Chemical Engineering Journal 235 (2014) 141–150

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Chemical Engineering Journal

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Review

Environmental remediation techniques of tributyltin contaminationin soil and water: A review

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.09.044

⇑ Corresponding author. Tel.: +61 8 830 25392.E-mail address: Jianhua.Du@unisa.edu.au (J. Du).

Jianhua Du a,⇑, Sreenivasulu Chadalavada b, Zuliang Chen a, Ravi Naidu a,b

a Centre for Environmental Risk Assessment and Remediation, Mawson Lakes Campus, University of South Australia, Mawson Lakes, SA 5095, Australiab CRC CARE (Cooperative Research Centre for Contamination Assessment and Remediation in the Environment), University of South Australia, Mawson Lakes, SA 5095, Australia

h i g h l i g h t s

� A critical review of remediation techniques for TBT contaminated soil and water.� The first review paper covers global TBT-antifouling paints legislation process.� Laboratory, pilot-scale and full-scale TBT clean-up techniques are reviewed.� Efficiency, feasibility and cost of the clean-up techniques are compared.

a r t i c l e i n f o

Article history:Received 12 July 2013Received in revised form 6 September 2013Accepted 7 September 2013Available online 18 September 2013

Keywords:TributyltinAntifoulingContaminationRemediationSoilWater

a b s t r a c t

Tributyltin (TBT) compounds are active constituents of organotin antifouling paints which were used toprevent the growth of ‘fouling’ organisms on marine structures and vessels since 1950s. Due to the wide-spread applications as antifouling agent on commercial and military marine vessels, TBT compoundsentered various ecosystems and are still being found in sewage sludge, sediments and waterways. Sincethe adverse toxic effects of TBT to non-targeted aquatic life were discovered in 1980s, significant efforthas been directed towards the clean-up of TBT-contaminated marine sediments and waterways. Mostof published research papers regarding TBT compounds mainly focus on their properties, environmentalfates, levels and toxicity. This paper firstly reviews the global TBT legislation development from 1980s to2008, and also presents a critical review of environmental disposal and remediation techniques of TBTcontaminated soil and water. The efficiency, feasibility and cost of recent TBT clean-up techniques bythermal treatment, biodegradation, advanced chemical oxidation and physio-chemical adsorption arealso critically reviewed.

� 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1422. Sources of TBT contaminant in the marine environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

2.1. TBT leached into seawater from antifouling paint applied to the hulls of ships. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1422.2. TBT-based antifouling paint scraped off from grounded ships or ice-breaker ship hulls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1432.3. Cleaning activities in shipyards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

3. The TBT ban . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1434. Remediation methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

4.1. Thermal treatment of TBT-contaminated sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1444.2. Steam stripping of TBT from contaminated sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1454.3. Biodegradation and phytoremediation of TBT from sediment/water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1454.4. Chemical/electrochemical oxidation of TBT contaminated water/sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1464.5. Solvent extraction/chemical washing of TBT contaminated water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1464.6. Coagulation and flocculation of TBT contaminated water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1484.7. Adsorption of TBT from water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

4.7.1. Natural clay absorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

142 J. Du et al. / Chemical Engineering Journal 235 (2014) 141–150

4.7.2. Organic modified absorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1485. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

Fig. 1. Pieces of antifoulant paint removed from the grounding site [22].

1. Introduction

Tributyltin (TBT) is a synthetic organotin compounds andwidely used from the 1950s to 2008 as the active constituent oforganotin antifouling paints on commercial and military marinevessels, as well as on small recreational watercraft [1]. Tributyltin(TBT) compounds conform to the following general formula(CH3CH2CH2CH2)3Sn-X, where X is an anion (normally a halogenion such as Cl�), or a covalently-bonded functional group. Worldproduction of organotin compounds increased from about40,000 tonne/year in 1985 and peaked at 50,000 tonne/year in1996 [2]. Ship fouling is the unwanted growth of biological mate-rials, such as barnacles, algae and other marine organisms, onships’ hulls immersed in seawater. Antifouling paints are appliedto ships’ hulls to prevent the growth of these marine organismsto reduce friction between the hulls of ships and water. Effectiveantifouling techniques using antifouling paints enable ships to tra-vel more smoothly and faster in water and so reduce fuel costs. Ithas been reported that a 1 mm thick layer of algal slime increaseshull friction by 80% and causes a 15% loss in ship speed, whereas a5% thickness increase in fouling for a tanker of 250,000 tons dead-weight will increase fuel usage by 17% [3]. Effective antifoulingpaints are thus very important for the shipping industry for botheconomic and environmental reasons. Copper-based (Cu2O) anti-fouling paints were used originally but they become ineffectivewithin a year and longer acting biocides are needed. TBT-basedantifouling paints swiftly kill marine organisms such as barnacles,algae and mussels and are much longer lasting than those based oncopper oxide.

TBT antifouling paint was introduced in the late 1950s and soonbecame widely adopted on a global scale as a very effective anti-fouling agent. However, adverse effects of TBT on oyster growthwere observed in oyster farms on the Atlantic coast of France in1980s [4] and it raised serious concerns on the toxicity of TBT tomarine ecosystems and waterways, even at very low concentra-tions. Research has demonstrated that lower level organismstypically show impairment at concentrations as low as 1 ng TBT/L while higher-level organisms show health impairment at concen-trations as low as 1 lg TBT/L [1]. In the last 40 years scientific stud-ies have shown that TBT-based anti-fouling paints release highlytoxic TBT into aquatic systems from relatively diffuse sources suchas TBT-based antifouling paint surfaces and from intense point-sources such as shipyards and hardstand areas within commercialharbours and marinas [5]. TBT compounds have the tendency toaccumulate in sewage sludge, sediments and biota. Degradationof TBT in the aquatic environment is a slow process. Half-livesrange from 1 to 3 weeks under optimal conditions to several yearsunder anaerobic conditions, and therefore TBT compounds pose asubstantial risk of toxicity and have significant chronic impactson non-targeted marine species, habitats and ecosystems. Humanhealth is therefore also at risk due to consumption of affectedseafood [6–13].

The environmental concentration levels, physical and chemicalproperties, toxicity, human exposure and distribution of TBT inaquatic systems have been well studied and documented [2,8,14–20]. However, most of published research and review papersregarding TBT compounds are mainly focusing on their properties,environmental fates, levels and toxicity. Since the discovery of theadverse impact of TBT antifouling agent on untargeted marine lives,

significant efforts have been put into clean-up of the TBT-contami-nated marine sediments and waterways. This paper gives an over-view of the development of international TBT legislation, theenvironmental sources and then focuses on laboratory, pilot-scalestudies and full-scale projects on remediation of TBT-contaminatedsediment and water in the past 20 years. The efficiency, feasibilityand cost of recent TBT clean-up techniques by thermal treatment,biodegradation, advanced chemical oxidation and physio-chemi-cal-adsorption are also overviewed and critically compared.

2. Sources of TBT contaminant in the marine environment

The principal source of TBT contamination of the marineenvironment is TBT-based antifouling paint. TBT can enter marinesystems through three major pathways:

2.1. TBT leached into seawater from antifouling paint applied to thehulls of ships

There are two main types of antifouling paint: free-associationand self-polishing antifouling paint [1]. Both of these antifoulingpaints consist of a film-forming material with a biocidal ingredient(TBT) and a pigment. TBT-based copolymer paints have a constantleach rate of 1.6 lg (Sn)/cm�2/day but the initial leaching rate of afreshly painted surface can be as high as 6 mg (Sn)/cm�2/day,which reduces to the designed constant rate after several weeks.Therefore a 3-day docking of a commercial ship at a harbour moor-ing can release more than 200 g TBT into the water and if freshlypainted, this amount can reach 600 g. As a result the dissolved

J. Du et al. / Chemical Engineering Journal 235 (2014) 141–150 143

TBT contamination in the surrounding water ranges from between100 and 200–600 ng (Sn) L�1, respectively [21].

2.2. TBT-based antifouling paint scraped off from grounded ships orice-breaker ship hulls

On 2nd November 2000, a 184 m Malaysian flagged vessel,Bunga Terarau Satu, ran aground on the Great Barrier Reef about40 km southeast of Cairns, Australia. After 13-days’ grounding,abrasive action of the hull over sand and reef substrate duringthe re-floating process scraped antifoulant paint containing TBTfrom the vessel’s hull. Fig. 1 shows the antifouling paint flakescollected by divers from the grounding site [22]. Due to theTBT-based antifouling paint scraped from the ship’s hull, as wellas TBT directly released from the ship’s hull, high levels of TBT,ranging from <1 to 17,000 lg Sn/kg were found in sediments inthe vicinity of the hull scar (within 5 m) [23]. Lower levels of TBTcontamination were recorded up to 250 m from the hull scar[22]. The release rate of TBT from free-association antifouling paintis uncontrollable and the paint has an effective period of about2 years. Self-polishing antifouling paint releases TBT relativelyslower and therefore has an antifouling lifetime of 5–7 years [1].Sediment type plays a major role in controlling the partitioningof TBT between bound and dissolved forms, with stronger bindingto sediments that contain higher organic carbon and have high cat-ion exchange capacities [24].

2.3. Cleaning activities in shipyards

Three activities in shipyards can produce TBT contaminatedwater [25]:

� When a ship is placed in dry dock, washing the hull with lowpressure water to remove any attached organisms/slime layeris conducted so that the condition of the hull paint can beinspected. The TBT contaminated organisms/slime release TBTinto the water.� After cleaning and inspection, if the hull is to be painted, hydro-

blasting or sandblasting are often used to remove either the sur-face paint coating or remove all the paint layers to bare metal.Any paint removed during the hydroblasting will produceTBT-containing water. The hydroblasting process generally pro-duces large amounts of contaminated water (over a millionlitres of water for cleaning a 300 m long vessel).� Replacement of water inside certain hull-mounted radar domes.

This process generates smaller volume of TBT contaminatedwater at less than 100,000 L.

Since most TBT-containing shipyard waters contain high con-centrations of suspended solids and TBT has a propensity for parti-tioning to particles, efforts to remove particulate matter have beendetermined to be essential [25].

3. The TBT ban

Concerns about the effects of TBT on marine life go back to1950s due to its toxicological impacts on non-target aquatic livingorganisms [17,19,26,27]. France was the first nation to introduceregulations in January 1982 banning TBT-based paints on vesselsthat are <25 m in length [18]. The use of TBT-based antifoulingpaints on small vessels was banned in the United Kingdom in1987 [28]. By March 1999, Canada, USA, most European countries,South Africa, Hong Kong, New Zealand and Australia required theapplication of all antifouling paints to be registered. Stricter rulesand regulations were introduced by the year 2000. USA, Canada,

all European Union countries and most of the European countriesoutside the EU, South Africa and Australia banned the applicationof TBT antifouling paints to vessels <25 m in length, while NewZealand and Japan were the first countries to completely ban theapplication of TBT antifouling on all vessels. For those vessels thathave antifouling paint already applied, USA, Canada, European Un-ion countries, Hong Kong and Australia set restrictions that the TBTantifouling paint must have a release rate of <4 lg TBT/cm2/day.USA required all antifouling paints applied in the USA to be regis-tered both federally with the Environmental Protection Agency (USEPA), and with each state authority. Canada required that all anti-fouling paints applied must be registered with Health Canada andTBT product removal/phase out plans must be submitted to HealthCanada to ensure products were withdrawn from the market by 1January 2003 [29–33].

In 2001 a diplomatic conference held under the auspices of theInternational Maritime Organisation (IMO), adopted an Interna-tional Convention on the Control of Harmful Antifouling Systemson Ships (AFS Convention) and proposed dates of 1 January 2003for the prohibition of the application of organotin compounds onships. By February 2003, Canada and all European Union countrieshad followed Japan and New Zealand in forbidding the applicationof TBT antifouling paint. Countries including Brazil, Antigua, Barbu-da and Denmark signed up the IMO’s AFS Convention without res-ervation while USA, Finland, Sweden, and Belgium signed upsubject to ratification. Australia signed the IMO–AFS Conventionon 19 August 2002 and ratified it on 9 January 2007. Australia’sOceans Policy came into effect on 1 January 2006. It proposed aban on the application of TBT on vessels in Australia unless theIMO ban came into force earlier, in which case Australia would fol-low the IMO’s timing [34–36]. An EU ban on the presence of TBT-based antifouling paint on ships’ hulls in EU ports came into effecton 1st January 2008 via Regulation (EC) No. 782/2003. The banmade it an offence for any ship visiting an EU port to have TBTpresent on its hull [37].

On 17 September 2008, the AFS Convention came into forceinternationally. IMO banned the application or re-application oforganotin compounds (TBT) and specified that ships either donot bear TBT compounds on their hulls or external parts or surfacesor have a coating that forms a barrier to prevent TBT leaching fromthe underlying non-compliant antifouling systems. All vessels withactive TBT on their hulls were forbidden from entering the portsand harbours of countries that had ratified the AFS Convention[38]. The signatories to the AFS Convention are Antigua and Barbu-da, Australia, Bahamas, Bulgaria, Cook Islands, Croatia, Cyprus,Denmark, France, Greece, Iceland, Japan, Kiribati, Latvia, Lithuania,Luxembourg, Marshall Island, Mexico, Netherland, Nigeria, Nor-way, Poland, Romania, Saint Kitts and Nevis, Sierra Leone, Slovenia,Spain, Sweden, and Tuvalu. One of the most important provisionsof the IMO–AFS Convention is contained in Article 6, which allowsfor the ban of future antifouling systems that pose a threat ofserious or irreversible damage to the marine environment and/orhuman health.

4. Remediation methods

TBT contamination can be generally divided into two catego-ries: TBT-impacted sediment or water. Dissolved TBT tends to beadsorbed or bound to the fine fractions of sediment such as silt,clay and humic matter, which tend to be attached to sand andgravel and form larger aggregates. These aggregates either suspendin the water column or deposit onto the surface sediments aroundharbours, shipyards or docks. Thus, sediments are potential envi-ronmental sinks for TBT. However, desorption experiments haveshown that TBT adsorption to solid particles is a reversible process

144 J. Du et al. / Chemical Engineering Journal 235 (2014) 141–150

and, therefore, sediments not only represent a sink for this toxiccompound but can also act as a store for renewed contamination[39]. As a result, elevated concentrations of TBT are often foundin effluents from shipyards and docks or in sediments around har-bours, shipyards or docks.

Under favourable conditions, tributyltin is readily degraded,both biologically and physio-chemically. Degradation and transfor-mation occur through a mechanism of sequential debutylation inwhich the three butyl chains are removed one by one, withdibutyltin and monobutyltin as intermediate forms. This processwill eventually lead to the liberation of inorganic tin. Recent TBTclean-up techniques generally involve thermal treatment (hightemperature treatment of up to 1100 �C), biodegradation, ad-vanced chemical oxidation and physio-chemical adsorption. Whendeciding which remediation method to apply in real applicationsthe major factors affecting the viability of TBT treatment are:

� Concentration of TBT contamination in water/sediment.� Volume of contaminated TBT material.� Nature of the TBT contamination.� Sediment particle sizes.� TBT removal efficiency.� Target concentration.� Availability of treatment facility.� Cost.

These factors need to be considered before deciding which oneof the following techniques is chosen and applied in field.

4.1. Thermal treatment of TBT-contaminated sediment

Thermal treatment is a remedial technique where solid materi-als such as sediments, soil or sludge, are heated to increase themobility and facilitate the extraction of organic contaminants.TBT compounds are thermally unstable and disintegrate intoDBT, MBT and inorganic tin at high temperature of around 900 �C.

A laboratory scale TBT treatment designed by Song et al. isshown in Fig. 2 [40]. A heat treatment system for ship hull sand-blast waste consisted of a vessel made of stainless steel tube, andan exhaust gas absorption consisting of a condenser with circulat-ing water and filters to remove some hazardous substances. It was

Fig. 2. Schematic diagram of the heat treatment system including the gas

found that the removal efficiency of total organotin compoundsfrom sandblast waste was >99% at 1000 �C treatment for 1 h, andthe tin-free sand produced by the heat treatment could be reusedas a building material or a cover soil for a landfill site.

Industrial scale thermal treatment is generally carried out intwo steps: a medium temperature thermal vaporisation desorptionstep followed by a secondary treatment unit to heat to tempera-tures >850 �C. A European Commission-funded Life-EnvironmentProgram (Life 02 ENV/B/000341) utilised full-scale thermal treat-ment on dredged sediment contaminated with TBT. The processwas performed in four phases [41]:

� Mechanical dewatering of the sediments which was facilitatedby adding quick lime or poly-electrolyte.� The dewatered sediments were fed into a medium temperature

thermal desorption unit.� The contaminated sediments are then heated in a rotary drum

to the point that the contaminants vaporise and become partof the gas stream.� The gas stream was fed to the secondary treatment unit via a

cyclone separator and heated to temperatures >850 �C for atleast 2 s, achieving an overall removal efficiency of >99.99%.

Another Life-Environment Program [42] found that at 450 �Cdegradation of TBT concentrations from 72.6 mg/kg to 0.29 mg/kg (TBT removal = 99%) in up to 15 min. It was found that higherTBT removal rates could be achieved at higher temperatures withincreased energy consumption. Despite the high efficiency of TBTremoval by thermal treatment, it is only a feasible remediationtechnique for highly TBT-contaminated sediments (>10 mg/kgTBT) due to the energy consumption required by dewatering andthe high temperature of the thermal treatment.

Mostofizadeh [43] tested thermal treatment under high pres-sure for energy saving reasons. The high pressure system mini-mised the high energy consumption in the normal thermaltreatment process. Temperature and dwell time were the mostimportant factors affecting the TBT degradation rate. The authornoted that a temperature of 230 �C, a pressure of 34.5 standardatm. and a dwell period of about 2 h are necessary for a safedecomposition of TBT with a removal rate of >99.9%. Followingimplementation of the pilot-scale tests two full-scale units with

absorption part (a) and the details of the heat treatment vessel (b).

J. Du et al. / Chemical Engineering Journal 235 (2014) 141–150 145

annual capacities of 85,000 and 170,000 tonnes were designed (butnot built) to estimate the energy consumption and costs. The totaltreatment cost was estimated to be €11.5–15 per tonne.

Thermal treatment is the most effective method to treatdredged sediments or soils that contain very high concentrationof TBT (>10 mg/kg TBT) in materials from around shipyards, har-bours and docks. This method is also a preferred option when con-centrations of other organic contaminants, such as PAH or mineraloil, are also present in the sediments, however, thermal treatmentdoes not degrade or remove the heavy metal contaminants, includ-ing tin, and leaves a solid residue requiring further treatment. It isworth pointing out that the high costs due to energy consumptionrequired by dewatering and high temperature thermal treatment isthe limitation for this method, especially when a large amount ofsediment needs to be processed and very low targeted concentra-tion of TBT is required.

4.2. Steam stripping of TBT from contaminated sediment

Many organic contaminants are adsorbed or bound to the finefractions of soil as silt, clay and humic matter, which tend to be at-tached to sand and gravel and form larger soil aggregates. TBT con-taminated sludge occurs all over the world due to dredgingactivities for example at harbours, shipyards or docks. As TBT hasa relatively high volatility combined with a relatively low boilingpoint of around 300 �C, it is possible to vaporise TBT from the solidphase to the gas phase. Therefore an alternative method to hightemperature thermal treatment is steam stripping to separateTBT from contaminated soil/sludge.

The steam-stripping process is based on the spontaneousvaporisation due to the change of temperature and pressureconditions [44]. The alteration of the physical–chemical conditionswithin the process results in a de-agglomeration of sedimentaggregates, desorption of the contaminants and vaporisation ofTBT contaminants and water. In 2001, Eschenbach et al. adoptedsteam stripper technique and designed a pilot scale treatmentplant (Fig. 3) to successfully treat two TBT-contaminated dredgedsediments sourced from a harbour in the North Sea and a shipyardin the Baltic Sea. Results showed that the steam stripping processwas able to decontaminate different materials almost completely(>98% removal). Drilling muds contaminated with mineral oilswere cleaned with an efficiency of 99% up to 100% [45].

Unlike thermal treatment in which the final product of TBT ismetallic tin remaining in the soil, the big advantage of stream

Fig. 3. Flow sheet of the pilot plant of t

stripping is that it actually physically separates adsorbed TBT fromcolloidal sludge, there is no requirement for off-gas treatment, onlya relatively small amount of waste stream with very concentratedorganics is generated; there is little organic contaminants residuesremaining in the cleaned-up soil and therefore the cleaned soil issuitable for application such as agricultural field. This method willalso removes concentrations of other organic contaminants, suchas PAH or mineral oil, present in the sediments. The energy con-sumption of steam stripping needs to be considered when scalingup this method.

4.3. Biodegradation and phytoremediation of TBT from sediment/water

Biosorption and biodegradation by microorganisms to removeTBT from contaminated sediments have attracted interest in recentyears. Biological treatment is generally effective at removing or-ganic industrial pollutants. However, since the purpose of TBT isto kill fouling organisms including bacteria, it may reduce theeffectiveness of conventional biological treatments [46]. Thereforebioremediation and plant uptake methods can be adopted onlywhen TBT contamination levels in sediments are moderate, inthe range of 1–10 mg/kg, and these remediation processes are gen-erally require a spanning long time period.

The processes involved in bioremediation are adsorption andbiodegradation of TBT which are the most important mechanismsfor TBT degradation in freshwater and estuarine sediments [47]. Ithas been demonstrated that microalgae, fungi and bacteria are ableto biosorb and debutylate TBT to less toxic dibutyltin, monobutyl-tin and inorganic tin [48–53]. Sakultantimetha et al. [54] foundthat in TBT spiked sediment the half-life of TBT in the controlsample (natural anaerobic attenuation) was 578 days and by intro-duction of bacterium Enterobacter cloacae, the half-life of TBT sig-nificantly dropped to 11 days in the stimulated sample (pH 7.5,aeration and incubated at 28 �C). Further stimulation by nutrientaddition (succinate, glycerol and L-arginine) resulted in a half-lifereduction to 9 and 10 days, respectively. Dowson et al. [47] foundthat TBT either debutylated to dibutyltin and monobutyltin in aer-obic sediments or degrades to DBT which subsequently desorbs tothe overlying water column. The bioavailability improvement sig-nificantly increased the rate and degraded amount of TBT. In anaer-obic sediment, the half-life of TBT was not discernible and appearsto be in the order of tens of years. Therefore by providing suitableconditions (aeration, temperature and pH), it is possible to enhance

he steam stripping technique [45].

Table 1Results from the slurry electrolysis with one of the high TBT concentration sediment.

Time (h) MBT (lg/kg) DBT (lg/kg) TBT (lg/kg)

0 890 4250 96,5000.5 730 3800 26,0002 1300 4800 22,0004 3200 6000 14,00024 14,000 2400 110

146 J. Du et al. / Chemical Engineering Journal 235 (2014) 141–150

TBT biodegradation and remediate TBT contaminated in a shorttime frame. It is also worth pointing out that all the above re-searches, the TBT concentrations were relatively low (less than100 lg/kg) so as reduce synergistic toxic effect to soil bacteria. Avery unique approach by Mathurasa et al. who exposed bacteriato rather high concentration of TBT (at a concentration range of1–100 mg Sn/L) into soil slurry spiked with anionic surfactant so-dium dihexylsulfosuccinate [55]. The authors found that when sur-factant concentration is below critical micelle concentration, thecomplex of TBT and surfactant monomers reduced the amountsof desorbed TBT exposing to the bacteria, promoted TBT bacterialdegradation and had low toxicity. Consequently, the presence ofanionic surfactant at low concentration could be beneficial forthe bioremediation of relatively high level of TBT contaminationin soil. While considerable amount of research has been conductedon the biodegradation TBT contaminated sediments, little has beenpublished on treatment processes for removal of TBT from theaqueous phase [56]. The published researches, not surprisingly,were also carried out at very low or sub-lethal concentrations(<100 lg/L) [57]. The addition of organic nutrient promoted degra-dation ability as expected, however, the TBT degradation was ob-served to be suppressed with the elapse of time when TBTconcentration was increased to relatively higher levels [50].

Phytoremediation is a method of using plants that are able totake up TBT which is metabolised and transformed into harmlesscarbon-compounds and tin inside the plant. Carvalho et al. [58]compared the capability of three common maritime plants halo-phytes for enhancing remediation of tributyltin from estuarinesediments in both ex situ and in the laboratory study. The lab studyfound that TBT levels were 30% lower after 9–12 months of plantexposure than those for non-vegetated sediments with identicalinitial composition. In the in situ study, which compared the levelsof TBT, DBT, and MBT, TBT and DBT were only detected in the non-vegetated sediment. It was concluded that the application of halo-phytes for TBT remediation in sediments seems to be efficient bothin situ and ex situ. The application of salt marsh plants in taking uppolycyclic aromatic hydrocarbons (PAHs), heavy metals and petro-leum hydrocarbons has also been reported [59–62].

Both biodegradation and phytoremediation methods are ma-tured methods for remediation of many kinds of contaminationeither in laboratory or full scale applications, however, the adop-tion of these methods for TBT contaminated soil is clearly re-strained by the local TBT concentrations, which is generallylimited to lower concentration range. It is for this reason that theapplication of these methods for the full-scale TBT remediation isnot yet to be reported to authors’ knowledge.

4.4. Chemical/electrochemical oxidation of TBT contaminated water/sediment

Since the distribution of organotin species in water/sedimentsystems depends on salinity, pH, organic matter content andtemperature. It is possible to remove the exchangeable part ofthe TBT compounds from the sediment by mechanical shakingand ion exchange and then to decompose them in the aqueousphase by means of chemical/electrochemical oxidation.

Chemical/electrochemical oxidation can break down organotincompounds by breaking the bonds between the metallic tin and or-ganic butyl group in an aqueous phase. Chemical oxidation isachieved by adding oxidising agents such as potassium permanga-nate or hydrogen peroxide. Electrochemical oxidation subjectsorganotin compounds to an electromotive force that generates rad-icals. Highly reactive radicals can attack the bonds between thecentral tin atom and butyl groups which oxidise TBT to tin dioxide.These methods have been studied and trialled in pilot scale to treatboth TBT-contaminated dredged sediments and aqueous phase.

Chemical oxidation of TBT wastewater was studied by Wang[63] who added potassium permanganate and an ionic coagulantAl2(SO)4 to a high concentration organotin wastewater from achemical plant. A tin dioxide sediment was produced after condi-tioning the pH of wastewater to around neutral. A pilot plant trialshowed a 99.71% reduction in total tin concentration from2402 mg/L to 6.97 mg/L. A 50% potassium permanganate solutionwas used in the Life-Environment Program (Life 02 ENV/B/000341) [41] in pilot-scale tests to oxide TBT, the results showedthe removal rates were >96%, while the most cost-effective resultsfor butyl-tins were obtained using a 15% potassium permanganatesolution in which TBT and DBT removal rates of around 90% wereobserved. It was also found that this method is more effective fordestroying TBT than other organotin compounds such as phenyl-tins. Yamashita et al. demonstrated that upon 24 h contacting withregenerated amorphous iron oxide, with some crystallised magne-tite content, the half-life of TBT at 60 �C shortened from at least4 days to only 4.2 h, and debutylation was confirmed to be thereduction mechanism as dibutyltin was found as the by-productof the reaction [64]. It is apparently that the concentration of oxi-dation agents is closely related to the TBT removal rate while cost-effective factors also need to be taken into account.

Electrochemical method for the elimination of organotins wereinvestigated by Arevalo and Cammano [65] who found that theorganotin concentrations in synthetic and real shipyard-wasteswere brought down from 25,000 ng Sn/L to 100 ng Sn/L. An earlierstudy carried out in 2001 by Stichnothe et al. [66] who collectedsediment samples from the harbour of Bremerhaven in northGermany in which TBT concentrations were recorded up to100,000 lg/kg. Slurry electrolysis of the suspended sedimentseemed to be more efficient than treating the leachate from a coun-ter-current extraction column followed by electrolysis. Theseresearchers found the desorption of TBT from sediment to the aque-ous phase took place rapidly and equilibrium was reached around1 h of mechanical stirring time. The results (Table 1) from slurryelectrolysis with one of the high TBT concentration sediments sug-gest that the detoxification mechanism is a stepwise splitting off ofthe butyl groups and that the detoxification seems to stop at MBT.After 24 h of electrolysis treatment the TBT concentration was re-duced from 96,500 to 110 lg/kg, and >99% of TBT was degradedinto MBT and DBT. Following the success of this study the sameauthors carried out a technical plant and a pilot plant (as shownin Fig. 4) investigation in 2005 [67]. During the pilot plant studythe TBT concentration in the fine-grained fraction obtained fromthe METHA plant in Hamburg could be reduced from 2600 lg/kgto 34 lg/kg within 1.14 h residence time. In the sediment fromBremerhaven contaminated with 194 lg/kg TBT, a residence timeof 0.85 h was necessary to reduce TBT concentration <100 lg/kg.Both results indicate that the treatment conditions were mainly re-flected by the residence time in the plant. The authors calculatedthe approximate cost to be around €15 per tonne.

4.5. Solvent extraction/chemical washing of TBT contaminated water

Tributyltin compounds generally have highly lipophilic charac-ters and therefore TBT has a very low polarity and low solubility in

Fig. 4. Layout of the Stichnothe et al. slurry electrolysis pilot plant [67].

J. Du et al. / Chemical Engineering Journal 235 (2014) 141–150 147

water but exhibits high affinity for organic matter. In principleorganic solvents (a wash solution) can be used to extract TBTcontamination from the aqueous phase.

The solubility of TBT in water is often estimated to be approxi-mately 4 mg/L, but measurements of TBT concentrations in dock-yard wastewaters reveal that concentrations very rarely exceed1 mg/L. The extraction process is carried out by dispersion of anappropriate solvent in the process water stream. The dispersionprovides an environment for mass transfer of the TBT-moleculesinto the solvent. Accordingly, the solvent is often dispersed intowater to form a high surface area and the solvent/TBT wastewaterdosage ratio is normally in the range of 1:50–100. The choice ofsolvent depends upon engineering and cost considerations, and lo-cal circumstances. For example, slops oil or crude oil can providereasonable performance are often locally available cheaply, andexisting facilities for its re-use or disposal are in place. Diesel oilhas been used and offers some possibilities of reuse in other appli-cations, thus effectively costing nothing for the material. Depend-ing on the solvent chosen approximately 1–2 tonnes solvent arerequired to treat 100 tonnes of effluent. Solvents with higher

Fig. 5. Schematic diagram of process for removal of TBT from dockyard wastewater[68].

partition coefficients can be used to reduce the total amount of sol-vent required. This may, however, cause problems with flammabil-ity or VOC controls, requiring special modification and possiblyadditional disposal costs [68].

Song et al. [40] studied solvent extraction to treat ship hullwashing wastewater and found that ship diesel was a better sol-vent for TBT extraction compared to the paint thinner, toluene,car diesel and ether. The optimised dosage of solvent for theextraction was 10 mL/L of wastewater. The extraction efficiencyof TBT was significantly affected by the agitation intensity. TheTBT in the wash wastewater was rapidly extracted within 1 h; after1 h extraction by diesel it dropped from around 4000 lg/L to2.8 lg/L, and this was further decreased to 0.8 lg/L after 5 hextraction. Abbott et al. [68] improved the design of solvent extrac-tion plant (as shown in Fig. 5) and successfully tested in prototypeat full scale.

The advantage of this design is that it can be arranged withtwo or more units either in series or parallel, offering the choiceof treating different volumes of wastes to different standardswithout further requirement of solvent. A prototype installationwas tested in 1997 with a treatment rate of up to 10 tonneper hour at Wear Dockyard, Sunderland, UK. The estimatedtreatment cost was approximately £10 per tonne of wastewaterwith further cost related to disposal of solid waste which wasgenerated from the treatment process. The quotation for the dis-posal of TBT-contaminated waste is around $90 per tonne byyear 1999.

Compared to other remediation methods solvent extraction is acompact and robust treatment that is not sensitive to aggressiveinfluent, and is capable of operation within the relatively hostiledockyard environment. Solvent can be sourced depending uponengineering, cost considerations and local circumstances. Cheapslops oil or crude oil were used to achieve reasonable performanceand recycling diesel oil after solvent extraction means low or nocost for the material. Therefore solvent extraction is an economic,compact, efficient and practical method to remove dissolved TBTfrom water.

148 J. Du et al. / Chemical Engineering Journal 235 (2014) 141–150

4.6. Coagulation and flocculation of TBT contaminated water

Coagulation and flocculation processes are used to separate sus-pended solids from water. All waters, especially surface waters,contain suspended particles. Since most TBT-containing shipyardwaters contain high concentrations of suspended solids, and TBThas a propensity for adsorption to particles, efforts to remove par-ticulate matter are considered to be essential [25]. Therefore thecoagulation and flocculation processes of the suspending particlesare important for removal of high levels of TBT from water.

Correct application of coagulation and flocculation processesand selection of the coagulants depend upon the charge, particlesize, shape and density of particles. Prasad and Schafran [25] testedtwo metal salt coagulants: aluminium sulphate and ferric sulphateat laboratory scale and results showed that on average 90% of TBTin shipyard waters could be removed by coagulation–flocculation–clarification under optimum conditions. No statistically significantdifference was found in TBT removal capabilities between the twometal salts when compared at equivalent metal doses and coagu-lation pH. The complete full-scale treatment plant averaged99.8% TBT removal over a period of 3 years. Ottosen et al. [69] con-cluded that about half of the TBT that was attached to small parti-cles in an original dockyard wastewater sample would be settledduring the flocculation process. A decrease in TBT concentrationfrom about 40 lg Sn/L to 20 lg Sn/L after coagulation with ironsalts (1.4 mL/L of a 1.0 M FeCl3 solution) was observed. Metal coag-ulants like aluminium and ferric compounds are widely used coag-ulants for water and wastewater treatment. Aluminium and ferriccoagulants always require attention to pH conditions and consider-ation of the alkalinity level in the raw and treated water. However,public health concerns accompanied by economic considerationspoint to iron salts as potential replacements for alum as the pri-mary coagulation agent [69].

Coagulation and flocculation treatments are economic andeffective ways to remove TBT attached to suspended fine particlesin water to a certain extent, but usually dissolved TBT remains inthe effluent. As a result coagulation and flocculation treatmentsare often used as a pre-treatment process followed by furthertreatment processes such as adsorption therefore is required tomeet strict TBT water discharge standards.

4.7. Adsorption of TBT from water

Adsorption methods are normally used as the final and vitalstep to remove the dissolved very low concentration of TBT(<1 lg/L) from the aqueous phase to meet the high standard dis-charge guideline. In practice they are used in conjunction with flo-tation, coagulation–flocculation and filtration in some full-scaleTBT wastewater treatment processes. Adsorption processes mayprove to be the most effective means to reduce TBT concentrationbecause the organotins are highly attracted to organic and inor-ganic particles in water [1].

An adsorbent for TBT can be generally divided into two parts:organic and inorganic. Terrestrial soils and aquatic sediments usu-ally contain considerable amounts of organic carbon, either as par-ticulate organic matter or attached to the mineral phase, e.g., bysorption. Association of TBT with organic matter in both soils andsediments is assumed to be an important parameter in controllingthe distribution and transport of TBT in terrestrial and aqueoussystems [39]. Both polar interaction and hydrophobic forces arethought to contribute to the adsorption of TBT to organic matter[70]. The common inorganic absorbents used for TBT adsorptionare fly ash, activated carbon, charcoal and soot [71–73], as wellas clay minerals due to their abundance and easily access. Clayminerals have been widely used as absorbents because of theirdominantly negative charged basal surface, high cation exchange

capacity, high specific surface area and large adsorption capacity.The dominant forms of TBT existing in water are TBT+ or TBTOHas TBT has a pKa value of 6.25 [74]. The dominating interactionof TBT with most minerals is sorption of TBT+ cations to nega-tively-charged surface sites through electrostatic interaction. Clayand modified clay adsorbents are covered in this review.

4.7.1. Natural clay absorbentsWeidenhaupt et al. [74] studied the sorption of TBT from aque-

ous solution to mineral surfaces in batch sorption experimentsusing clay minerals (kaolinites, montmorillonites, illites), and alu-minium, iron, and silicon (hydr)oxides. For all minerals investi-gated the sorption kinetics of TBT was fast (within 1 h) andsorption was reversible. For clay minerals sorption of TBT is dom-inated by cation exchange of the TBT+ species. Adsorption of TBTsby homoionic clays increases with decreasing selectivity coeffi-cients of the exchangeable cations (Na+ > K+� Cs+, Ba2+, Ca2+,Mg2+). A maximal TBT+ absorption value of about 11 mmol/kgwas extrapolated from the data using a Langmuir isotherm. On asurface area basis TBT sorption to montmorillonite and illite issmaller than to kaolinite, consistent with the surface charge densi-ties of the clays and the absence of TBT+ intercalation. Hoch andSchwesig [39] concluded that numerous environmental parame-ters influence the adsorption process of TBT, such as solid/solutionratio, clay content and salinity. Another important factor governingTBT adsorption is pH, because it affects both the TBT species in theaqueous phase as well as the surface properties of the mineralphase. The maximum TBT adsorption onto clays was always foundto be at around pH 6–7. Fox et al. [75] investigated the removal ofTBT from wastewater in a pilot scale study in a series of treatmentsthat included dissolved air flotation (removal of suspended parti-cles), sand filtration (removal of fine particles) and acidified acti-vated carbon (removal of dissolved TBT). This process reducedthe TBT concentrations in the wastewater from between 8300and 480,000 ng/L to 41 and 2100 ng/L. A similar process (coagula-tion-clarification, filtration and activated carbon adsorption) in afull-scale treatment plant was carried out by Prasad and Schafranfound the complete full-scale treatment plant averaged 99.8% ofTBT removal over a period of 3 years [25]. Whereas relatively highpercentages removal were achieved, the total treatment processdid not consistently reduce the TBT concentration from above1,000,000 ng/L to below 50 ng/L, which is the effluent concentra-tion level required by the US EPA. The most important parameterdetermining the cost of the adsorption–flocculation process is theadsorbent cost. Vreysen et al. [76] screened several commerciallyavailable adsorbents and optimised a mixture of two absorbents(a bentonite clay based adsorbent, Südflock P294 and a powderedactivated carbon Norit SAE Super) to remove dibutyltin, tributyltin,Cu and Zn from shipyard wastewaters by a one-step adsorption–flocculation method. The results based on bentonite sorbent andpowered activated carbon with real dockyard wastewater suc-ceeded in reaching a discharge limit of 100 ng Sn/L in the effluent.The sorption-flocculation process was economically favourable atrelatively low organotin concentrations (<10 lg Sn/L). At mediumto high organotin concentrations the installation of a granular acti-vated carbon column would be preferred.

4.7.2. Organic modified absorbentsDifferent forms of organic matter have various functional

groups and therefore are able to adsorb different pollutants. As aresult, the content of organic matter in non-organic absorbents willstrongly influence their capability to retain TBT.

Poerschman et al. [70] investigated the adsorption of sixorganotin compounds to particulate organic matter at pH 7.5(which is higher than the pKa = 6.25 of TBT) to avoid the existenceof cationic organotin species. The authors found that the sorption

J. Du et al. / Chemical Engineering Journal 235 (2014) 141–150 149

coefficient increased with increasing degree of alkylation of theorganotins (monobutyltin < dibutyltin < tributyltin), which sug-gested that hydrophobic force governed the binding of TBT underthe given conditions. However, the sorption coefficients of organo-tins were higher in comparison to the non-polar polycyclicaromatic hydrocarbon, fluorene, whose adsorption is only due tohydrophobic forces. Therefore, the authors assumed that bothpolar interactions and hydrophobic forces between the organotinmolecules and the organic matter must be taken into account[70]. Comparing natural Wyoming Na-montmorillonite, kaoliniteKGa and quartz sand, Hoch and Schwesig [39] found that the stron-gest effect on TBT adsorption was achieved by introducing organicmatter into the reaction system, either as dissolved organic matteror particulate organic matter. Their laboratory experimental datashowed TBT is more strongly adsorbed to organic matter than tothe mineral phases. The addition of 5% particulate organic matterto the solid phase yielded a 50-fold increase of adsorption co-effi-ciency from 51 up to 2700 L/kg when compared to the pure claymineral.

5. Summary

Since the concerns about the toxicity of TBT to marine ecosys-tems, waterways and humans were raised in the 1980s, many na-tional and international regulations are moving toward stricteststandards and a complete ban of application of TBT antifoulingpaint came into force in 2008. Many laboratory researches, pilot-scale and full-scale remediation works have been carried out toremediate the TBT-contaminated marine sediments and wastewa-ter. These efforts lead to the development and implement of prac-tical and cost-effective methods to clean up TBT contaminationfrom marine sediments and wastewater.

These clean-up techniques include thermal treatment, steamstripping, biodegradation, phytoremediation, chemical/electro-chemical oxidation, solvent extraction, coagulation/flocculationand adsorption. Thermal treatment is suitable for treating high le-vel of TBT (>10 mg/kg TBT) contaminated sediment and has beenproved to be very successful in full-scale treatment. However, chal-lenge of thermal treatment is the expensive energy cost related tothe high temperature (850–1000 �C) and the treated soil also re-quires further disposal at landfill. Thermal treatment combinedwith high pressure can lower the temperature further to 230 �Cbut a much longer dwell time of 2 h is required. Steam strippingis less energy-demanding than thermal incineration and generatesonly a relatively small amount of TBT concentrated waste stream.The treated soil may be reused for agricultural purpose rather thangoes to landfill. However steam stripping has not been tested infull scale application. Chemical/electrical oxidation combined withcoagulation has been tested in both laboratory and pilot scale totreat intermediate levels of TBT-contaminated sediment. Bio/phy-toremediation was proved to be effective under optimised condi-tion where the TBT level in the sediment is in the moderaterange. However, at the time of this review, biodegradation andphytoremediation have not been tested above laboratory scale,which is probably due to the difficulty in controlling the optimalconditions in full-scale application. Debutylation seems to be thedetoxification mechanism for both biological degradation andchemical/electrochemical oxidation. The later techniques havebeen tested in both laboratory pilot scale and full scale with satis-factory TBT removal rate and low cost of around €15 per tonne ofsediment. Solvent extraction is suitable treating high TBT concen-tration and the full-scale water treatment showing a high removalefficiency of >99.9%. Solvent extraction is subject to engineeringand cost considerations, and local circumstances. Adsorption (withGAC or natural/modified clay) in conjunction with flotation, coag-ulation–flocculation and filtration are generally used as the final

step to remove the dissolved TBT and ensure the final effluent meetthe strictest TBT water discharge standards. The cost of adsorbentis normal high and the adsorption capacity of TBT at ultra-trace le-vel is also very limited, therefore there are ongoing researches andefforts devoted to developing alternative materials such as modi-fied clay adsorbents.

The authors hope this critical review of highlights in the re-search and development in treating TBT-contaminated soil andwater in the past 20 years will be beneficial for fellow researchersand engineers for improving and developing economic, robust andreliable remediation techniques.

Acknowledgment

The authors greatly appreciate Australian Department of De-fence for their support in this work.

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