Synthesis and characterization of adsorbents for the elimination of nitrates and bromates from water aiming to develop a continuous oxyanion water elimination system

It is known that the excess of oxyanions such as NO 3 and BrO 3 in drinking water affects its quality. In this work, three adsorbents (montmorillonite (Mt), silica (Si), and diatomaceous earth) loaded with hexadecyl(H) and octadecyl-trimethylammonium (O) were used to remove these oxyanions from aqueous solutions by adsorption. In batch systems, the highest NO 3 removal was obtained with Mt modified with H and O (Mt-H and Mt-O), attaining 33% and 50%, respectively, while for BrO 3 removal Si modified with H and O, Si-H and Si-O samples, reached 38% and 42%, respectively. A direct relationship between the adsorption capacity of NO 3 and BrO 3 and the mass of the adsorbent was found in column filtration tests with Mt-O and Mt-H samples in standard solution and real groundwater samples. The adsorption capacity of the column, in the groundwater sample, remained constant after two reuses.The results obtained are promising for the development of a continuous oxyanion removal system containing the low-cost clay Mt modified with either H or O.


INTRODUCTION
Different compounds affect the quality of water intended for human consumption. Among them, there are several oxyanions such as NO À 3 , NO À 2 , BrO À 3 , ClO À 3 , ClO À 4 (Chaplin et al. ). In particular, NO À 3 is converted to NO À 2 in the human body and can cause oxygen depletion in the blood (Ward et al. ). In addition, BrO À 3 is generated from Br À by the ozonation of water to disinfect it and has been classified as a potent carcinogen by the International Agency for Research on Cancer (Moore & Chen ). Amorphous silica or silica gel is produced by the acidification of sodium silicate solutions, generating a gelatinous precipitate that is washed and dehydrated to produce microporous silica. Silica gel is a substance of crystalline appearance, high surface area, and is porous, inert, non-toxic and odorless, chemically stable and insoluble in water or any other solvent. These properties qualify it to act as a selective adsorbent against different molecules (Gammoudi et al. ).
Diatomaceous earth comes from sedimentary rocks of biogenic origin in whose composition amorphous silica predominates. It consists of skeletons of aquatic organisms called diatoms. This material has a very complex structure, with numerous microscopic pores, cavities, and channels, which gives it a large specific surface, high porosity, and low density. It also has the advantage of being abundant and inexpensive, so its use as a commercial adsorbent has been explored (Sriram et al. ).
In this sense, this study proposes the modification of an Argentinian Mt, amorphous silica, and diatomaceous earth with two cationic surfactants of different chain length, to evaluate their potential in the remediation of NO À 3 and BrO À 3 ions present in water. The objective is to design a system that allows the adsorption of these anions on different inexpensive materials so that they can be eliminated from groundwater.  The amount of adsorbed NO À 3 , Q ADS , (mg NO À 3 g À1 clay) was determined according to:

Batch adsorption studies
where C i and C e are the initial and equilibrium anion concentration, respectively, V is the anion solution volume (mL) and m is the adsorbent mass (mg).
The adsorbent that attained the highest NO À 3 and BrO À 3 adsorption in aqueous solutions was also evaluated in water samples taken from the Puelche Aquifer, and the NO À 3 concentration of these water samples was measured before the adsorption test (

Effect of pH
The effect of pH on NO À 3 removal by Mt-H was evaluated by using 0.1 g Mt-H and 50 mL NO À 3 solution 100 mg L À1 (pH 7.7) added into the flasks. The pH of the solution was adjusted to 3.2 and 5.0 using HCl 0.1 M. The flasks were shaken for 24 h, and the solutions recovered after the centrifugation process of these samples were analyzed by IC.

Adsorption isotherms
To perform the adsorption isotherms, NO À 3 solutions were prepared by diluting a 1,000 mg L À1 stock solution in deionized water. The adsorption of NO À 3 was conducted using the best adsorbent determined in the batch adsorption studies (Mt-H sample). Then, 0.1 g of adsorbent and 50 mL Isotherms were fitted using different mathematical models widely applied in the liquid/solid adsorption processes, Langmuir, Freundlich, and Sips models.
The Langmuir model assumes monolayer adsorption on finite, identical, and equivalent sites. However, it does not predict lateral interactions or steric hindrance, even between adjacent adsorbate molecules (Foo & Hameed ). The mathematical expression of the Langmuir isotherm model is: where Q ADS (mg g À1 ) is the NO À 3 adsorbed amount, Q max (mg g À1 ) is the theoretical maximum adsorption capacity, This model describes non-ideal and reversible adsorption on a heterogeneous surface (Foo & Hameed ). The expression of the Freundlich equation is the following: where K F (L g À1 ) 1/n is the Freundlich constant related to the adsorbed capacity, and 1/n is a dimensionless number that where K S (L mg À1 ) is the Sips constant or affinity coefficient (Sandy et al. ).

Column filtration studies
The columns of different lengths were filled with a mixture of commercial quartz sand (Cicarelli, particle size 0.106-0.850 mm) and 2 wt% of the adsorbents. To prevent the filling loss, glass wool was placed in the lower and upper parts of the column. Then, the sand and adsorbent mixture, previously mixed manually, was slowly added to the column (filling length: 2.5, 3.0, and 3.5 cm).
The column was conditioned with the passage of a slow flow of deionized water from the bottom to top, to prevent the formation of preferential paths during elution. After the column was conditioned, a constant rate flow at 2.6 mL min À1 of a solution containing 100 mg L À1 of NO À 3 or 50 mg L À1 of BrO À 3 was passed through the column, and aliquots were taken at different filtered volumes to evaluate the removal of the respective anions. Figure S2 (Supplementary Material) shows the column diagram of the system used to remove the anions. In the diagram, there is a reservoir that contains the solution of NO À 3 or BrO À 3 that is passed through the column using a peristaltic pump.

Desorption experiments
Desorption experiments were carried out immediately after the adsorption by passing a solution of 1 M NaCl through the column at a flow rate of 2.5 mL min À1 . This column was reused with a new solution of NO À 3 100 mg L À1 . The effluent was collected at regular intervals, and this procedure was repeated twice.

Adsorbent characterization
The morphology of some absorbents was analyzed by SEM before and after the adsorption process, to observe changes in these materials.

Batch adsorption studies
Figure 2(a) and 2(b) show the removal (%R) of NO À 3 and BrO À 3 respectively. The results obtained showed a great increase in NO À 3 and BrO À 3 removal using the surfactantmodified adsorbents, which is almost zero in the unmodified adsorbents. This behavior could be related to the surface charge sign of each adsorbent, which is positive for those modified with surfactants (anion attraction) and negative for the unmodified ones (anion repulsion).
In the results obtained from Figure 2, it is important to note that there is a relation between the amount of surfactant loaded (see Table 1) and anion removal. In addition, higher NO À 3 removal was obtained with the surfactant with the largest chain length (O) in agreement with the results found in previous work (Jaworski et al. ).
The highest NO À 3 removal was achieved using the Mt modified with both surfactants (Mt-H and Mt-O), while the highest BrO À 3 removal was obtained with silica samples (Si-H1, Si-H1.3 and Si-O1).

Effect of pH
The effect of the pH on NO À 3 removal in a batch system using the adsorbent Mt-H was studied (Figure 3). The NO À 3 removal was performed at three different pH values: 3.2, 5.0, and 7.7. As shown in Figure 3, there are no significant changes in NO À 3 removal with the different pH values evaluated. This could be because the surface charge remains practically constant in all pH ranges (Figure 1).

Adsorption isotherms
The adsorption isotherm of NO À 3 on the Mt-H sample is shown in Figure 4. The amount of NO À 3 adsorbed increases rapidly at low initial concentrations. As the concentration increases, the saturation of the system is reached, and the adsorption remains constant. The Langmuir, Freundlich, and Sips isotherm   models were applied to determine the related parameters for NO À 3 adsorption on the Mt-H sample in aqueous media. The Langmuir, Freundlich, and Sips parameters obtained are summarized in Table 2. The best fit to the experimental adsorption points was obtained by the Sips model. This model predicts that the adsorption system, at low concentrations of NO À 3 , would behave in a heterogeneous form, as predicted also by the Freundlich model. However, at high NO À 3 concentrations, the adsorption would occur in a homogeneous form, and as a monolayer.
The Q max obtained with Sips fit was 18 mg g À1 , similar to the experimental value found, and the 1/n value was close to 1, which could indicate low surface heterogeneity.

Column adsorption studies
As was described previously, the synthesized adsorbents were evaluated in a batch system (Figure 2) using NO À 3 and BrO À 3 solutions prepared in deionized water. However, the use of batch systems to remove oxyanions from a large volume of water would be expensive. As a technological application, the column filtration systems are considered a better economic alternative. Column filtration systems using organo-Mt and sand mixtures have been previously reported to yield good results in perchlorate and thiophanate-methyl removal (Nir et al. ; Flores et al. ).
The first experiments were done with the column of length 2.5 cm, and the adsorbents in a proportion of 2 wt% concerning filling weight. The adsorbents used were D-H1, Si-H1, Mt-H, and Mt-O. With the column containing D-H1, the adsorbent eluted along the column with the NO À 3 solution, and with the column containing Si-H1, the surfactant was released from the silica and eluted through the column. Therefore, the results obtained with the adsorbents based on diatomaceous earth and silica are not shown in the column filtration studies. The column study using the Mt without surfactant is not included either. Due to the high water retention of this clay, the column swells, and it is difficult to take water samples.
The column filtration results of NO À 3 and BrO À 3 performed with Mt-H and Mt-O samples are presented in

Influence of the filling height
In order to evaluate the influence of the operational conditions on NO À 3 removal, the column filtration experiments were carried out at different filling heights ( Figure 6). The adsorption capacity increased with the filling length for the column that reached saturation (15.47 mg g À1 and 19.15 mg g À1 , for 2.5 cm and 3.0 cm, respectively). For the column with the highest filling height (3.5 cm), the saturation capacity was not reached in the filtered volume analyzed ( Figure 6) and for this reason, NO À 3 adsorption capacity was lower than that obtained for the shorter columns (11.71 mg g À1 ). The general trend is an increase in the adsorption capacity of NO À 3 with the increase in filling length, indicating that the longer contact time between NO À 3 and the adsorbent, generated by the length of the column filling, produced the increase of the adsorption capacity (Wu et al. ).

Desorption and reuse assay
To test the reusability of the Mt-H column, NaCl 1M was used as eluent. The NO À 3 was removed and then, the column was reused with a fresh NO À 3 solution. These adsorption-desorption cycles were repeated twice (Figure 7).
The obtained adsorption capacity values were 19.15 mg g À1 and up to 21.74 mg g À1 for initial adsorption, cycle 1 and 2, respectively.
In the first cycle, NO À 3 desorption was 72% after the passage of 37 mL of NaCl. This column was reused with a fresh NO À 3 solution, and the adsorption capacity increased up to 13.5%. In the second desorption cycle, its value was 51% after passing the same volume of 1 M NaCl as in the first cycle. However, by reusing the column, the adsorption capacity remained constant.
In order to further investigate the applicability of the Mt-H columns in environmental conditions, the column (filling height: 2.5 cm) was used with groundwater extracted from one of the largest aquifers in Argentina (Figure 8). The groundwater characteristics were: NO À 3 initial concentration of 60 mg L À1 and the presence of mostly bicarbonate.