Removal of Boron by Surfactant Modified Zeolitic Tuff from Northeastern Greece

Boron adsorption on surfactant modified zeolite (SMZ) was investigated, in the perspective of using SMZ for B removal from irrigation waters, since SMZ has been proven efficient adsorbent for many anions. A calcareous clinoptilolite was used and a sub-sample was treated with HCl. Then the charge of the external surface of the untreated and treated zeolite was modified and B adsorption was studied. The SMZ adsorbed B and its adsorption capacity was increased with HCl treatment and pH adjustment at the strongly alkaline range. However, in all cases the relative amounts of B removed from solution were much lower than other anions reported in the literature. In addition, B concentration in the equilibrium solution was above 3 mg L, which is considered a critical limit for B in irrigation waters in respect to the risk of B phytotoxicity. It was concluded that SMZ is not an effective means for B removal from irrigation waters and thus its use for B water treatment is not recommended.


Introduction
High B concentration in irrigation water can lead to B accumulation in soils causing phytotoxicity. Consequently, B concentration in water is one of the parameters that determine the suitability of water for irrigation (Nable et al., 1997). In general, B concentrations in irrigation water higher than 3 mg L -1 can cause adverse effects on most crops (FAO, 1985). Specifically, according to Scofield (1936) the permissible B concentration in irrigation water is ≤ 1 mg L -1 for sensitive crops, ≤ 2 mg L -1 for semi-tolerant crops and ≤ 3 mg L -1 for tolerant crops. Furthermore, according to Keren and Bingham (1985) the permissible B concentration ranges in irrigation water are 0.3 to 1 mg L -1 for sensitive crops, 1 to 2 mg L -1 for semi-tolerant crops and 2 to 4 mg L -1 for tolerant crops. Several studies concerning treatments of water to reduce B concentration are reported in the literature which include chemical methods, such as the adsorption using adsorbents like lime, alum, ferric salts (Waggot, 1969) and compounds of Ca, Mg and Na (Dionisiou et al., 2006;Okay et al., 1985), precipitation, membrane filtration, reverse osmosis, use of B selective resins and use of aquatic plants (Chieng & Chong, 2012). Among the previously mentioned methods, adsorption is attracting more attention, due to its simplicity and low cost (Ozturk & Kavak, 2005).
The objectives of this study were to initially investigate B adsorption on a SMZ, under different conditions of zeolite's pretreatment, prior its surface modification, and of pH and B concentration in the external solution, in the perspective of using this material for B removal from irrigation waters.

Pretreatment and Surface Modification of the Zeolite
The zeolite used was a clinoptilolite with the chemical formula (Ca 1.5 K 1.4 Mg 0.6 Na 0.5 ) (Al 6.2 Si 29.8 O 72 )·20H 2 O, obtained from the Greek region Petrota, a village of the prefecture of Evros in Greece (Filippidis, 2010;Filippidis & Kantiranis, 2007) and its chemical composition, as it was determined by the Department of Mineralogy-Petrology-Economic Geology of the Aristotle University of Thessaloniki, using X-ray diffraction analysis is presented in Table 1. The zeolite was sieved through a 2 mm sieve and this material was used for all analyses and B adsorption experiments, after its pretreatment and modification of the surface charge. The total and the external CEC of the zeolite were determined employing the method of Ming and Dixon (1987) and were 226 and 10 cmol c kg -1 , respectively (Table 1).
Certain quantity of the natural zeolite was treated with 1 Μ HCl once, to improve its subsequent surface modification and thus to increase its adsorbing capacity for B. Specifically, a quantity of the zeolite, sufficient to conduct the following experiments, was equilibrated with 1 M HCl, at a ratio 1 / 20 (w / v), initially for 2 h, at 60 ± 1 °C and then for 24 h, at 25 ± 1 °C, with intermittent shaking. Then, the zeolite was washed with distilled water and dried at 100 ± 1 °C.
The modification of the surface charge of the untreated and treated zeolite was accomplished using hexadecyltrimethylammonium bromide (C 19 H 42 BrN or HDTA-Br), according to Bowman et al. (2000). Specifically, a quantity of the untreated and treated zeolite, sufficient to conduct all the B adsorption experiments, was equilibrated with 0.05 M HDTMA-Br, at a ratio 1 / 4 (w / v), for 24 h, at 25 ± 1 °C, with intermittent shaking. The solution of the 0.05 M HDTMA-Br was added to the zeolite at an amount of the HDTMA-Br equivalent to twice the external CEC of the zeolite. At the end of the equilibration time, the SMZ was washed with distilled water and air-dried. External Cation Exchange Capacity (cmol c kg -1 ) 10 SiO 2 (g kg -1 ) † 679 Al 2 O 3 (g kg -1 ) † 120 CaO (g kg -1 ) † 31 MgO (g kg -1 ) † 8.9 Na 2 O (g kg -1 ) † 5.1 Si/Al † 4.8 † The chemical composition of the zeolite was determined by the Department of Mineralogy-Petrology-Economic Geology of the Aristotle University of Thessaloniki.

Boron Adsorption Experiments
Initially, B adsorption experiments were conducted employing the natural zeolite, without any treatment or surface charge modification. However, the results showed that the amounts of the B adsorbed on the natural zeolite were negligible (data not shown). Thus, only the B adsorption experiments on the SMZ (untreated and treated) are described below and their results are presented in the subsequent section. www.ccsenet.org/jas Journal of Agricultural Science Vol. 5, No. 12; Boron adsorption data were obtained by a slight modification of the technique used by Li et al. (2007), at two pH values, namely 7.0 ± 0.1 and 9.5 ± 0.1 for the untreated SMZ and 3.0 ± 0.1 and 9.5 ± 0.1 for the treated SMZ. Specifically, five g sub-samples of the SMZ (untreated or treated), in two replications, were placed in polypropylene centrifuge tubes with 20 mL of deionized water containing B at concentrations 0, 5, 10 and 15 mg L -1 , as H 3 BO 3 . The suspensions were equilibrated for 24 h, at 25 ± 1 °C, with intermittent shaking. At the end of the equilibration period the tubes were centrifuged and the supernatant liquid was filtered and analyzed for B by the azomethine-H method (John et al., 1975), using a UV-Vis spectrometer (Lambda 5, Perkin-Elmer). Adsorbed B was calculated as the difference between the amount added and that found in solution at equilibrium.
For the amounts of adsorbed B, within each B concentration in the external solution, analysis of variance (ANOVA) was conducted and LSD test was used for mean comparisons.

Results and Discussion
The amounts of B adsorbed on the untreated SMZ at the alkaline pH were significantly higher than the amounts of B adsorbed at the neutral pH, in all cases of the initial B concentration in the external solution studied (Table 2).
Less than 17 % of the added B was removed by the SMZ when the pH of the equilibrium solution was 7.0 ± 0.1, whereas 24 % of the added B was removed at pH 9.5 ± 0.1.

It is well known that B exists in solution with two chemical forms mainly, i.e. boric acid (H 3 BO 3 ) and borates anions [Β(ΟΗ) 4 -]
, and the proportion of these two forms is pH depended. Since pKa of H 3 BO 3 is 9.24, H 3 BO 3 is the dominant B chemical species at acidic and neutral pH, whereas as pH increases to the alkaline range, Β(ΟΗ) 4 − are increased at appreciable amounts and become the dominant B chemical species at the strongly alkaline pH range (pH = 9.24) (Goldberg, 1993).
According to Li and Bowman (1997) the anion adsorption mechanism on SMZ (with HDTMA-Br) is anion exchange, meaning the anions are exchanged for Broriginating from the HDTMA-Br on the surface of the zeolite and held by electrostatic bond. Based on the above, the lower amounts of B adsorbed on the untreated SMZ at pH 7.0 ± 0.1 compared to the amounts adsorbed at pH 9.5 ± 0.1, could be attributed to the different B chemical species. Apparently, the SMZ exhibited lower affinity for the neutral H 3 BO 3 , which is the dominant B chemical species at a neutral pH value, than the univalent Β(ΟΗ) 4 -, which is the dominant B chemical species at the strongly alkaline pH range.
Similarly to the untreated SMZ, the amounts of B adsorbed on the treated SMZ at the alkaline pH were significantly higher than the amounts of B adsorbed at the acidic pH, in all cases of the initial B concentration in the external solution studied (Table 2). Less than 16 % of the added B was removed by the SMZ when the pH of the equilibrium solution was 3.0 ± 0.1, whereas 14-47 % of the added B (depending on the initial B concentration in the external solution) was removed at pH 9.5 ± 0.1. Moreover, at the alkaline pH value, the amounts of B adsorbed on the treated SMZ were significantly higher than the untreated SMZ, in almost all cases (Table 2). This was an indication that the acid treatment of the zeolite prior its surface charge modification increased SMZ's adsorption capacity for Β(ΟΗ) 4 -, which is the predominant B species at a such alkaline pH value.
In addition, in all cases of the initial B concentration in the external solution, B in the equilibrium solution was ranged from 3.8 ± 0.2 to 12.6 ± 0.3 mg L -1 , meaning it was higher than the value of 3 mg B L -1 , which is considered a critical limit for B in irrigation waters, since B concentrations higher than this value are expected to have undesirable effects on most crops, according to Scofield (1936), Keren and Bingham (1985) and FAO (1985). www.ccsenet.org/jas Journal of Agricultural Science Vol. 5, No. 12;

Conclusion
In conclusion, the SMZ adsorbed B and its adsorption capacity increased with pH increase and treatment with HCl (before its surface modification). However, in all cases, the relative amounts of B adsorbed on the SMZ were low compared to the amounts of other anions adsorbed on the same material or to B amounts adsorbed on other materials. Furthermore, B concentration in the equilibrium solution was above the critical limit of 3 mg L -1 for irrigation waters. Consequently, it seemed that SMZ is not an effective means for B removal from irrigation waters; therefore its use for water treatment in respect to B is not recommended.