J Radioanal Nucl Chem (2010) 286:287–293 DOI 10.1007/s10967-010-0652-z Radiolysis of 1-naphthol in aqueous solutions Thang M Ngo • Nam M Hoang • Tram T M Tran Received: 29 May 2010 / Published online: 22 June 2010 Ó Akade´miai Kiado´, Budapest, Hungary 2010 Abstract The effects of absorbed doses, initial pH and 1-naphthol concentration onto its radiolysis in aqueous sulphuric and hydrochloric acids by gamma rays from 60Co were investigated Under the experimental conditions, 1-naphthol degradation yields increased with increasing the absorbed doses (0.3–3.0 kGy) and with decreasing the initial 1-naphthol concentration (20–1 ppm) It was found out that the hydrated electrons did not play any significant roles in 1-naphthol radiolysis, as the degradation yields were higher at pH0 * 0.46 compared to those at pH0 * 2.0–5.0 The corresponding radiolytic yields G(-1-naphthol) were (6.13 ± 1.00)) 10-2 and (5.11 ± 0.22) 10-2 lmol/J in sulphuric acids, (15.61 ± 3.85) 10-2 and (4.76 ± 0.48) 10-2 lmol/J in hydrochloric acids 1-Naphthol degradation rates could be described by the kinetic equations of pseudo-first-order reactions An empirical relation between the observed reaction constants kD and the initial 1-naphthol concentrations was established, enabling to predict the absorbed doses required for a given treatment efficiency Three products of 1-naphthol degradation were revealed using an HPLC/UV procedure Keywords Gamma-radiolysis Á 1-Naphthol Á Water treatment T M Ngo (&) Á N M Hoang Á T T M Tran Faculty of Chemical Engineering, Ho Chi Minh City University of Technology, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Vietnam e-mail: nmthang@hcmut.edu.vn Introduction Recently, irradiation has gained more attention as a potential technological solution in water and wastewater treatment [1] For its practical application, knowledge about the radiolysis of pollutants in aqueous solutions is necessary Although the economically feasible radiation sources are still lacked, the decomposition of hydrocarbons, aromatic compounds and some herbicides by irradiation has been reported [1–8] In our previous work [7], carbaryl residues at concentrations up to *40 ppm in water could be readily decreased by bremsstrahlung at absorbed doses less than *3 kGy The rates of carbaryl radiolysis could be described by kinetic equations of pseudo-first-order reactions Some UV-absorbing products of carbaryl radiolysis were recognized by means of an HPLC/UV procedure However, no traces of 1-naphthol were revealed in all irradiated samples, though it had been considered the main product of carbaryl degradation under natural conditions [9] On the other side, 1-naphthol has been frequently used in chemical industries, e.g., in production of dyes, plastics, synthetic rubber, plant protecting formulations, etc The toxicity of 1-naphthol is considered similar to that of naphthalene and carbaryl [9, 10] Due to the presence of a hydroxyl group in its molecular structure, 1-naphthol solubility in water as well as its mobility in natural aquifers is enhanced The available literature data focused on sorption of 1-naphthol residues using different adsorbents, e.g., humins [11], biochars [12], polymers [13–15], carbon nanotubes [16], and composite silica [17] Besides, 1-naphthol degradation or transformation by photochemical [18–20] and catalytic oxidation methods [21–28] were investigated, too This paper dealt with radiolysis of 1-naphthol in aqueous solutions by gamma-irradiation The effects of 123 288 T M Ngo et al absorbed doses, initial 1-naphthol concentrations and pHvalues onto the degradation yields and degradation rates were investigated Experimental procedure 1-Naphthol C8H10O (p.a., Merck), concentrated H2SO4 (Merck), acetonitrile (HPLC grade, J.T Baker), and another reagents (p.a., China) were used as purchased Aqueous solutions were prepared and diluted according to the standard method for the examination of water and wastewater The pHs of solution were measured by a pH-meter METTLE TOLEDO model MP220, and adjusted by dropping diluted H2SO4, HCl or NaOH solutions The stability of 1-naphthol in solutions was controlled by measuring absorption spectra with an UV–VIS 1800 Spectrophotometer (Shimazdu) and cm quartz cell Twenty milliliters of aerated aqueous solutions were sealed in plastic bottles and irradiated by a 60Co source in Nuclear Research Institute Da Lat at temperature 28 °C and dose rate 6.20 kGy/h, the absorbed doses varied from 0.3 to 3.0 kGy (kJ/kg) 1-Naphthol concentrations in the reference and irradiated samples were analyzed by means of HPLC/UV The apparatus was Agillent 1100, operated with a Gemini reversed phase C-18 column (5 lm, 250 mm 4.6 mm, Phenomenex), and an UV detector set up at 212 nm The mobile phase was acetonitrile/water = 60/40 (v/v) solution, flowing at mL/min The analysis followed a calibration procedure using solutions of pure 1-naphthol at concentrations 0.16–32 ppm Concentrations of 1-naphthol in samples were determined from the peak areas according to the calibration line Degradation yields were determined as the ratios of peak areas acquired from irradiated (SD) and reference (S0) samples according to the equation: R% ¼ S0 À SD Â 100 S0 ð1Þ Fig UV-spectra of aqueous 1-naphthol solutions Fig Calibration line for analysing 1-naphthol in samples by an HPLC/UV procedure low as about 0.02 ppm 1-naphthol (*0.139 lmol/L) without any preconcentration steps A calibration line was obtained for 1-naphthol concentrations up to about 30 ppm (Fig 2) Results and discussions HPLC/UV procedure for 1-naphthol analysis Concentrations of 1-naphthol in aqueous samples are often analysed by HPLC/UV, with the UV-detectors setting up at 274 nm [13–15] or 254 nm [23, 28] However, such procedures suffered low sensitivity or required a preconcentration step by solid-phase extraction Figure revealed that the wavelength 212 nm is much more sensitive for quantifying 1-naphthol in ppm-levels Operating at this wavelength enabled to reproduce peak areas at concentrations as 123 Influence of absorbed doses and initial 1-naphthol concentrations Setting the initial solution pH * 5.0—a typical value for natural surface water, the initial 1-naphthol concentrations were varied in the range 1–20 ppm Throughout this concentration range, 1-naphthol was almost completely degraded in sulphuric and hydrochloric acids by irradiation with absorbed doses up to 3.0 kGy (Fig 3) This result is comparable with the almost total degradation of alachor in Radiolysis of 1-naphthol in aqueous solutions 289 Fig Effects of 1-naphthol initial concentrations on its degradation yields by gamma-irradiation in diluted sulphuric (a) and hydrochloric (b) acids aqueous solution irradiated by gamma rays from 60Co with an absorbed dosis about kGy [2] The degradation yields of 1-naphthol increased with decreasing its initial concentrations and with increasing the absorbed doses Similar trends were observed in our previous work dealing with carbaryl radiolysis in aqueous solutions [7] Figure demonstrates that irradiation is a promising alternative method for treating 1-naphthol at initial concentrations B*1 ppm, which resulted *98% degradation yields by absorbing doses 0.5–0.7 kGy Table indicates that the radiolytic yields of 1-naphthol obtained in this work were higher than those of carbaryl under similar experimental conditions These results strongly support our previous finding that 1-naphthol traces were not found in irradiated carbaryl solutions [7] Because even when carbaryl radiolysis generated 1-naphthol, the later itself would promptly undergo radiolysis, too Figure demonstrates that in almost all cases, the degradations yields of 1-naphthol in diluted sulphuric were comparable with the corresponding values in hydrochloric acids The later was about 22% enhanced comparing to the former only at low initial 1-naphthol concentration (*1 ppm) and low absorbed dosis (*0.3 kGy) It means that in this case, chloride ions themselves and/or their interactions with products of water radiolysis significantly contributed to 1-naphthol degradation In fact, these interactions are well known in the literature [29]: Cl ỵ OH ỵ H3 Oỵ ! Cl ỵ H2 O k ¼ 1:7 Â 1010 L2 molÀ2 sÀ1 2ị Cl ỵ Cl ! Cl 3ị Reaction (2) consumes some OH radicals and could negatively affect the degradation yields of 1-naphthol On the other side, both atomic and ion radicals Cl; ClÀ are strong oxidants, which could positively contribute to the degradation yields of 1-naphthol At very low concentrations (*10-5 mol/L) of ClÀ and H3 Oỵ ; these effects appeared very weak and could not be revealed at higher absorbed doses or higher 1-naphthol concentrations At very low 1-naphthol concentrations and absorbed doses, the probability for 1-naphthol molecules and OH radicals to collide each other became substantially lower In such cases, reactions (2) and (3) increased 1-naphthol degradation yields, while they transformed OH radicals to active oxidants Cl; ClÀ It is worth to note that even UV-photolysis of 1-naphthol was enhanced in chloride solutions In our previous work [20], 1-naphthol photodegradation yields were increased by about 10–20% in the presence of 0.1 mol/L Cl- While UV is substantially weaker than gamma-irradiation, the effect of chloride ions could be observed at a higher initial concentration (20 ppm 1-naphthol) and longer UV-photolysis times (1–4 h) Presenting the experimental results in terms of ln(C/C0) versus D (Fig 4), it is clear that under the experimental Table Comparison of radiolytic yields of 1-naphthol and carbaryl pH0 * 2–5 G(-1-naphthol), 102 lmol/J G(-carbaryl), 102 lmol/J pH0 * 0.46 HCl H2SO4 HCl 4.76 ± 0.48 5.11 ± 0.22 15.61 ± 3.85 – 2.28 ± 0.31 H2SO4 – 6.13 ± 1.00 3.42 ± 0.62 123 290 T M Ngo et al Influence of initial pH-values Figure 6a demonstrates that 1-naphthol degradation yields in sulphate medium is only slightly pH-dependent, when the absorbed doses B1.0 kGy This finding correlated to the almost constant yields of water radiolysis products throughout this pH-range [2] As at pH * 0.46, the disappearance of hydrated electrons and the increased radiolytic yield of hydrogen atoms did not affect the degradation yields, their roles in 1-naphthol radiolysis could be considered negligible Similar results were obtained when carbaryl radiolysis was investigated in sulphate medium [7] Considering that in more concentrated sulphuric acids, reactions (4) could take place [29], one can conclude that reactions (4) did not affect 1-naphthol degradation by hydroxyl radicals Fig Effects of the initial sample pH on 1-naphthol degradation yields by gamma-irradiation in 0.4 M sulphuric (a) and hydrochloric (b) acids conditions investigated, 1-naphthol radiolysis obeyed the kinetic rules of pseudo-first-order reactions, with the observed reaction constants kD depending on its initial concentrations Under the experimental conditions investigated, this dependence proved to be linear in semi-logarithmic plots (Fig 5) Similar results were obtained in radiolysis of aqueous solution of carbaryl [7] From the practical point of view, the empirical equation obtained is meaningful: For a given initial 1-naphthol concentration, it serves to generate a corresponding reaction constant The later further serves to calculate absorbed doses/irradiation time for the required treatment efficiency À H2 SO4 =HSO ỵ OH ! HSO4 ị = SO4 ị þ H2 O k % 106 L molÀ1 sÀ1 ð4Þ In hydrochloric acids, however, a tendency to increase 1-naphthol degradation yields with increasing the initial HCl concentration could be recognized even in the range 10-5–10-2 M, reaching significantly higher values at *0.4 M HCl (Fig 6b) This effect could be ascribed to the increase of both chloride- and hydroxonium ion concentrations, which underwent reactions (2) and (3) and therefore enhanced 1-naphthol degradation yields The different results acquired in sulphuric and hydrochloric acids might be due to the difference in reaction constants, which is about four orders of magnitude between reactions (2) and (4) Degradation products Fig Reaction rates of 1-naphthol radiolysis in investigated samples 123 Besides the degradation yields and rates, the intermediates and end-products have gained more attentions when considering a treatment method [2, 7, 21, 30, 31] For their identification and/or characterization, applications of suitable analytical methods and theoretical calculations are necessary However, even the simple HPLC/UV procedure used in this work enabled to make some remarks about 1-naphthol radiolytic products Figure shows at least three UV-absorbing degradation products of 1-naphthol radiolysis with retention time shorter, i.e., they are more polarized than its precursor There are no peaks with retention time longer than that of 1-naphthol All peaks of degradation products increased with increasing absorbed doses up to 3.0 kGy, except for the peak with retention time \*2 min, which turned to decrease at absorbed doses C*1.0 kGy Because this HPLC/UV procedure generated a peak with retention time *1.5 for diluted samples od NaHCO3 as well as of NaNO3, we suppose 1-naphthol Radiolysis of 1-naphthol in aqueous solutions 291 Fig Empirical relation ln kD versus C0,naphthol Fig Chromatograms of 1-naphthol (20 ppm) in irradiated 10-6 M H2SO4 solutions Fig Chromatograms of 1-naphthol (10 ppm) in irradiated 0.4 M HCl solutions 123 292 was partly mineralised to NaHCO3, which at higher doses disappeared as CO2 escaped from the irradiated samples Nevertheless, the remaining two peaks with retention times *2.2–3.6 indicated that a great deal of degradation products remains organics which seem more resistant towards irradiation comparing to 1-naphthol Some paper reported dimers and trimers as products of catalytic oxidation of 1-naphthol However, their oxidising conditions were milder, and their conclusions based solely on the UV–VIS spectra [26–28] Similar results were obtained at another pH0 (2.0–5.0) as well as with solution 0.4 M H2SO4 However, the results obtained with solution 0.4 M HCl were somewhat different (Fig 8) As already discussed above, 1-naphthol degradation yields and degradation rates in hydrochloric acids were higher due to reactions (2) and (3) In principle, products of reactions (2) and (3) could generate some chlororganic derivates which are not desirable in water treatment processes Figure shows that the earliest peak (t \ min) almost disappeared even at low absorbed doses On the other side, some small peaks of less polarized products were observed Fortunately, they disappeared at doses C1 kGy The remaining peaks appeared at similar retention times and their areas increased with increasing the absorbed doses as in solutions H2SO4 This finding indicates that the degradation products of 1-naphthol in both hydrochloric and sulphuric acids remained almost the same However, more evidences are necessary to confirm this statement, while it is very important from the environment point of view Conclusion Under the experimental conditions investigated, up to *20 ppm 1-naphthol in aqueous solutions could be effectively treated by c irradiation at low absorbed doses B3.0 kGy The degradation yields increased with increasing doses and with decreasing initial 1-naphthol concentration In both sulphuric and hydrochloric acids, 1-naphthol degradation yields were not 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