the form of organic polyanions, they should be repelled from the negatively charged tuff surface. The interaction is promoted by the Ca^sup 2+^ ion, which, because of its relative high charge density, acts as a bridge between the mineral and the organic anions. In our opinion, the reduction of adsorption capability observed in the figure for high values of humic acid concentration in solution reflects intramolecular association/dissociation and a change in the volume of the macromolecule rather than a reduction of the affinity for the tuff. Previous experiments carried out in our laboratory using nonpurified humic acid (20% ash, according to the manufacturer) did not show any reduction in adsorbitivity at a high humic acid concentration (De Nisi , 2004).
Ionic Strength Effect. Figure 2 shows the sorption isotherms carried out in solutions containing different amounts of potassium chloride, from O to 0.04 M. In the concentration range explored, the adsorption capability increases with the salt concentration.
Figure 3 shows a comparison of the effects of potassium chloride with the effects of sodium chloride on the humic acid sorption isotherms. At low values of humic acid concentration, the two isotherms are almost coincident with each other. Differences in adsorption capability appear significant as humic acid concentration increases. In 0.01-M sodium chloride and pH 7.4, which are typical values of groundwater, the partition coefficient for the low concentration of humic acid in the Ca^sup 2+^-enriched NYT/water system, defined by the slope at the low humic acid concentration of the experimental data in Figure 3, was approximately 1000 L/kg, against the value of approximately 100 L/kg for the tuff in the natural form (Figure 1).
pH Effect. Figure 4 reports the sorption isoterms at pH 8.5,7.4, and 6.3, in the presence of a 0.01-M sodium ion. The binding between the Ca^sup 2+^-enriched tuff and humic acid depends markedly on pH. The greatest affinity was recorded at pH 7.4. According to the works of Abate and Masini (2003), the negative charges on the humic acid molecules increase from approximately 2.5 mmol/g at pH 6.3 to approximately 3.8 at pH 8.5. This increase should, on one hand, lead to an increase in the interaction of the macromolecule with the Ca^sup 2+^ ions located on the tuff surface and, on the other hand, lead to a molecular swelling because of the increase of the electrostatic repulsion between ionized groups (Avena et al., 1999). This multiplicity of effects can justify the observed lack of monotonic dependence of the humic acid/NYT binding on the pH.
Small-Column Studies. Figure 5a shows the breakthrough curve for the sorption of humic acid onto Ca^sup 2+^-enriched NYT, obtained by eluting a small column (see Experimental Conditions) with a humic acid solution containing 0.01-M sodium chloride and 0.01-M Tris buffer, at pH 7.4. These experimental conditions have been selected on the basis of the above results and considering the typical composition of groundwater.
In the first 150 pore volumes, there was a reduction of approximately 50% in the amount of dissolved humic acid. This number of pore volumes, therefore, is then the retardation factor, according to the widespread definition (Li and Bowman, 2001). With a concentration in the inflow of 45 mg/L humic acid, 1 g of tuff absorbed approximately 1.2 mg of humic acid during the elution of these first 150 pore volumes. Figure 5b reports the concentration in the eluate after the eluant had been replaced with an aqueous solution that did not contain either sodium chloride or humic acid. Approximately 75% of the adsorbed material was desorbed. There is a rapid increase of the ratio of humic acid concentration in the effluent to the humic acid concentration in the influent (C/Co) ratio to values greater than 1, followed by a rapid reduction. The increase is a result of the marked reduction of the sorption capacity with decreasing ionic strength, as reported in the pH Effect section.
The data collected in the present investigation strongly support the possible use of Ca^sup 2+^-enriched NYT for removing humic acid from water. In fact, the measurements carried out in equilibrium closed systems and by flowing humic acid solutions through a small bed provide evidence that this cheap material has a very high capacity to sorb humic substances from water. At pH 7.4 and a sodium chloride concentration of 0.01 M, which are typical values of groundwater, the partition coefficient at low values of humic acid concentration was approximately 1000 L/kg. Moreover, by eluting the tuff with water that does not contain salts, the partition coefficient value markedly decreases, promoting the release of humic acid. This way, the adsorbing material can easily be regenerated after its use.
This work was financed by grants from the Italian Ministry of University and of Scientific Research and Technology.
Submitted for publication July 28, 2005; revised manuscript submitted December 14, 2005; accepted for publication March 1, 2006.
The deadline to submit Discussions of this paper is June 15, 2007.
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S. Capasso1*, C. Colella2, E. Coppola3, P. Iovino4, S. Salvestrini5
1* Full Professor, Department of Environmental Sciences, Second University of Naples, Caserta, Italy; Dipartimento di Scienze Ambientali, Seconda Universit degli Studi di Napoli, via Vivaldi 43, 81100 Caserta, Italy; e-mail: [email protected]
2 Full Professor, Department of Materials and Production Engineering, University of Naples, Federico II, Italy.
3 Researcher, Department of Environmental Sciences, Second University of Naples, Caserta, Italy.
4 Ph.D. Student, Department of Environmental Sciences, Second University of Naples, Caserta, Italy.
5 Researcher, Department of Environmental Sciences, Second University of Naples, Caserta, Italy.
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