Photooxidation of Hg(0) in Artificial and Natural Waters

Photooxidation of Hg(0)in Artificial and Natural Waters

J A N I C K D .L A L O N D E ,*,?M A R C A M Y O T ,?A N N E M .L .K R A E P I E L ,?A N D F R A N C ¸O I S M .M .M O R E L §INRS-Eau,Universite ´du Que ´bec,2800rue Einstein,CP 7500Sainte-Foy (Que ´bec),G1V 4C7Canada,E Äcole et Observatoire des Sciences de la Terre,Universite ´Louis Pasteur,1rue Blessig,67084Strasbourg,France,and Department of Geosciences,Princeton University,Guyot Hall,Princeton,New Jersey 08544

The oxidation of volatile aqueous Hg(0)in aquatic systems may be important in reducing fluxes of Hg out of aquatic systems.Here we report the results of laboratory and field experiments designed to identify the parameters that control the photooxidation of Hg(0)(aq)and to assess the possible importance of this process in aquatic systems.The concentrations of elemental and total Hg were measured as a function of time in both artificial and natural waters irradiated with a UV-B lamp.No change in Hg speciation was observed in dark controls,while a significant decrease in Hg(0)was observed in UV-B irradiated artificial solutions containing both chloride ions and

benzoquinone.Significant photooxidation rates were also measured in natural samples spiked with Hg(0)(aq);the

photooxidation of Hg(0)then follows pseudo first-order kinetics (k )0.6h -1).These results indicate that the previously observed Hg(II)photoreduction rates in natural waters could represent a net balance between Hg(II)photoreduction and Hg(0)photooxidation.As calculated from Hg(0)

photooxidation rates,the dominant Hg(0)sink is likely to be photooxidation rather than volatilization from the water column during summer days.

Introduction

Redox reactions of mercury are important in determining the fate of Hg in aquatic systems.The total mercury con-centration in a water body is partly controlled by direct depo-sition from the atmosphere of the oxidized form,Hg(II),and by volatilization of the reduced form,Hg(0)(1).The loss of Hg from the water column is therefore enhanced by in situ Hg(II)reduction and decreased by Hg(0)oxidation.

While reduction,particularly photoreduction,of Hg(II)in aquatic systems has been fairly well studied (2-6),oxi-dation has been largely ignored because Hg(0)is typically considered to be unreactive.Nevertheless,recent laboratory experiments have demonstrated that,in oxygenated solutions containing sufficient chloride concentrations,liquid Hg(0)is effectively oxidized in the dark (7,8).But it is not known if these results obtained with liquid drops of Hg(0)are relevant to natural waters containing low concentrations of dissolved Hg(0),Hg(0)(aq).Furthermore,the reaction mechanisms are unclear.

In a field study in the coastal waters of the Gulf of Mexico,Amyot et al.(9)have observed that Hg(0)(aq)can undergo (dark)oxidation at a significant rate.In experiments with river water,these authors showed that the oxidation rate of Hg(0)(aq),like that of liquid Hg(0),is clearly enhanced in the presence of high Cl -concentrations and also appears to depend on the presence of unidentified particles or colloids that can be removed by filtration.These experiments were partly confounded,however,by the loss of mercury from solution,either by volatilization of Hg(0)to the headspace of the container and/or adsorption of Hg(II)on the container walls.

In experiments designed to study the photoreduction of Hg(II)in field samples,it is common to observe a plateau in Hg(0)concentrations after 1-4h (5,6)and sometimes even a decrease in [Hg(0)(aq)]after the initial increase (9).Similar complex kinetics have previously been observed in the photoreduction of Fe(II)in natural waters and been shown to result from the accumulation of photoproduced oxidants (10).It thus seems possible that,in addition to dark oxidation,Hg(0)may be subject to photooxidation in natural waters.Here we report the results of systematic laboratory and field experiments designed to determine the principal param-eters that control the photooxidation of Hg(0)(aq)such as the concentrations of chloride and potential oxidants and to assess the possible importance of this process in aquatic systems.

Experimental Section

Preparation of Solutions.Clean techniques were used during the experiments.The glassware was thoroughly cleaned by soaking in nitric (15%v/v;Baker instra-analyzed reagent,J.T.Baker,Phillisburg)and hydrochloric acid (2%v/v;J.T.Baker,Phillisburg)for approximately 24h.Gloves (hypoclean powder free latex gloves,Safeskin,San Diego,CA)were worn at all times.

To observe Hg(0)photooxidation in the laboratory,we prepared synthetic solutions with ultrapure water (>18M ?cm -1)buffered at pH 8with a phosphate buffer (NaH 2PO 4and Na 2HPO 4,both analytical reagents,BDH Inc.,Toronto,Canada)cleaned by passing it through an ion-exchange resin column (Chelex-100Na-form,type styrene lattice with iminodiacetic acid exchange groups;75-150µm,BioRad Laboratories).The solutions were spiked with Hg(0)(aq).Solutions of Hg(0)(aq)were prepared by bubbling Milli-Q water with a N 2gas flow containing Hg(0).Hg(0)was incorporated into the gas by letting it flow over a drop of liquid Hg (99.9999%Hg,reagent ACS,Aldrich,Milwaukee,WI)placed at the bottom of a U-shaped glass tube.The concentrations of Hg(0)(aq)obtained by this method were 90(40nM.This solution was then used to spike the water sample with Hg(0)(aq).Final Hg concentrations varied from 0.2to 2.0nM.

To test the importance of the concentrations of chloride and semiquinones on Hg(0)oxidation,we conducted ex-periments in the presence and absence of 0.5M KCl (reagent ACS,ACP,Montreal,Canada)and 0,0.16,and 0.32mM p -benzoquinone (Acros,NJ).

Sampling Sites,Water Collection,and Ancillary Data Collection.Three sampling sites along the http://doc.guandang.netwrence River were chosen for their different salinity (Table 1;Figure 1).Th …… 此处隐藏：26390字，全部文档内容请下载后查看。喜欢就下载吧 ……