Removal of heavy metals from water by electron-beam treatment in the presence of an hydroxyl radical scavenger

摘要

Mendeleev Communications Electronic Version, Issue 2. 1997 (pp. 47ndash;86) Removal of heavy metals from water by electron-beam treatment in the presence of an hydroxyl radical scavenger Alexei K. Pikaev,*a Lidiya I. Kartasheva,a Tatrsquo;yana P. Zhestkova,a Tamara K. Yurik,a Vladimir N. Chulkov,a Oleg A. Didenko,a Duk Kyung Kim,b Yuri Kimb and Bumsoo Hanb a Institute of Physical Chemistry, Russian Academy of Sciences, 117915 Moscow, Russian Federation.Fax: +7 095 335 1778 b Samsung Heavy Industries Co., Daeduk Ramp;D Center, Daejeon, 305-600, Republic of Korea. Fax: +82 042 865 4314 Electron-beam treatment in combination with the use of formate as an hydroxyl radical scavenger and subsequent filtration has been developed for the removal of heavy metals (cadmium, lead, chromium) from water.One of the most important environmental problems is the removal of heavy metals from waste water. We used an electron-beam technique, which is applicable for the purification of various waste waters, (see e.g. refs. 1ndash;4) to remove these metals from the water. The method is based on radiation-chemical reduction of the metal ions to their respective metals or to lower oxidation state ions which can then be removed by filtration.Two requirements are necessary: the absence of oxygen in the water (in the case of CdII and PbII) and scavenging of OH radicals which can oxidize the reduced metal ions. The reduction, upon electron-beam treatment, can be a result of reactions of the ions with hydrated electrons eaq ndash; and H atoms formed from water radiolysis.5 For example, in the case of CdII ions, it is possible to write: If the water is saturated with air, oxygen reacts with eaq ndash; and H equations (4)ndash;(6), partially or completely suppressing reactions (1) and (2).Since the reactions of HO2 and O2 ndash; with CdII and PbII are comparatively slow, upon electron-beam treatment they can combine forming hydrogen peroxide: and/or react with reduced metal ions: Hydrogen peroxide can also oxidize such ions: The reduction takes part after the consumption of oxygen, and so to decrease the required dose air should be removed from water (for example, by bubbling inert gas) before and/or during electron-beam treatment.Hydroxyl radicals formed from water radiolysis and in reaction (9) react with reduced ions: In the case of PbII ions, hydroxyl radicals can react with them forming PbIII (see e.g.ref. 5): Then reaction (12) can proceed: Hydrogen peroxide is also a water radiolysis product; together with the H2O2 produced from reactions (7) and (8) it can oxidize reduced ions. The overall effect will be that the metal ions will not be reduced to the free metal. To exclude the negative influence of hydroxyl radicals it is possible to use a scavenger which converts the hydroxyl radicals into reducing species. Such a scavenger is formate ion.In reaction with hydroxyl radicals it gives COOndash; radical ion which can then reduce metal ions: The carbon dioxide produced in reactions (14)ndash;(16) is not toxic, yet the use of other hydroxyl radical scavengers can give rise to the formation of toxic compounds.For instance, ethyl and isopropyl alcohols which are also utilized as scavengers can form acetaldehyde and acetone, respectively. In addition, R HOH radicals formed can oxidize CdI. We studied the removal of cadmium, lead and chromium from model aqueous solutions. The first two metals were initially in the form of bivalent ions, and chromium was in the form of a chromate ion.CdII and PbII were reduced to Cd0 and Pb0, and CrVI was reduced to CrIII. The source of the ionizing radiation was a linear U-12 electron accelerator (electron energy 5 MeV, pulse duration 2.3 ms). Doses were measured with an ordinary or modified Fricke dosimeter (in a dependence on dose rate).6 The total dose was varied by changing the amount of electron pulses or by changing the irradiation duration when a series of pulses (400Hz) were used.The concentrations of cadmium and lead were determined using a Perkin-Elmer lsquo;Plasma-40rsquo; atom absorption spectrometer. Analysis of CrVI was performed spectrophotometrically from the optical density of the solution at 373nm (pH 10ndash;11, molar extinction coefficient 461 m2 molndash;1,7,8 the solution was filtered if CrIII hydroxide precipitate was formed) using a lsquo;Specord M-40rsquo; spectrophotometer.All reagents were analytical grade. The solutions were aerated and deaerated. The removal of air from the solutions was conducted by bubbling purified argon for 30ndash;40min. The precipitates formed upon irradiation were CdII + eaq ndash; CdI CdII + H CdI + H+ CdI + CdI CdII + Cd0 (1) (2) (3) eaq ndash; + O2 O2 ndash; H + O2 HO2 HO2 H+ + O2 ndash; (4) (5) (6) HO2 + O2 ndash; + H+ H2O2 + O2 (7) CdI + HO2 + H+ CdII + H2O2 (8) CdI + H2O2 CdII + OH + OHndash; (9) CdI + OH CdII + OHndash; (10) PbII + OH PbIII + OHndash; (11) PbIII + PbI 2PbII (12) HCOOndash; + OH COOndash; + H2O CdII + COOndash; CdI + CO2 CdI + COOndash; Cd0 + CO2 COOndash; + O2 CO2 + O2 ndash; (13) (14) (15) (16) C middot; Figure 1 Dependence of CdII concentration in deaerated aqueous solution containing 5times;10ndash;3 mol dmndash;3 formate on the dose of electron radiation. 2.5 2.0 1.5 1.0 0.5 0 1 2 3 4 CdII /mg dmndash;3 Dose/kGyMendeleev Communications Electronic Version, Issue 2. 1997 (pp. 47ndash;86) filtrated using glass filters or were centrifuged. Figure 1 shows the dependence of the change in CdII concentration in the deaerated solution containing ca. 2 mg dmndash;3 CdII and 5times;10ndash;3 mol dmndash;3 formate on the dose of electron radiation.A dose of ca. 3.5 kGy leads to a decrease in CdII concentration below the permitted level of 0.1 mg dmndash;3.9 Note that irradiation of aerated solution at such a concentration of CdII did not give a positive result. At sufficiently high concentrations of CdII (for instance, at 50mgdmndash;3) the required removal level is reached even in aerated solutions.Under this condition reactions (1) and (2) can compete with the interaction of oxygen with eaq ndash; and H. Similar results were obtained with solutions of PbII. The dependence of the decrease in PbII concentration in aerated solution containing ca. 5 mg dmndash;3 PbII in the absence of formate (curve 1) and in the presence of 10ndash;2 mol dmndash;3 formate (curve 2) on the dose of electron irradiation is shown in Figure 2.In the absence of formate the removal of lead does not take place. The induction period is characteristic of the dependence expressed by curve 2. Apparently, it is caused by the presence of oxygen in the solution, and is finished after oxygen consumption. The dose required to remove ca. 5 mg dmndash;3 PbII to the concentration less than 1 mg dmndash;3 (a permitted level of lead content9 in disposed waste waters) is ca. 0.7 kGy. Slightly higher doses (1.0ndash;1.2 kGy) are required for the removal of 10ndash;20 mg dmndash;3 PbII. Chromate in aqueous solution is reduced to CrIII upon electron-beam irradiation. The process occurs in deaerated and aerated solutions, in the absence and the presence of formate (see Figure 3). However, in aerated solutions in the absence of formate only 10ndash;30 reduction occurs.Such a low degree of reduction seems to be caused by reactions (4)ndash;(6); HO2 and O2 ndash; radicals do not reduce CrVI ions in a neutral medium, but oxidize transient reduced species. In the presence of formate the process is very effective even in aerated solutions. Apparently, the COOmiddot;ndash; radical ion formed in reaction (13) is a stronger reducing agent towards CrVI than HO2 and O2 ndash; radicals.Note that the residual CrVI after a radiation dose of ca. 3.5 kGy is ca. 0.05 mg dmndash;3. Chromium(III) is precipitated as the hydroxide. The medium for precipitation of CrIII in this form is slightly-alkaline (pH 8.5ndash;9.5, see e.g. ref. 10). Hence, electron-beam treatment in combination with the use of formate as an hydroxyl scavenger and subsequent filtration or centrifugation can be used for the removal of cadmium, lead and chromium from water.References 1 A. K. Pikaev, Khim. Vys. Energ., 1994, 28, 5 High Energy Chem. (Engl. Transl.), 1994, 28, 5. 2 R. J. Woods and A. K. Pikaev, Applied Radiation Chemistry: Radiation Processing, Wiley, New York, 1994. 3 A. K. Pikaev, Usp. Khim., 1995, 64, 609 (Russ.Chem. Rev., 1995, 64, 569). 4 C. N. Kurucz, T. D. Waite and W. J. Cooper, Radiat. Phys. Chem., 1995, 45, 299. 5 A. K. Pikaev, Sovremennaya radiatsionnaya khimiya. Radioliz zhidkostei i gazov (Modern Radiation Chemistry. Radiolysis of Liquids and Gases), Nauka, Moscow, 1986 (in Russian). 6 A. K. Pikaev, Dosimetriya v radiatsionnoi khimii (Dosimetry in Radiation Chemistry), Nauka, Moscow, 1975 (in Russian). 7 A.K. Lavrukhina and L. V. Yukina, Analiticheskaya khimiya khroma (Analytical Chemistry of Chromium), Nauka, Moscow, 1979 (in Russian). 8 Yu. Yu. Lurrsquo;e, Rukovodstvo po analiticheskoi khimii (Handbook of Analytical Chemistry), Khimiya, Moscow, 1979 (in Russian). 9 Ya. M. Grushko, Vrednye neorganicheskie soedineniya v promyshlennykh stochnykh vodakh (Toxic Inorganic Compounds in Industrial Waste Waters), Khimiya, Leningrad, 1979 (in Russian). 10 V. E. Ternovtsev and V. M. Pukhachev, Ochistka promyshlennykh stochnykh vod (Purification of Industrial Waste Waters), Budivelrsquo;nik, Kiev, 1986 (in Russian). Figure 2 Dependence of PbII concentration in aerated aqueous solution in the absence of formate (1) and in the presence of 10ndash;2 moldmndash;3 formate (2) on the dose of electron radiation. 5 4 3 2 1 0 0.5 1.0 1.5 PbII/mg dmndash;3 Dose/kGy 1 2 Figure 3 Dependence of CrVI concentration in aerated neutral aqueous solutions in the absence of formate (1) and in the presence of 9.6times;10ndash;3 mol dmndash;3 formate (2) and in deaerated neutral aqueous solution (3) in the absence of formate on the dose of electron radiation. 6 4 2 0 1 2 3 4 CrVI /mg dmndash;3 Dose/kGy 1 2 3 Received: Moscow, 15th November 1996 Cambridge, 7th January 1997; Com. 6/08036G