DSpace at VNU: Radiolysis of 1-naphthol in aqueous solutions

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 on

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 from60Co

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

signifi-cant 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)) 9 10-2 and (5.11 ±

0.22) 9 10-2 lmol/J in sulphuric acids, (15.61 ± 3.85)

9 10-2and (4.76 ± 0.48) 9 10-2lmol/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

estab-lished, 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 1-Naphthol 

Water treatment

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 hydrocar-bons, aromatic compounds and some herbicides by irradi-ation 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 pro-duction 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 aque-ous solutions by gamma-irradiation The effects of

T M Ngo ( &)  N M Hoang  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

DOI 10.1007/s10967-010-0652-z

METTLE TOLEDO model MP220, and adjusted by

drop-ping diluted H2SO4, HCl or NaOH solutions The stability of

1-naphthol in solutions was controlled by measuring

absorption spectra with an UV–VIS 1800

Spectrophotom-eter (Shimazdu) and 1 cm quartz cell

Twenty milliliters of aerated aqueous solutions were

sealed in plastic bottles and irradiated by a60Co 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

irradi-ated 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 9 4.6 mm,

Phenomenex), and an UV detector set up at 212 nm The

mobile phase was acetonitrile/water = 60/40 (v/v)

solu-tion, flowing at 1 mL/min The analysis followed a

cali-bration 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  SD

S0

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

pro-cedures suffered low sensitivity or required a

preconcen-tration step by solid-phase extraction Figure1revealed that

the wavelength 212 nm is much more sensitive for

quanti-fying 1-naphthol in ppm-levels Operating at this

wave-length enabled to reproduce peak areas at concentrations as

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)

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 con-centration 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

Fig 1 UV-spectra of aqueous 1-naphthol solutions

Fig 2 Calibration line for analysing 1-naphthol in samples by an HPLC/UV procedure

aqueous solution irradiated by gamma rays from60Co with

an absorbed dosis about 5 kGy [2] The degradation yields

of 1-naphthol increased with decreasing its initial

concen-trations and with increasing the absorbed doses Similar

trends were observed in our previous work dealing with

carbaryl radiolysis in aqueous solutions [7] Figure3

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

Table1indicates 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

Figure3 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

inter-actions are well known in the literature [29]:

ClþOHþ H3Oþ!Cl þ 2 H2O

Reaction (2) consumes someOH radicals and could nega-tively affect the degradation yields of 1-naphthol On the other side, both atomic and ion radicalsCl;Cl2 are strong oxidants, which could positively contribute to the degrada-tion yields of 1-naphthol At very low concentradegrada-tions (*10-5mol/L) of Cland H3Oþ; 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 proba-bility for 1-naphthol molecules andOH radicals to collide each other became substantially lower In such cases, reac-tions (2) and (3) increased 1-naphthol degradation yields, while they transformed OH radicals to active oxidants

Cl;Cl2 It is worth to note that even UV-photolysis of 1-naphthol was enhanced in chloride solutions In our pre-vious 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-irradi-ation, 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

Fig 3 Effects of 1-naphthol initial concentrations on its degradation yields by gamma-irradiation in diluted sulphuric (a) and hydrochloric (b) acids

Table 1 Comparison of radiolytic yields of 1-naphthol and carbaryl

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

inves-tigated, this dependence proved to be linear in

semi-loga-rithmic 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

reactions (4) did not affect 1-naphthol degradation by hydroxyl radicals

H2SO4=HSO4 þOH!ðHSO4Þ =ðSO

4Þ þ H2O

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

10-5–10-2M, 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 con-centrations, 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

Besides the degradation yields and rates, the intermediates and end-products have gained more attentions when con-sidering a treatment method [2, 7, 21, 30, 31] For their identification and/or characterization, applications of suit-able 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 Figure7 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 min for diluted samples

od NaHCO3as well as of NaNO3, we suppose 1-naphthol

Fig 4 Effects of the initial sample pH on 1-naphthol degradation

yields by gamma-irradiation in 0.4 M sulphuric (a) and hydrochloric

(b) acids

Fig 5 Reaction rates of 1-naphthol radiolysis in investigated

samples

Fig 6 Empirical relation ln kDversus C0,naphthol

Fig 7 Chromatograms of 1-naphthol (20 ppm) in irradiated 5 9 10-6M H2SO4solutions

Fig 8 Chromatograms of 1-naphthol (10 ppm) in irradiated 0.4 M HCl solutions

tion 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

pro-cesses Figure8 shows that the earliest peak (t \ 2 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

absor-bed doses as in solutions H2SO4 This finding indicates that

the degradation products of 1-naphthol in both

hydro-chloric 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

effec-tively 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

deg-radation yields were not significantly affected by the initial

pH-values in the range 2.0–5.0 The degradation rates obey

kinetics of pseudo-first-order reactions with the observed

reaction constants kD, which were found proportional to

initial 1-naphthol concentrations in semi-logarithmic plots

From this empirical equation, one can calculate the

corre-sponding constants kDand the absorbed doses necessary to

reach the predetermined treating efficiencies The

chro-matograms revealed three UV-absorbing degradation

prod-ucts which are more polarized compared to 1-naphthol

Efforts are carried out to identify the degradation products

and to investigate effects of other inorganic and organic

components in samples

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