bench-scale evaluation of advanced oxidation processes for treatment of a cyanide-contaminated...

Bench-Scale Evaluation ofAdvanced Oxidation Processes forTreatment of aCyanide-Contaminated Wastewaterfrom an Engine ManufacturingFacilityJack Ford,a Rafael Hernandez,b and Mark Zappiba School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0100b Dave C. Swalm School of Chemical Engineering, Mississippi State University, Mississippi State, MS 39762;[email protected] (for correspondence)

Published online 6 September 2005 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ep.10098

Advanced oxidation processes (AOPs) are aggressivechemical oxidation techniques that are characterizedby the formation of hydroxyl radicals. This treatabilitystudy evaluated the performance of the following AOPsfor treating cyanide-contaminated wastewater froman engine manufacturing company: low-pressure ul-traviolet (LPUV) light/ozone (O3), medium-pressure UV(MPUV)/hydrogen peroxide (H2O2), MPUV/O3/H2O2,and peroxone (combination of O3 and H2O2). Abench-scale reactor was used to examine the relativeeffectiveness of each AOP for removal of both cyanideand total organic carbon (TOC) from the wastewater.Initial cyanide concentration of the water was 0.6mg/L. The experiments were run for 1 h. The LPUV/O3

process removed 18 and 60% of the initial cyanide andTOC in the wastewater, respectively. The changes inTOC observed with the evaluated peroxone schemes(5% O3/100 mg/L H2O2 and 1% O3/100 mg/L H2O2)were almost identical to those seen with the LPUV/O3

system. The combination of medium-pressure UV and500 mg/L H2O2 performed best among the AOPs in thestudy with respect to cyanide removal. Surprisingly lowremoval of TOC was observed, indicating a low poten-tial for hydroxyl radicals to remove the organic frac-tion from this wastewater. A combination of all theresults indicates that the main mechanisms of TOC andcyanide removal are volatilization and photolysis, re-spectively. The results of the treatability study were usedin designing a water treatment system currently inoperation at the engine manufacturing facility. © 2005American Institute of Chemical Engineers Environ Prog, 25:32–38, 2006

Keywords: cyanide-contaminated wastewater, ad-vanced oxidation processes, peroxone

INTRODUCTIONAn engine manufacturing company was using nitro-

carburizing processes to add fatigue strength, hardness,and weather resistance to the finished parts. This pro-cess uses alkali cyanates to add carbon and nitrogen to© 2005 American Institute of Chemical Engineers

32 April 2006 Environmental Progress (Vol.25, No.1)

the parts, creating a coating that has properties moresimilar to a ceramic than to a metal. As a result of thisnitrocarburizing process, cyanide compounds weretransferred into the process rinse waters. The existingcyanide treatment process for this wastewater streamconsisted of a two-stage alkaline chlorination to con-vert cyanide to cyanate, and then further destroy thecyanate. The existing wastewater treatment processfailed to meet the treatment goals, so a treatability studywas performed to determine the relative removal ca-pacity of several oxidation processes for the removal ofboth cyanide and cyanate from the wastewater.

Cyanide is used in a variety of industries includingmetals mining, iron and steel works, and production ofplastics and pesticides [1]. Effects of chronic exposureto cyanide include thyroid problems and nerve dam-age, whereas acute exposure can damage the heart andbrain or even result in death [2]. Because of theseadverse health effects, the EPA has set the maximumcontaminant level (MCL) for cyanide in drinking waterat 0.2 mg/L. Oxidation is the most commonly usedcommercial method of treating cyanide-contaminatedwastewater [1]. Various oxidation methods exist to treatcyanide, with alkaline chlorination being one of thecommonly used technologies. Other suitable oxidationtreatments for low concentrations of cyanide includebiological treatment, anodic oxidation, and hydrogenperoxide or ozonation [3].

The overall objective of this study was to perform alaboratory experiment that featured the side-by-sidecomparison of several candidate advanced oxidationprocesses for the removal of the cyanide content fromthe wastewater. Five experiments were selected fromthis study for discussion in this report. The selectedtreatments are representative of the other treatments inthe treatability study.

THEORYAdvanced oxidation processes (AOPs) are defined

as those oxidation processes that make use of radicalspecies as the primary means of degrading a targetedpollutant [4]. AOPs are well documented in literatureand have proven more effective than primary oxidation[5–8]. Hydroxyl radicals can react millions of timesfaster than ozone molecules alone [9]. These powerfuloxidizer species can be formed by various reactionsbetween ozone, hydrogen peroxide, and ultravioletradiation [10].

Hydroxyl radicals react with other molecules to cre-ate more radical species. The free radicals produced actas carriers of the chain reaction [11]. These radical chainreactions can be represented by the following reactionsof an organic RH, in the presence of the OH radical [12]:

OH• � RH 3 H2O � R• (1)

R• � H2O2 3 ROH � OH• (2)

Oxidation processes may be divided into twogroups based on the method of radical production:lighted and dark AOPs [4]. Lighted AOPs use lightsources, often UV lamps, to produce radicals by the

photolytic degradation of oxygen containing speciesinto hydroxyl radicals. Dark AOPs do not use light toproduce hydroxyl radicals. Examples of dark AOPsinclude peroxone (hydrogen peroxide reacted withozone) and Fenton’s reagent (iron-based degradationof hydrogen peroxide into hydroxyl radicals).

To produce significant amounts of hydroxyl radicalsusing lighted techniques, the ultraviolet lamp shouldemit the most photons in the frequency range at whichthe chemical oxidants absorb UV radiation. Low-pres-sure UV lamp emission is nearly monochromatic at 254nm [13], which corresponds nicely to the UV absorptionfor ozone. Hydrogen peroxide absorbs photons over awider frequency range; therefore, it is well suited to thepolychromatic emission range between 200 to 300 nmof medium-pressure UV lamps [13].

Hydroxyl radicals are nonselective in nature andreact freely with a wide variety of contaminants [9]. Thisnonselectivity makes OH radicals susceptible to scav-enging reactions in water [14]. Scavenging reactions areany reactions that consume the hydroxyl radical but donot result in the degradation of the contaminant ofconcern [8]. Examples of these radical scavengers in-clude bicarbonate ions, organic contaminants, and ex-cess O3 or H2O2 [14, 15].

UV/H2O2 ProcessesHydrogen peroxide can react in the presence of UV

light to form the hydroxyl radical [12]:

H2O2 � h� 3 2OH• (3)

This reaction can cause a chain of reactions that con-tribute to the formation of additional radical species inthe UV/H2O2 process [12].

OH• � H2O2 3 H2O � HO2• (4)

HO2• � H2O2 3 H2O � O2 � OH• (5)

2HO2• 3 H2O2 � O2 (6)

The radical species created can accelerate the degrada-tion of H2O2 and the contaminant [14].

The UV/H2O2 process is the most direct techniquefor the formation of hydroxyl radicals. This process istypically faster than other AOPs because of increasedradical formation and higher-intensity UV light [8]. Onepotential disadvantage of this process can arise whenthe wastewater to be treated has low UV transmissivity.High levels of manganese or iron in the wastewaterbeing treated can result in fouling of the quartz UV-lamp housing. Another disadvantage is the capital andmaintenance cost of UV lamps.

UV/O3 ProcessesDissolved ozone reacts in the presence of UV light to

produce hydrogen peroxide [15]:

O3 � H2O � h� 3 H2O2 � O2 (7)

Environmental Progress (Vol.25, No.1) April 2006 33

Then, the hydrogen peroxide reacts with O3 to producehydroxyl radicals (see Eqs. 8–10). Ozone is known todegrade faster at high pH values (see Eq. 8), resulting inmore hydroxyl radical formation and faster contami-nant removal [9].

The UV/ozone process is an AOP with a long recordof successfully treating wastewaters and groundwaters[16]. A clear disadvantage of ozone treatment is thecapital and operating cost of generating ozone on site,given that ozone is toxic at relatively low concentra-tions, and additional safety measures must be taken. Aswith the other lighted AOPs, the effectiveness of thisprocess can be limited by the UV transmissivity of thewastewater.

H2O2/O3 ProcessesRadical formations from ozone and hydrogen per-

oxide are believed to follow the following reactionscheme [5, 15, 17]:

O3 � OH � 3 HO2� � O2 (8)

H2O27 HO2� � H � (9)

O3 � HO2� 3 OH• � O2

.� O2 (10)

These steps initiate radical chain reactions involvingthe above species and other radical- and ionic-interme-diates [18]. In the presence of UV, these reactions, andthe reactions presented in the previous sections, canoccur.

The major advantage of this process is that it is notaffected by the UV transmissivity of the wastewater [8].Disadvantages of this process: it is less aggressive andgenerally slower than other AOPs [4].

Cyanide OxidationCyanide is oxidized directly by ozone to the cyanate

ion by the following mechanism [9]:

CN � � O3 � H2O 3 CNO � � O2 � H2O (11)

Continued treatment breaks the carbon–nitrogen bond,yielding carbonate and ammonia [9]. The ammonia isfurther oxidized to nitrate by ozone [3]:

CNO � � OH � � H2O 3 CO3� 2 � NH3 (12)

NH3 � 4O3 3 NO3� � H2O � 4O2 � H � (13)

Cyanide also reacts with the hydroxyl radical, start-ing a chain reaction [19]:

CN � � OH• 3 HO• �OCAN (14)

HO• �OCAN � H � 3 HOOC•ANH (15)

HOOC•ANH 3 OAC•ONH2 (16)

This radical reacts with oxygen to form a peroxyl rad-ical, which decomposes to yield cyanic acid [19]:

OAC•ONH2 � O2 3 •OOC�O�NH2 (17)

•OOC�O�NH2 3 HO2• � HOCN (18)

Cyanic acid then dissociates in water and reacts as inEqs. 12 and 13 to form carbonate and nitrate. The esterradical produced in Eq. 17 may take part in saponifi-cation reactions with the alkali cyanides and cyanicacid (which will likely generate foam).

METHODS AND MATERIALSThe experimental setup consisted of a cylindrical

round-bottom borosilicate 500-mL reactor made by AceGlass (Figure 1). The reactor had three ports at the top:one in the center and two smaller ones to the sides.Fitted rubber stoppers were used to ensure that thesystem remained airtight. The system was agitated us-ing a magnetic stirring plate and a stirring bar in thereactor. At the beginning of each experiment, the reac-tor was filled with 500 mL of cyanide-contaminatedwater from the facility. Liquid samples for all experi-ments were taken at 0, 5, 10, 20, 40, and 60 min.Approximately 10 mL of sample was withdrawn fromthe reactor at each sampling interval.

Four parameters were measured for each time inter-val: cyanide concentration, hydrogen peroxide concen-tration, total organic carbon (TOC), and pH. In addi-tion, the concentration of ozone in the gas streamentering the reactor was measured to verify the ozonedosage. TOC and pH were monitored to confirm thatthe destruction of cyanide did not create additionaltreatment issues such as extreme pH and to determinewhether the other organics in the waste stream weredestroyed.

Figure 1. Experimental setup.


UV/H2O2 ExperimentsThe UV/H2O2 experiments used an Ace-Hanovia

medium-pressure quartz mercury arc lamp (Model S)rated at 25.18 W total output power, with 1.10-W out-put at 254 nm (referred to herein as the MPUV lamp).The lamp was placed in a quartz immersion well in thecenter of the reactor. Another port was used for liquidsampling and hydrogen peroxide addition. The re-maining port was sealed with a stopper to keep thereactor airtight. The hydrogen peroxide was injectedinto the reactor at time zero and monitored throughoutthe experiment.

UV/O3 ExperimentsThe UV/O3 experiments used both the MPUV lamp

and a low-pressure UV lamp (referred to herein as theLPUV lamp) in the quartz immersion well in the centerport. Both lamps were both independently tested inconjunction with ozonation (Ace-Hanovia photochem-ical immersion lamp with 3.5-W output at 254 nm). Oneof the side ports of the reactor was used for ozoneinput and off-gas removal. Teflon® tubing ran from theozone generator through this port to a sparging disc inthe bottom of the reactor. Another tube carried off-gases from the headspace of the reactor to the ozonemonitor. The remaining port was used for liquid sam-pling during the experiment.

After taking initial samples, the ozone generator wasactivated and the experiment was initiated. Ozone wasgenerated using an Ozonology LC-1234, and the initialgas-phase ozone concentration was measured using aPCI Ozone & Control Systems Model HC meter. Ozonewas sparged continuously at 2.5 standard cubic feet perhour (SCFH).

H2O2/O3 ExperimentsThe setup for the H2O2/O3 experiments varied only

slightly from the UV/O3 experiments. The quartz im-mersion well was still in place to duplicate the flowpatterns from the earlier experiments, but the lamp wasremoved. Two side ports were used: one for ozoneinput and off-gas removal and the other for samplingand peroxide injection.

Analytical MethodsHydrogen peroxide, obtained from Sigma (30%

w/w), was quantified during the experiment using EMScience hydrogen peroxide–test strips and the accom-panying RQ Flex meter. An Accumet AP61 meter withan Accumet pH probe was used to analyze the pH ofthe samples. TOC was determined using a ShimadzuTOC-5000A Total Organic Carbon Analyzer. Cyanidesamples were analyzed with a spectrophotometer us-ing U.S. EPA Method 335.2.

RESULTS

MPUV/H2O2 SystemThis experiment used the medium-pressure UV

lamp, a 500 mg/L H2O2 dose, and no ozone addition.During the experiment, cyanide concentrations de-creased from 0.60 mg/L initially to 0.10 mg/L after 20min (Figure 2). The final concentration of cyanide inthe liquid phase was 0.14 mg/L. Hydrogen peroxidealso decreased, starting at 457.5 mg/L and decreasing toa final concentration of 127.5 mg/L. The drop in hy-drogen peroxide concentration indicates that a signifi-cant amount of the hydrogen peroxide had reactedwith UV light to form hydroxyl radicals, yet surprisinglylow removal of the TOC was observed, indicating a lowpotential for hydroxyl radicals to remove the organicfraction from this wastewater. In terms of cyanide re-moval, this system provided significant removal, likelyas a result of the selection of the MPUV lamp, whichhas a higher potential of photolytically activating thecyanide molecule compared to the LPUV lamp, used inthe LPUV/O3 experiments. As a point of note, the pHremained relatively constant throughout the experi-ment, suggesting no production of organic acids.

LPUV/O3 SystemThis experiment tested a system composed of a

low-pressure UV, continuous 5% O3 sparging at a rateof 2.5 SCFH, and no H2O2 addition. The sparging ofozone into the reactor produced significant foaming inthe reactor containing the wastewater. The foamingwas greatest during the first 10 min of the experiment,

Figure 2. MPUV/hydrogen peroxide results.


and then slowly declined until about 20 min, when it allbut ceased. The foaming resulted in a nearly 40% liquidvolume reduction (overflow out of the reactor by in-creased volume from the resulting gas-holdup volumein the reactor because of foaming). As mentioned ear-lier, the ester radical produced during oxidation likelytakes part in saponification reactions with the alkalicyanides and cyanic acid that will generate foam. How-ever, sufficient volume remained to complete the test,except that it was decided to sample at 60 min and skipthe 40-min sampling because of the reduced volume ofwater left in the reactor. It is hypothesized that theincrease in cyanide observed during this testing is at-tributable to organocyanide-complexes breaking downby the ozone, thus liberating increased amounts of freecyanide within the water (increases apparent cyanidelevels from analysis). This increase in cyanide concen-tration occurs over the first 10 min of testing, starting at0.56 mg/L and reaching 0.78 mg/L by 10 min (seeFigure 3). After that time, the concentration leveled off,ending up at 0.64 mg/L by the 60-min sampling. TheTOC in the reactor decreased from 95.98 to 38.87 mg/Lduring the test run. The MPUV/H2O2 system is knownto produce appreciable levels of hydroxyl radicals. The

MPUV/H2O2 results indicate that the TOC in the cya-nide-contaminated wastewater is relatively unreactivewith the hydroxyl radicals. Therefore, the TOC re-moved by the LPUV/O3 system represents an amountof either organics mineralization by reaction with theozone or volatilization of volatile organics caused bythe sparging of the ozonated gas into the reactor. It isclear that the very narrow band emissions and muchlower stream of cyanide-reactive UV photons emittedfrom the LPUV lamp compared to the emissions fromthe MPUV (comparing Figures 2 and 3) were not nearlysufficient to degrade appreciable amounts of the cya-nide. The system pH dropped from 9.14 to 8.28, whichis believed to be a fairly insignificant drop that is notexpected to impact the oxidative character of the testsolution.

PeroxoneThis experiment evaluated peroxone treatment sys-

tems of 100 mg/L H2O2, 5% O3, or 1% O3 at 2.5 SCFH(two ozone application doses were tested to evaluatethe impact of overall system oxidation potential on theremoval of the TOC). Note that the peroxone systemrepresents a dark oxidative mechanism that does not

Figure 4. Peroxone results.

Figure 3. LPUV/ozone results.


provide UV light for photolysis. In this experimentalset, the 1% ozonated system was run for only 30 min ascompared to the 60-min run time used for the 5%system. Foaming was observed throughout the first 20min of the 5% O3 sparged test; however, foaming wasobserved throughout the entire 30 min of the 1% O3sparged test. The changes in TOC observed with bothperoxone systems tested were almost identical to thoseseen with the LPUV/ozone system (see Figure 4). After20 min, very little H2O2 remained in the 5% ozonatedreactor, resulting in the system essentially becoming anozonation system after this period of testing. On theother hand, the 1% ozonated system had detectablehydrogen peroxide levels throughout the 30 min oftesting. The more persistent levels of hydrogen perox-ide in the 1% ozonated system are expected because ofthe much lesser amounts of ozone available withinsolution to react with the hydrogen peroxide to ulti-mately form radical species. In essence, the 5% ozon-ated peroxone system can be considered a much moreaggressive oxidative system compared to the 1% ozo-nated system because of greater radical species pro-duction. Based on this premise and the fact that levelsof TOC removal achieved within both peroxone sys-tems tested were almost identical, it can be concludedthat TOC removal was not attributed to ozone and/orradical oxidation, but rather to volatilization. As withthe LPUV/ozone system, neither peroxone system re-sulted in significant cyanide removal from the waste-water, again indicating little apparent reactivity of thecyanide to radical oxidizer species or the parent oxi-dizers.

MPUV/H2O2/O3

This test used the MPUV lamp with 5% O3 applied ata rate of 2.5 SCFH sparged into the reactor and dosedwith 500 mg/L of hydrogen peroxide as an attempt tomaximize performance with respect to both cyanideand TOC removal because this system is believed to beone of the most aggressive treatment systems availableusing the equipment and oxidizers tested within thisstudy. Foaming again occurred, but subsided withinonly 10 min. There does appear to be a relationship

between foam formation and the aggressiveness of theoxidizer system tested. The lesser systems in terms ofoverall oxidation potential resulted in longer foamingevents, suggesting that either an intermediate wasformed that foams during gas sparging or a foamingagent was originally in the water before treatment. Ineither case, the more aggressive system did reduce thetreatment time to reach foam cessation. This experi-ment showed cyanide degradation similar to that ob-served with the MPUV/hydrogen peroxide only system.The cyanide concentrations were reduced from 0.60 to0.14 mg/L within the first 20 min of the experiment (seeFigure 5), providing additional evidence that cynanideremoval was primarily the result of photolysis and notoxidation. The pH decreased from 9.21 to 8.26, againconsidered of no consequence to system performance.

CONCLUSIONSThe results of this treatability study indicate that for

cyanide removal from the wastewater tested, photoly-sis was the predominant removal mechanism and notoxidation. Additionally, TOC removal was a result ofvolatilization, but if overall TOC removal is of interest,air sparging instead of ozonation may be a more eco-nomical means of removing the TOC. These results doprovide sufficient evidence that significant cyanide andTOC removal could be obtained using a MPUV/ozona-tion system and that cyanide removal alone could beachieved using the MPUV/hydrogen peroxide system.It is also concluded that the provision of hydrogenperoxide into this system is likely of little consequencewith respect to improving system performance in termsof cyanide removal and, in fact, will adversely impacttreatment cost because of the increased cost of dosinghydrogen peroxide and very likely reduce system per-formance because of the competition of hydrogen per-oxide and cyanide for the UV photons.

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