Available online at www.sciencedirect.com

www.elsevier.com/locate/jphotobiol

Journal of Photochemistry and Photobiology B: Biology 91 (2008) 117–124

Photoreduction of mercury(II) in the presence of algae,Anabaena cylindrical

Lin Deng a,*, Feng Wu b, Nansheng Deng b, Yuegang Zuo c

a Department of Municipal Engineering, Southeast University, Nanjing 210096, PR Chinab School of Resources and Environmental Science, Wuhan University, Wuhan 430072, PR China

c Department of Chemistry and Biochemistry, University of Massachusetts Dartmouth, 285 Old Westport Road, North Dartmouth, MA 02747-2300, USA

Received 4 October 2007; received in revised form 17 February 2008; accepted 17 February 2008Available online 21 February 2008

Abstract

The photochemical and biological reduction of Hg(II) in the presence of algae, anabaena cylindrical, was investigated under the irra-diation of metal halide lamps placed in cooling trap for maintaining constant temperature by water circulation (k P 365 nm, 250 W).The photoreduction rate of Hg(II) increased with increasing algae concentration. The addition of Fe(III) and humic substances intothe suspensions of algae also enhanced the photoreduction of Hg(II). Alteration of pH value affected the photoreduction of Hg(II) inaqueous solution with or without the addition of algae. The concentration of dissolved gaseous mercury (DGM), the reduced productof Hg(II), increased with increasing exposure time and then gradually approached to a steady state. The influence of initial Hg(II) con-centration on the photoreduction of Hg(II) with algae was studied by irradiating the suspensions of anabaena cylindrical at pH 7.0 withinitial concentrations (C0) of Hg(II) at 50, 100, 120, 150 and 180 lg L�1, respectively, the light-induced reduction of Hg(II) followed theapparent pseudo first-order kinetics. The initial photoreduction rate could be expressed by the equation: rA = 0.0871 + 0.00129C0, with acorrelation coefficient R = 0.9994. The overall mercury mass balance study on the photo-reductive process revealed that more than39.86% of Hg(II) from the algal suspension was reduced to volatile metallic mercury.� 2008 Elsevier B.V. All rights reserved.

Keywords: Photoreduction; Anabaena cylindrical; Hg(II); Fe(III); Humic substances

1. Introduction

Mercury (Hg) is a toxic element and has no knownessential biological function [1]. Hg and other metals can-not be degraded biologically or chemically in the environ-ment as many organic pollutants often are. However,mercury or its compounds can be changed from one formto another in chemical reactions. These reactions can resultin more toxic species such as methyl mercury or some stim-ulating species inducing mercury oxidase activity such asH2O2, formed in the biological or abiotic methylation pro-cesses of inorganic mercury in natural environment [2–5].

1011-1344/$ - see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.jphotobiol.2008.02.005

* Corresponding author. Tel.: +86 25 83790750; fax: +86 25 83791619.E-mail address: dlwhu@163.com (L. Deng).

Further researches suggest that the more recent Hg occur-rences in many regions resulted from atmospheric deposi-tion of Hg from anthropogenic and natural sourcesthrough long-distance atmospheric transport [6,7]. Oncedeposited in natural waters, Hg undergoes an aquaticredox cycling between oxidized mercury (Hg(II)) and ele-mental mercury (Hg(0)). Among the Hg(II) pool are toxic,bioaccumulative CH3Hg+ and (CH3)2Hg, and inorganicHg(II) species, while Hg(0) dominates dissolved gaseousmercury (DGM) in freshwater, which is subject to evasionback to the atmosphere [7,8]. Mason et al. [9] estimatedthat the evasion from ocean could globally add more than2000 tons of Hg to the atmosphere annually.

Ferric carboxylate complexes, especially ferric polycar-boxylate like oxalate and citrate complexes are photo-chemically reactive. The photolysis of these ferriccarboxylate complexes produces ferrous ions and other

118 L. Deng et al. / Journal of Photochemistry and Photobiology B: Biology 91 (2008) 117–124

reactive species in aqueous solutions [10–12]. Researchesand prospects on the applications of Fe(III)-oxalate andFe(III)-citrate complexes in photochemical treatments ofwastewater were also discussed in the literature [13]. Inrecent years it was believed that Fe(III)-oxalate complexesundergo photolysis to produce not only reactive reductivespecies of C2O��4 , CO��2 and ferrous ions but also reactiveoxidative species such as HO�2=O��2 , H2O2 and �OH.Khindaria et al. [14] reported that the CO��2 radicalformed was able to reduce CCl4. Huston and Pignatello[15] had successfully degraded carbon tetrachloride andhexachloroethane in UV-illuminated (300–400 nm) acidicoxic or anoxic solutions containing Fe(III) and oxalate.Hug et al. [16] investigated Fe(III) catalyzed photochemi-cal reduction of Cr(VI) by oxalate and citrate in aqueoussolution. Wu et al. [17] reported photoreduction of Cr(VI)in aqueous solutions containing ferric and carboxylateIons. It was believed that Fe(II), HO�2=O��2 and H2O2 werereductants for Cr(VI), and the reaction product in thepresence of oxalate was mainly soluble Cr(III)-oxalate.Fe(III)-hydroxo complexes are also photochemically reac-tive in aqueous media [18]. Its role in mediating photo-chemical redox cycling of heavy metals in naturalaqueous systems is well established [18–20]. Zhang andLindberg [21] studied sunlight and Fe(III)-induced photo-chemical production of dissolved gaseous mercury inFreshwater. These studies suggest that sunlight andFe(III)-induced photochemical reduction of Hg(II) couldbe one of the mechanisms responsible for natural photo-chemical production of DGM in freshwaters.

There have been many publications on the photo-reduc-tion of Hg(II) in aqueous solutions, while few researchworks have been reported on the reduction of Hg(II) inthe system containing algae complexes. The algae com-plexes mainly include Fe(III)-algae, HA-algae, the secre-tion of algae, senescent algae and the health algae, etc.The present work will focus on the photoreduction ofHg(II) in the presence of algae complexes under the irradi-ation of metal halide lamps. The metal halide lamps placedin cooling trap for maintaining constant temperature bywater circulation. The mechanisms involved in this studymay also act in the observed photochemical or photobio-logical production of DGM in the sunlit natural freshwa-ters [22] since algae are commonly present in naturalaquatic systems. This research will provide informationfor predicting the photochemical reduction of Hg(II) andthe formation of DGM in natural water in the presenceof algae complexes.

2. Experimental section

2.1. Chemicals and reagents

Humic substances were purchased from Aldrich Chem.Co. (Milwaukee, WI, USA). Double-distilled water andN2 (98.5%) were used in all experiments. HgCl2 was analyt-ical grade and used as Hg(II) for all experiments.

2.2. Preparation of algae

The algae used in the experiments were anabaena cylin-

drical, and obtained from the Wuhan Hydrobiology Insti-tute of Chinese Academy of Sciences (Wuhan, PRChina). The algae were grown in culture medium at25 �C using 24-h light cycle in a culturing room equippedwith constant temperature air-conditioner. The light inten-sity of culturing room was 2000 Lux.

The culture medium for anabaena cylindrical consistedof (NH4)2SO4, [Ca(H2PO4)2 � 2H2O + CaSO4 � H2O],MgSO4 � 7H2O, NaHCO3, KCl, FeCl3, H3BO3,MnCl2 � 4H2O, ZnSO4 � 7H2O, MoO3, CuSO4 � 5H2O, soilextract; the medium was adjusted to pH 7.0–7.2 by usingNa2CO3. The algae were cultured in axenic medium. Whenthe conditions were such that the algae were growing in alogarithmic growth phase and the density of algae was high(normally 12–14 d of culture), the algae were taken for usein experiments after being washed [23].

To remove colloidal ferric hydroxide particles thatmight have adsorbed on the algae cells, a modified proce-dure from Zepp and Schlotzhauer [24] was used in experi-ments. This procedure involved washing the cells by gentleagitation for 30 min with 0.01 M aqueous ascorbic acidadjusted to pH 3.0. Then the algae were washed with dou-ble-distilled water three times. The resulting algae suspen-sion was obtained in this way. The cell counting wascarried out under inverted microscope at 400� and thedensity of algae (cells L�1) was calculated. Thus, the algaewere prepared for subsequent use. Different concentrationsof algae were gained through diluting washed algae withdouble-distilled water. These experiments were carriedout at a room temperature of 26 ± 2 �C.

2.3. Irradiation procedure

Irradiation experiments were carried out in a cylindricalreactor (20 cm length, 10 cm diameter, 1.0 cm wall thick-ness) (see Fig. 1a), with metal halide lamps (k P 365 nm,Changzhou Shangzi Lamp Co. Ltd., China) placed in cool-ing trap for maintaining constant temperature by water cir-culation. After the radiations of metal halide lamps drillthrough water circulation, UVB was absorbed into watercirculation. The rest wavelengths of metal halide lampsare longer than 365 nm. The cylindrical reactor was notsealed in the uppermost part. The cylindrical reactor wasplaced in a box. HCl and NaOH were used to adjust thepH values of solutions. The cylindrical reactor containingsolutions were kept in the dark before and after irradiation.Different concentrations of Hg(II) and humic substances,Fe(III) or algae were mixed thoroughly and transferredinto the cylindrical reactor. The light intensity at the posi-tion of the cylindrical reactor was 159,000 Lux, which wasdetected using Digit Lux meter (TES 1332, Taiwan, China).At different time intervals during the irradiation, sampleswith Hg(II) and Fe(III), humic acid or algae were takenfrom the cylindrical reactor. In the solution with algae,

Fig. 1. Cylindrical reactors a and b. (a) The cylindrical reactor was not sealed in the uppermost part. Different concentrations of mercury(II) and humicsubstances, Fe3+ or algae were mixed thoroughly and transferred into the cylindrical reactor to study the influencing factors on the photoreduction ofmercury(II) in aqueous solution with or without algae. (b) The cylindrical reactor was sealed in the uppermost part. The experiments for the overallmercury mass balance were performed to study the release of the volatile mercury from the photochemical processes in the cylindrical reactor.

0 1 2 3 4 5 6 70.0

0.1

0.2

0.3

0.4

0.5

0.6

% m

ercu

ry r

educ

ed in

sol

utio

n

Exposure time (h)

Hg(II) Hg(II)+UV-light Hg(II)+A.cylindrica Hg(II)+A.cylindrica+UV-light

Fig. 2. Mercury(II) concentration change in the control experiments.Light source: metal halide lamps (250 W), C0 (Hg2+) = 100 lg L�1,anabaena cylindrical: 1.7 � 109 cells L�1.

L. Deng et al. / Journal of Photochemistry and Photobiology B: Biology 91 (2008) 117–124 119

samples were digested with HCl–HNO3 (1 + 1). Then thesamples and a Teflon-coated stir bar were placed in acapped and secured glass flask and heated to 90 �C in awater bath for a minimum of 1 h. Upon cooling the sam-ples were analyzed by a cold vapor atomic absorption spec-trometer (AAS). Samples without algae were analyzeddirectly.

The experiments for the overall mercury mass balancewere also performed to study the release of the volatilemercury from the photochemical processes in anothercylindrical reactor (20 cm length, 9 cm inside diameter,11 cm outside diameter, 1.0 cm wall thickness) (seeFig. 1b). This cylindrical reactor was sealed in the upper-most part. Irradiation experiments were carried out in thecylindrical reactor, with metal halide lamps (k P 365 nm,Changzhou Shangzi Lamp Co. Ltd., China) placed in cool-ing trap for maintaining constant temperature by water cir-culation, and reaction solutions pumped with N2 at a fixedflow rate throughout the experiment. The mercury vaporproduced upon irradiation was taken into an imbibingflask by N2 that was filled with 200 mL KBr–KBrO4 solu-tions and 2.0 mL H2SO4 [25].

2.4. Chemical analyses

The Hg(II) concentrations were determined by a coldvapor AAS. The calibration equation for Hg(II) wasApeak area = 95.14CHg(II) � 6.24 (r = 0.992), where CHg(II)

was the concentration of Hg(II) in the range of 0–200.0 lg L�1. All vitreous apparatus were dipped inHNO3/water (the volume ratio was 1:1) overnight.

During the course of the overall mercury mass balanceexperiments in the presence of algae, 5 mL sample wastaken from the imbibing flask at 1-h time intervals andNH2OH � HCl–NaCl solution was added slowly until theyellow color of the sample disappeared. Then the sampleswere stood for 5 min until clarified. Finally the samples

were determined by a cold vapor AAS. All the experimentswere triplicate. Only results with error less than 5% werepresented.

3. Results and discussion

3.1. Photoreduction of Hg(II) both in the absence and

presence of algae

Fig. 2 shows the typical time series for the photochemi-cal reduction of Hg(II) both in the absence and in the pres-ence of anabaena cylindrical as well as the correspondingdark controls. In the dark, in the absence or presence ofalgae, only small amount of Hg(II) (7.2% or 13.8%) waslost from the suspension within 7 h due to physical absorp-tion to the inner surface of the reactor and the slow absorp-tion and reduction by biological and thermal chemicalprocesses [26]. Under the metal halide lamps, 12.1% of

00.0

0.1

0.2

0.3

0.4

0.5

0.6

% m

ercu

ry r

educ

ed in

sol

utio

n

Exposure time (h)

pH=9 pH=7 pH=5

1 2 3 4 5 6 7

Fig. 4. PH effect on photoreduction of mercury(II) with algae. Lightsource: metal halide lamps (250 W), C0 (Hg2+) = 100 lg L�1, anabaena

cylindrical: 1.7 � 109 cells L�1.

120 L. Deng et al. / Journal of Photochemistry and Photobiology B: Biology 91 (2008) 117–124

Hg(II) was disappeared after 7-h irradiation even in theabsence of algae due to the slow thermal reduction, absorp-tion and the direct photoreduction of Hg(II). The directphotoreduction of Hg(OH)2 could be responsible for theincreased disappearance of Hg(II) [27].

HgðOHÞ2 þ light! Hg0ðaqÞ þ other products ð1Þ

In the presence of anabaena cylindrical, 43.0% of Hg(II)disappeared from the suspension after 7-h irradiation un-der the metal halide lamps, indicating that the photolysisof anabaena cylindrical could promote the reduction ofHg(II). The algae, anabaena cylindrical, might undergophotolysis to generate smaller organic molecules and freeelectrons [28], which could reduce Hg(II). The enzymaticreaction at the algae cell surface could be another possiblemechanism involved in the reduction of Hg(II) [29].

3.2. Effect of pH on the photoreduction of Hg(II) in aqueous

solution with or without algae

The photoreduction experiment was performed inHg(II) aqueous solutions at several pH values between 5and 9. As shown in Fig. 3, there was an increase in the pho-toreduction efficiency of Hg(II) with increasing pH. At pH9, up to 15.8% of Hg(II) was removed from the suspensionafter 7-h light irradiation. The observed pH effect demon-strate that Hg(OH)2 could be the reactive species of Hg(II)involved in the direct photochemical reduction of Hg(II) inaqueous solution.

Experiments were also carried out to study the pH effecton the photoreduction of Hg(II) in aqueous suspension ofanabaena cylindrical under metal halide lamps. The resultsshowed that the photoreduction of Hg(II) increased withincreasing pH value in the range of 5.0–9.0. At pH 9,63.7% of Hg(II) was disappeared from the algal suspensionafter 7-h irradiation (Fig. 4). In the aqueous suspension ofanabaena cylindrical, the photochemical and biological pro-cesses are more complex. Besides the higher concentrationof Hg(OH)2 at higher pH value leads to a higher photore-duction of Hg(II), the enhanced production of dissolved

00.00

0.05

0.10

0.15

0.20

% m

ercu

ry r

educ

ed in

sol

utio

n

Exposure time (h)

pH=5 pH=7 pH=9

1 2 3 4 5 6 7

Fig. 3. PH effect on photoreduction of mercury(II). Light source: metalhalide lamps (250 W), C0 (Hg2+) = 100 lg L�1.

organic matter and free electrons [28] also accelerate theconversion of Hg(II) to Hg(0). The results showed thatthe photoreduction of Hg(II) increased with increasingpH value in the range of 5.0–9.0, which was in consistentwith those reported in previous studies [30–33]. Duringthe course of irradiation, the algae cell gave off dissolvedorganic matter(DOM). The structural differences inDOM could also affect the photoreduction of Hg(II) [34].

There were two reasons that pH 7.0 was selected in thefollowing experiments because natural water has a pHvalue close to neutral, and the pH value of the algal suspen-sion is also about 7. Thus no pH adjustment was neededfor the tested suspensions.

3.3. Effects of algae concentration on the photoreduction of

Hg(II)

To test the effects of anabaena cylindrical on the photo-reduction of Hg(II), suspensions of anabaena cylindrical atthree initial algal concentrations, 1.6 � 109, 3.5 � 109 and4.6 � 109 cells L�1, at pH 7.0 were illuminated under metal

0 1 2 3 4 5 6 70.0

0.1

0.2

0.3

0.4

0.5

0.6

% m

ercu

ry r

educ

ed in

sol

utio

n

Exposure time (h)

1.6×109 cells/L3.5×109 cells/L

4.6×109 cells/L

Fig. 5. Algae concentrations effect on photoreduction of mercury(II) insolution Light source: metal halide lamps (250 W), C0

(Hg2+) = 100 lg L�1, pH 7.0.

L. Deng et al. / Journal of Photochemistry and Photobiology B: Biology 91 (2008) 117–124 121

halide lamps up to 7 h. As shown in Fig. 5, the faster pho-toreduction of Hg(II) is with the higher algal concentra-tion. After 7-h irradiation, 55.7% of Hg(II) was removedfrom the suspension under the initial algal concentration4.6 � 109 cells L�1; 51.9% of Hg(II) disappeared at initialalgal concentration of 3.5 � 109 cells L�1; 35.0% removalof Hg(II) at the algal concentration of 1.6 � 109 cells L�1.A higher algal concentration results in a faster photopro-duction of free electrons and small dissolved organic mat-ter [28]. The dissolved organic matter, like humic andfulvic acid, could serve as photosensitizers and acceleratethe photoreduction of Hg(II) [35].

3.4. Effects of humic substances on the photoreduction ofHg(II) in algal suspensions

In these experiments, commercial humic substances wereused to study effects of humic substances on the photore-duction of Hg(II) in algal suspensions. Humic substancesaffected the photoreduction of Hg(II) as shown in Fig. 6.In the presence of 4.0 mg L�1 humic substances, 7.8%Hg(II) was removed from a suspension of anabaena cylin-drical stood in dark for 7 h. Under 7-h irradiation, 52.8%Hg(II) disappeared in the presence of 4.0 mg L�1 humicsubstances, while only 26.8% Hg(II) was removed in theabsence of humic substances. These results indicated thathumic substances could promote the reduction of Hg(II)in algal suspension. Several research groups have reportedthat humic substances could promote the reduction ofHg(II) in aqueous solution [30–33].

Humic substances are a ubiquitous and heterogeneousgroup of organic compounds that play an important rolein the fate and transport of many pollutants. Humic sub-stances contain many acidic functional groups that arecapable of binding cations, such as carboxylic acids, phe-nolic, keto, and thiol groups [36]. Humic substances areoperationally divided into Humic acid, fulvic acid, andhumin based on solubility [37]. Other investigators [38,39]have shown that commercial humic acids tend to have

0 1 2 3 4 5 6 70.0

0.1

0.2

0.3

0.4

0.5

0.6

% m

ercu

ry r

educ

ed in

sol

utio

n

Exposure time (h)

A.cylindrica+UV-light+Humid acid A.cylindrica+UV-light A.cylindrica+Humid acid

Fig. 6. Humid acid effect on photoreduction of mercury(II) with algaeLight source: metal halide lamps (250 W), pH 7.0, C0

(Hg2+) = 100 lg L�1, anabaena cylindrical: 1.4 � 109 cells L�1.

more nonpolar moieties than natural humic substancesand soil-derived substances are less polar than those iso-lated from aquatic systems. In all cases, soil and commer-cial humic substances were found to be better sorbentsthan their aquatic counterparts [39,40], and these observa-tions have been largely attributed to polarity effects.

3.5. Effects of Fe(III) on photoreduction of Hg(II) in algal

suspensions

Fe(III) may also influence the photoreduction of Hg(II)as shown in Fig. 7. In the dark, 18.9% Hg(II) disappearedin the suspension of anabaena cylindrical containing11.2 mg L�1 Fe(III). After 7-h light irradiation, 50.2% ofHg(II) was removed from suspension of anabaena cylindri-

cal without Fe(III), while 68.9% of Hg(II) disappeared inthe presence of 11.2 mg L�1 Fe(III). Fe(III) could obvi-ously accelerate the photoreduction of Hg(II) in algal sus-pensions under metal halide lamps irradiation. The abioticmechanisms involved for the effects of Fe(III) on the pho-toreduction of Hg(II) include light-induced photochemicalproduction of highly reducing organic free radicals throughphotolysis of Fe(III)-organo coordination compounds[Fe(III)-Org] and subsequent reaction of Hg(II) with theorganic and inorganic free radicals formed [21,22,41]:

FeðIIIÞ þ natural organic acids ! FeðIIIÞ-Org ð2ÞFeðIIIÞ �Orgþ hv! FeðIIÞ þ organic free radicals ð3Þorganic free radicalsþHgðIIÞ ! Hgð0Þ þ productsþ CO2

ð4Þorganic free radicalsþO2 ! �O�2 þ productsþ CO2 ð5ÞDOCþ hv! DOC� ð6ÞDOC� þO2 ! �DOCþ þ �O�2 ð7Þ�O�2 þ �O

�2 þHþ ! H2O2 þO2 ð8Þ

H2O2 þ FeðIIÞ ! �OHþOH� þ FeðIIIÞ ð9Þ�OHþHgð0Þ ! HgðIIÞ þOH� ð10Þ�O�2 þHgðIIÞ ! Hgð0Þ þO2 ð11Þ

0 1 2 3 4 5 6 70.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

% m

ercu

ry r

educ

ed in

sol

utio

n

Exposure time (h)

A.cylindrica+UV-light+Fe(III) A.cylindrica+UV-light A.cylindrica+Fe(III)

Fig. 7. Fe3+ effect on photoreduction of mercury(II) with algae Lightsource: metal halide lamps (250 W), pH 7.0, C0 (Hg2+) = 100 lg L�1,anabaena cylindrical: 3.5 � 109 cells L�1.

Table 1Photoreduction kinetics of different initial concentrations of mercury(II)in algal solution

C0 (lg L�1) rA ¼ d½c�dt (lg L�1 min�1) Kinetics equation

180 0.318 rA = 0.08711 + 0.00129C0

150 0.283120 0.240100 0.21350 0.153

Light source: high-pressure mercury lamp (250 W), pH 7.0, anabaena

cylindrical: 3.5 � 109 cells L�1.

122 L. Deng et al. / Journal of Photochemistry and Photobiology B: Biology 91 (2008) 117–124

where �O�2 is a superoxide free radical. In addition to reac-tion (9), �OH may also be produced in natural water by di-rect photolysis of DOC [42]:

DOCþH2Oþ hv! �OHþ products ð12ÞFe(III) is highly photochemically reactive in aqueous media[18]. Its role in mediating photochemical redox cycling ofheavy metals in natural aqueous systems is well established[18–20]. Photochemical reduction of heavy metal ions (e.g.,Cr(VI) [16]) by highly reducing organic free radicals, pro-duced through photolysis of Fe(III)-organo coordinationcompounds has been implicated to be one of the mostimportant mechanisms for the redox cycling of these metals[10]. The same mechanisms may also act in the observedlight-induced photochemical production of DGM in algalsuspensions and natural water because both algae andFe(III) are commonly present in nature aquaticenvironment.

3.6. Effects of initial concentrations of Hg(II) on the

photoreduction of Hg(II) in algal suspensions

The influence of initial Hg(II) concentration on the pho-toreduction of Hg(II) with algae was examined by irradiat-ing the suspensions of anabaena cylindrical at pH 7.0 withinitial concentrations (C0) of Hg(II) at 50, 100, 120, 150and 180 lg L�1, respectively. The irradiation of metalhalide lamps could evidently induce photoreduction ofHg(II) in aqueous solution with algae. The disappearanceof Hg(II) in term of percentage of the initial Hg(II) concen-tration was higher at lower initial Hg(II) concentration. Inthis experiment, rA = d[C]/dt (the change in [C] vs. t) couldbe measured from the slope of the exponential curve atearly times in the reaction. The instantaneous initial veloc-ity � Slope at initial point. The velocity (formation ofproduct/unit time) of a first-order reaction also can bederived from the equation: rA = k[C]. Thus a plot of rA

vs. [C] could be obtained by plotting the instantaneous ini-tial velocity of the reaction, as determined by the slope ofthe tangent to the curve at the initial time against the [C].This plot was a straight line whose slope was equal to k,the reduction rate increased with increasing of initial con-centration of Hg(II). It must be done at the instantaneousinitial velocity because the concentration of Hg(II) wasalways changing. The experiment datum was fitted with arange of initial concentrations from 50 lg L�1 to

Table 2Mercury mass balance

Every term mercury (%) Sample one S

HgT0 100 1LHgT11 38.98 3DHgT11 39.86 4DHgT 11.86 1DHgT + DHgT11 + LHgT11 90.7 9

Light source: high-pressure mercury lamp (250 W), pH 7.0, C0 (Hg2+) = 50 lg

180 lg L�1 at pH 7.0. The initial rates for each concentra-tion were determined from the pseudo-first-order rate con-stants and initial concentrations. When the initialphotoreduction rate as listed in Table 1 was plotted as afunction of initial concentration of Hg(II), a linear rela-tionship was achieved. The photoreduction rate of Hg(II)followed the apparent pseudo-first-order kinetics. The ini-tial photoreduction rate of Hg(II) under the conditionscould be expressed by the equation: rA = 0.0871 +0.00129C0 with a correlation coefficient R = 0.9994.

3.7. Mercury mass balance on the photoreduction of Hg(II)

in algal suspensions

To confirm whether the disappeared Hg(II) in the sus-pensions was predominantly reduced to volatile metallicmercury during the reactions, 50 lg L�1 mercuric chloridein the suspensions of anabaena cylindrical at three differentalgal concentrations (2.2 � 109, 2.8 � 109 and 3.2 �109 cells L�1) at pH 7.0 in a new cylindrical reactor (seeFig. 1b) was exposed to the irradiation of metal halidelamps for 11 h. The dark control was made at an algal con-centration of 2.2 � 109 cells L�1 anabaena cylindrical. Thevolatile metallic Hg(0) was blown out from the reactionsuspensions by using pure N2 gas and absorbed into animbibing flask filled with 2 mL H2SO4 and 200 mL KBr–KBrO4 solutions and subsequently quantitated using a coldvapor AAS. The photoreduction of Hg(II) increased withincreasing algae concentration in consistent with the resultspresented in Fig. 5. To further confirm the decrease in mer-cury concentration in the irradiated samples, we measuredthe percentages of total mercury in the initial algae suspen-sion (HgT0), in the algae suspension after 11 h (LHgT11), in

ample two Sample three Sample four

00 100 1000.52 26.35 20.808.94 53.47 56.716.49 17.78 18.735.75 97.60 96.24

L�1.

L. Deng et al. / Journal of Photochemistry and Photobiology B: Biology 91 (2008) 117–124 123

the solution left in the imbibing flask after 11 h (DHgT11)and the total mercury taken from a imbibing flask everyhour (DHgT). The results for the overall mass balance stud-ied were given in Table 2. On average, more than 90.7% ofinitial Hg in the suspension was well balanced, more than39.86% of Hg(II) from the algal suspension was reducedto volatile metallic mercury. O’Driscoll et al. used a similartechnique to study the gross photo-reduction mercury massbalance if Hg(0) was bubbled out as it was formed, andfound that of the total mercury available in each samplea mean of 37.8% was reduced during the experiments [43].

4. Conclusions

The results obtained in this study had shown that algae,anabaena cylindrical, could significantly accelerate thephotochemical reduction of Hg(II) under the irradiationof metal halide lamps placed in cooling trap for maintain-ing constant temperature by water circulation(k P 365 nm, 250 W). The photoreduction of Hg(II)increased with increasing concentration of algae,Fe(III)and humic substance. Increasing pH value alsoled to a higher photoreduction rate of Hg(II) in algae sus-pensions. DGM increased with increasing exposure timeand then appeared to approach a steady state in the irra-diated suspensions. The influence of initial Hg(II) concen-tration on the photoreduction of Hg(II) with algae wasstudied by irradiating the suspensions of anabaena cylin-

drical at pH 7.0 with initial concentrations (C0) of Hg(II)at 50, 100, 120, 150 and 180 lg L�1, respectively, The pho-toreduction of Hg(II) followed the apparent pseudo-first-order kinetics and the initial photoreduction rate couldbe expressed by the equation: rA = 0.0871 + 0.00129 C0,with a correlation coefficient R = 0.9994 under the condi-tions studied. The overall mercury mass balance study onthe photo-reductive process revealed that more than39.86% of Hg(II) from the algal suspension was reducedto volatile metallic mercury.

Acknowledgements

This work was financed by the Natural Science Founda-tion of PR China (No. 20477031) and Hubei Biomass-Re-source Chemistry and Environmental Biotechnology KeyLaboratory (HBRCEBL2007002). Authors gratefullyacknowledge the reviewers of this article.

References

[1] P. Allen, Mercury accumulation profile and their modification byinteraction with cadmium and lead in the soft tissues of the cichlidOreochromis aureus, Bull. Environ. Contam. Toxicol. 53 (1994) 684–692.

[2] J.W.M. Rudd, Sources of methylmercury to freshwater ecosystems: areview, Water Air Soil Pollut. 80 (1995) 697–713.

[3] Y. Zuo, S. Pang, Photochemical methylation of inorganic mercury inthe presence of sulfhydryl compounds, Acta Sci. Circum. 5 (1985)239–243.

[4] V. Celo, D.S. Lean, S.L. Scott, Abiotic methylation of mercury in theaquatic environment, Sci. Total Environ. 368 (2006) 126–137.

[5] Stevend. Siciliano, Nelsonj. O’Driscoll, D.R.S. Lean, Microbialreduction and oxidation of mercury in freshwater lakes, Environ.Sci. Technol. 36 (2002) 3064–3068.

[6] L. Poissant, M. Amyot, M. Pilote, D. Lean, Mercury water–airexchange over the upper St. Lawrence river and Lake Ontario,Environ. Sci. Technol. 34 (2000) 3069–3078.

[7] W.H. Schroeder, J. Munthe, Atmospheric mercury – an overview,Atmos. Environ. 32 (1998) 809–822.

[8] N. O’ D riscoll, D.R.S. Lean, L.L. Loseto, R. Carignan, S.D.Siciliano, Effect of dissolved organic carbon on the photoproductionof dissolved gaseous mercury in lakes: potential impacts of forestry,Environ. Sci. Technol. 38 (2004) 2664–2672.

[9] R.P. Mason, W.F. Fitzgerald, F.M.M. Morel, The biogeochemicalcycling of elemental mercury: anthropogenic influences, Geochim.Cosmochim. Acta 58 (1994) 3191–3198.

[10] Y. Zuo, J. Hoigne, Formation of hydrogen peroxide and depletion ofoxalic acid in atmospheric water by photolysis of iron(III)-oxalatocomplexes, Environ. Sci. Technol. 26 (1992) 1014–1022.

[11] B.C. Faust, R.G. Zepp, Photochemistry of aqueous iron(III)-polycarboxylate complexes: roles in the chemistry of atmosphericand surface waters, Environ. Sci. Technol. 27 (1993) 2517–2522.

[12] Y.G. Zuo, J. Hoigne, Photochemical decomposition of oxalic,glyoxalic and pyruvic acid catalysed by iron in atmospheric waters,Atmos. Environ. 28 (1994) 1231–1239.

[13] F. Wu, N.S. Deng, Y.G. Zuo, Photochemical properties of ferric-oxalate complexes and their effects on photodegradation of organiccompounds in natural aqueous phase, Adv. Environ. Sci. 2 (1999) 79–91.

[14] A. Khindaria, T.A. Grover, S.D. Aust, Reductive dehalogenation ofaliphatic halocarbons by lignin peroxidase of phanerochaete chrysos-

porium, Environ. Sci. Technol. 29 (1995) 719–725.[15] P.L. Huston, J.J. Pignatello, Reductive of perchloroalkanes by

ferrioxalate-generated carboxylate radical preceding mineralizationby the photo-fenton reaction, Environ. Sci. Technol. 30 (1996) 3457–3463.

[16] S.J. Hug, H.U. Laubscher, B.R. James, Iron(III) catalyzed photo-chemical reduction of chromium(VI) by oxalate and citrate inaqueous solutions, Environ. Sci. Technol. 31 (1997) 160–170.

[17] F. Wu, L. Zhang, M. Xiao, N.S. Deng, Photoreduction ofchromium(VI) in aqueous solutions containing ferric and carboxylateions, Fresen. Environ. Bull. 14 (2005) 612–615.

[18] B.C. Faust, In aquatic and surface photochemistry, in: G.R. Helz,R.G. Zepp, D.G. Crosby (Eds.), Lewis Publishers, Boca Raton, FL,1994, pp. 3–37.

[19] W. Stumm, J.J. Morgan, Aquatic Chemistry: Chemical Equilibria andRates in Natural Waters, John Wiley & Sons, New York, 1996, pp.726–759.

[20] P.L. Brezonik, Chemical Kinetics and Process Dynamics in AquaticSystems, Lewis Publishers, Boca Raton, FL, 1994, pp. 643–729.

[21] H. Zhang, Stevee. Lindberg, Sunlight and iron(III)-induced photo-chemical production of dissolved gaseous mercury in freshwaterenviron, Environ. Sci. Technol. 35 (2001) 928–935.

[22] H. Zhang, S.E.J. Lindberg, Processes influencing the emission ofmercury from soils: a conceptual model, Geophys. Res. 104 (1999)21889–21896.

[23] X.L. Liu, F. Wu, N.S. Deng, Photoproduction of hydroxyl radicals inaqueous solution with algae under high-pressure mercury lamp,Environ. Sci. Technol. 38 (2004) 296–299.

[24] R.G. Zepp, P.F. Schlotzhauer, Influence of algae on photolysis ratesof chemicals in water, Environ. Sci. Technol. 17 (1983) 462–468.

[25] Q. Fan, Quick-speed measure mercury in waste water by counterac-tion of procedure with KBrO3–KBr, Chinese J. Lab. Technol. 10(2000) 43–44.

[26] Y. Deng, J. Liu, study on correlation between distribution of mercuriccomplexes and adsorption of Hg(II) on metal oxides, Environ. Chem.5 (1986) 7–11.

124 L. Deng et al. / Journal of Photochemistry and Photobiology B: Biology 91 (2008) 117–124

[27] N.S. Deng, F. Wu, Environment Photochemistry, Chemical IndustryPress, Beijing, 2003, pp. 234–238.

[28] M. Amyot, G. Mierle, D.R.S. Lean, D.J. McQueen, Sunlight-inducedformation of dissolved gaseous mercury in lake waters, Environ. Sci.Technol. 28 (1994) 2366–2371.

[29] R.P. Mason, F.M.M. Morel, H.F. Hemond, in: Proceedings of theInternational Conference on Heavy Metals in the Environment, vol.2, Toronto, CEP Consultants Ltd., Dinburgh, 1993, pp. 293–296.

[30] J.J. Alberts, J.E. Schindler, R.W. Miller, D.E. Nutter, Elementalmercury evolution mediated by humic acid, Science 184 (1974) 895–897.

[31] R.K. Skogerboe, S.A. Wilson, Reduction of ionic species by fulvicacid, Anal. Chem. 53 (1981) 228–232.

[32] B. Allard, I. Arsenie, Abiotic reduction of mercury by humicsubstances in aquatic system – an important process for the mercurycycle, Water Air Soil Pollut. 56 (1991) 457–464.

[33] A. Matthiessen, Reduction of divalent mercury by humic substances-kinetic and quantitative aspects, Sci. Total Environ. 213 (1998) 177–183.

[34] N.J. O’Driscoll, S.D. Siciliano, D. Peak, R. Carignan, D.R.S. Lean,The influence of forestry activity on the structure of dissolved organicmatter in lakes: Implications for mercury photoreactions, Sci. TotalEnviron. 366 (2006) 880–893.

[35] R. Franke, C. Franke, Model reactor for photocatalytic degradationof persistent chemicals in ponds and waste water, Chemosphere 39(1999) 2651–2659.

[36] M.H.B. Hayes, P. MacCarthy, R.L. Malcolm, R.S. Swift, HumicSubstances II. In Search of Structure, John Wiley Chichester, NewYork, 1989, pp. 121–127.

[37] E.M. Thurman, R.L. Malcolm, Preparative isolation ofaquatic humic substances, Environ. Sci. Technol. 15 (1981) 463–466.

[38] Y.P. Chin, W.J. Weber, Estimating the effects of dispersed organicpolymers on the sorption of contaminants by natural solids. I.Apredictive thermodynamic humic substance organic solute interactionmodel, Environ. Sci. Technol. 23 (1989) 978–984.

[39] T.D. Gauthier, W.R. Seitz, C.L. Grant, Effects of structural andcompositional variations of dissolved humic materials on pyrene Kocvalues, Environ. Sci. Technol. 21 (1987) 243–248.

[40] C.T. Chiou, D.E. Kile, T.I. Brinton, R.L. Malcolm, J.A. Leenheer, P.MacCarthy, A comparison of water solubility enhancements oforganic solutes by aquatic humic materials and commercial humicacids, Environ. Sci. Technol. 21 (1987) 1231–1234.

[41] S.O. Pehkonen, C.J.J. Lin, Air waste manage, Association 48 (1998)144–150.

[42] P.P. Vaughan, N.V. Blough, Photochemical formation of hydroxylradical by constituents of natural waters, Environ. Sci. Technol. 32(1998) 2947–2953.

[43] N.J. O’Driscoll, S.D. Siciliano, D.R.S. Lean, M. Amyot, Grossphoto-reduction kinetics of mercury in temperate freshwater lakesand rivers: Application to a general model for DGM dynamics,Environ. Sci. Technol. 40 (2006) 837–843.