Archives of Petroleum and Environmental Biotechnology

Volume 2017; Issue 01
7 Aug 2017

Efficient Removal of Heavy metals from Oil-field Produced Water: A Review of Currently Available Techniques

Review Article

Rakhi N. Mehta1and Dayanand Saini2*

1Chemical Engineering Department, Sarvajanik College of Engineering andTechnology, Surat,India.
2Department of Physics & Engineering, California State University, USA.

*Corresponding author:Dayanand Saini, Department of Physics and Engineering, California State University, Bakersfield, CA, USA, Tel: +661 654-2845; Fax+661 654-2693;Email:

Received Date: 2January, 2017; Accepted Date:10 February,2017; Published Date:1  




Suggested Citation



Oil and gas production operations from depleted oil and gas fields often result in the production of enormous amount of water. Even though, disposal of this oil-field produced water itself is a challenging task, however at the same time, treated oil-field produced water can provide an unconventional source of water that could be used for certain beneficial reuses.


The present study reports on the review of various currently available oil-field water treatment technologies with a focus on heavy metals removal technologies.It appears that the photo catalyticmethods are the most promising methods for removing heavy metals from oil-field produced water. However, several factors including the initial metal concentration, the component of the wastewater, capital investment and operational cost, plant flexibility and reliability, environmental impact, and utility of the treated water have a great influence on the selection of the most suitable treatment techniques.




Extraction of oil and gas from underground reservoirs often is accompanied by water or brine, which is referred to as produced water. Produced water is the largest waste-stream of oil and gas exploration which includes water trapped in underground formations and water injected into the stratum to drive out the crude oil [1]. In early stages of oil production, water content is usually low but can rise to as high as 80% during the later years of the well [2]. As reservoirs mature, especially if secondary or tertiary recovery methods are used, the quantity of water climbs and often exceeds the volume of the hydrocarbons before the reservoir is exhausted. It has been observed that in some of the oil-fields this ratio is 1:15 where 15 parts of water accompany just 1 part of oil. Global produced water production is estimated at about 250 million barrels per day compared with about 80 million barrels per day of oil [3].The chemical composition and behaviour of produced water varies when compared with the surface waters because they are constrained within an aquifer and has distinctive characteristics due to the presence of organic and inorganic matters, high salinity, BTEX, PAH, heavy metals etc. which can cause toxicity to the environment[4].Generally, produced water is composed of dissolved and dispersed oil components, dissolved formation minerals, productionchemicals, dissolved gases (including CO2 and H2S) and produced solids. There is a wide variation in the level of its organic and inorganic composition due to geological formation, lifetime of the reservoir and the type of hydrocarbon produced. Also, this produced water contains various microorganisms which result in microbial corrosion of the inner surfaces of pipesby forming biofilms on the metal surfaces [5].The constituents of produced water vary and can differ from well to well with pH in the range of 6-8.5 [6]. Produced water is increasingly being considered to supplement limited freshwater resources in many parts of the US as well as other countries[7]. The cost of producing, handling, and disposing of the produced water often defines the economic lifetime of a field and the actual hydrocarbon reserves; therefore, understanding and predicting the aspects, behaviour, and problems induced by the produced-water flow is important.



Composition of oil-field produced water


Oil field produced water generally consists of dispersed and dissolved oil components those are mixture ofhydrocarbons including BTEX (benzene,toluene,ethylbenzeneandxylene), PAHs (polyaromatic hydrocarbons) and phenols. Dissolved oils are the polar constituent organic compounds in produced water, while small droplets of oil suspended in the aqueous phase are called dispersed oil [8-10]. BTEX, phenols, aliphatic hydrocarbons, carboxylic acid and low molecular weight aromatic compound are classified as dissolvedoil, while less-soluble PAHs and heavy alkyl phenols are present in produced water as dispersed oil [11]. Dissolved and dispersed oil has to be removed in order to utilize this produced water for some efficient reuse.


Dissolved inorganic compounds or minerals are usually highin concentration, and classified as cations and anions, naturallyoccurring radioactive materials and heavy metals. Cations andanions play a significant role in the chemistry of producedwater. Na and Cl2are responsible for salinity, ranging from afew milligrams per litre to 300000 mg/L[12]. Cl2, SO4,CO3, HCO, Na, K, Ca2, Ba2, Mg2, Fe2 and Sr2affect conductivity and scale-forming potential. Typical oilfieldproduced water contains heavy metals in varied concentrations,depending on the formation geology and the age of oil well and its concentration is usually higher than those of receiving water (for enhanced oil recovery) and those found in sea water[13].226Ra and 228Ra are the most abundant naturally occurringradioactive elements present sometimes in oilfield produced water[13].Radioactivity of produced water results primarily fromradium that is co-precipitated with barium sulphate (scale)or other types of scales [14].


Current scenario of oil-field produced water disposal


At present,oil and gas operators manage the produced water by following one or more of the options. Some try to avoid production of water by blocking the water fractures by polymer gel or downhole water separator;but this option is not always possible as water would generally be produced during secondary and tertiary extraction process of oil. Another is injecting the produced water back into the formations. However, this demands transportation of water, and treatment to reduce fouling and bacterial growth. In the long term, the stored produced water may pollute the underground waters. Also, some follows its discharge in to the environment if it meets onshore and offshore discharge regulations. Petroleum industry may try reusing minimally treated produced water for drilling and work over operations. Finally, it could be utilized for beneficial domestic purposes such as for irrigation, wildlife consumption and industrial water, but this may involve significant treatment [3,15].


Environmental concerns and the prospect of beneficial uses have driven research into the treatment of produced water. Current conventional treatment technologies are targeted at removal of heavy metals, oil and grease, suspended solids, and desalination, which often lead to the generation of large volumes of secondary waste. For instance, heavy metals are removed as sludge using current treatment technologies [16]. This article reviews current produced water efficient, eco-friendly and cost effective treatment technologies for removal of heavy metals thereby rendering the produced water for reuse with minimal negative impact on the environment.


Pre-treatment methods for oil-field produced waters


The first pre-treatment process of the oil-field produced waters containing traces of oil is the oil-water separation. Theconventional rectangular-channel separators, developed by the American PetroleumInstitute (API) are wildly used for this purpose, and their design criteria are summarized inthe publication API, 1990[17]. Many other separators had been developed based on the oil-waterseparation theory and some of them, as the parallel plate and corrugated plate separators,had been implemented in the petroleum refineries [18]. The oilseparators removeonly the fraction of free oil, whereas the emulsified and the dissolved oil remain in the separator effluent in the form of oil-water emulsions. These emulsions could be destabilized using chemical destabilizers followed by separation using dissolved air flotation technique (DAF)[19-21]. Different biologicaltreatment processes that have been utilized for treating produced water are aeratedponds, activated sludge, biological contactors, sequential bath reactors and moving bedreactors[22-25]. Initial researches that had been done for recycling of the biologically treated refinery effluent involved the use of activated carbon adsorption alone or in combination with ozonation or sandfiltration [26-28].Themembrane technology development allowed additional options, such as ultrafiltration andreverse osmosis[29-32].The implementation of the advanced treatment technology allowedreusing of the biologically treated wastewater and freshwater savings in the refineries.


Effects of heavy metals


Heavy metals are generally considered to be those whose densityexceeds 5 g per cubic centimetre and atomic weights between 63.5 and 200.6.  Most heavy metals are well-known toxic and carcinogenic agents and it represent a serious threat to the human population and the fauna and flora of the receiving water bodies. Heavy metals have a great tendency to bio-accumulate and end up as permanent additions to the environment and have detrimental effect on human health [33].Oncethey enter the food chain, large concentrations of heavymetals may accumulate in the human body. If the metals concentrations are ingested beyond the permitted value, they can cause serious health disorders. A large number of elementsfall into this category, but the ones discussed here are those of relevance in the environmental context. Arsenicis usually regarded as a hazardous heavy metal even thoughit is actually a semi-metal. Heavy metals cause serious healtheffects, including reduced growth and development, cancer,organ damage, nervous system damage, and in extreme cases,death. Exposure to some metals, such as mercury and lead,may also cause development of autoimmunity, in which aperson’s immune system attacks its own cells. Thiscan leadto joint diseases such as rheumatoid arthritis, and diseases ofthe kidneys, circulatory system, nervous system, and damagingof foetus’s brain. Wastewater regulationswere established to minimize human and environmental exposureto hazardous chemicals. This includes limits on the typesand concentration of heavy metals that may be present in thedischarged wastewater[34-35].


Produced water management techniques


The general objectives for operators treating produced water are: de-oiling (removal of dispersed oil and grease), desalination, removal of suspended particles (salts) and sand, removal of soluble organics, removal of dissolved gases, removing traces of heavy metals, removal of naturally occurring radioactive materials (NORM), disinfection and softening (to remove excess water hardness) [15]. In this current review,different methods to remove heavy metals from oil-field produced waters have been discussed in detail. These methods includechemical precipitation, coagulation, flocculation,ion-exchange, adsorption, membrane filtration,Reverse Osmosis, floatation, electrochemicaltreatment technologies, Electrodialysis, Photocatalysis and biological methods.


Chemical Precipitation, Coagulation and Flocculation


The conventional method for heavy metal removal from industrial wastewater generally involves achemical precipitation process[36-39].Of the varioustreatment methods employed to remove heavymetals, hydroxide precipitation is the most commontreatment technology. Heavy metals are removed byadding alkali such as caustic, lime or soda ash toadjust the wastewater pH to the point where themetals exhibit a minimum solubility. Then a proper solid-liquid separation technique removes the metal precipitation such as sedimentationand filtration. The conventional heavy metalremoval process has some inherent shortcomingssuch as requiring a large area of land, a sludgedewatering facility, skilful operators and multiplebasin configurations[40].The conceptual mechanism of heavy metal removal by chemical precipitation is presented in Eq. (1) M2+ + 2(OH)  ↔ M(OH)2,[41].


Where, M2+ and OH represent the dissolved metal ions and the precipitant, respectively, while M(OH)2 is the insoluble metal hydroxide. Adjustment of pH to the basic conditions (pH range of 9–11) is the major parameter that significantly improves heavy metal removal by chemical precipitation. Lime and limestone are the most commonly employed precipitant agents due to their availability and low-cost in most countries [42- 43]. Lime precipitation can be employed to effectively treat inorganic effluent with a metal concentration of higher than 1000 mg/L and other advantages of using lime precipitation include the simplicity of the process, inexpensive equipment requirement, and convenient and safe operations. To enhance lime precipitation, fly ashwas used as a seed material[43]. The fly ash- limecarbonationtreatment increased the particle size of the precipitateand improved the efficiency of heavy metal removal. In hydroxide precipitation process, the addition of coagulantssuch as alum, iron salts, and organic polymers can enhance theremoval of heavy metals from wastewater Another researcher employed chemical coagulation and precipitation by lime to treatsynthetic wastewater[44].Sulphide precipitation is also an effective process for the treatmentof toxic heavy metal ions. One of the primary advantages ofusing sulphides is that the solubility’s of the metal sulphide precipitatesare dramatically lower than hydroxide precipitates and sulphide precipitates are not amphoteric. Also, this process can achieve a high degree of metal removal overa broad pH range compared with hydroxide precipitation. Metalsulphidesludge also exhibit better thickening and dewateringcharacteristics than the corresponding metal hydroxide sludge.However, there are potential dangers in the use of sulphide precipitation process as it many times results into evolution of toxic H2S fumes. However, chemical precipitation has been successful in combination with other methods and a reported literature shows that sulphide precipitation can reuse and recover heavy metalions by employing nanofiltration as a second step[45].There are some reports on chemical precipitation in combination with ion-exchange treatments. Here nickel was removed with the help of ion-exchange in combination of chemical precipitation [46].Another novel method is nucleation precipitation which is a simple cost effective method to strip off heavy metals in a fluidized sand bed. In operation, the metal-bearing wastewater is pumped through afluidized sand column with a simultaneous injection of carbonatesolution to raise pH metal precipitation to occur and then deposit onthe sand surface (nucleated precipitation) rather than to formdiscrete metal original sand grains are 0.2-0.3mm in diameter, butquickly grow to a much larger size (up to 2 or 3mm) uponcontinuous coating of metal precipitates. The larger coated sandparticles sink to the bed bottom from which they can be easilyremoved and new sand can then be added from the top of the stripper.Since the coated particle contains a high level of metal mineral, theycan be collected for mineral recovery[47].However, chemical precipitation requires large amounts of chemicals to reduce metals to an acceptable level for discharge along with excessive sludge formation that requires further treatment, slow metal precipitation, poor settling, the aggregation of metal precipitates, and the long-term environmental impacts of sludge disposal [48]. Coagulation is one of the most important methods for wastewater treatment, but the main objects of coagulation are only theHydrophobic colloids and suspended particles. Flocculation is the action of polymers to form bridges between the flocs and bind the particles into large agglomerates or clumps. Once suspended particles are flocculated into larger particles, they can usually be removed or separated by filtration, straining or floatation. Today many kinds of flocculants, such as PAC, polyferric sulphate (PFS) and polyacrylamide (PAM), are widely used in the treatment of wastewater; however, it is nearly impracticable to remove heavy metal very well from wastewater directly by these current flocculants.


Ion-Exchange Method


Ion exchange may be defined as the exchange of ions between the substrate and surrounding medium. The most useful ion exchange reaction is reversible, whereby the reaction is reversible and the ion exchanger can be regenerated and reused many times. Generally, resins aremanufactured in the spherical, stress and strain free form to resist physical degradation.They are stable at high temperatures and applicable over a wide pH range. Ion exchangeresins, which are completely insoluble in most aqueous and organic solutions, consist of across linked polymer matrix to which charged functional groups are attached by covalent bonds [49]. Typical ion exchangers are produced with a particle size distribution in the range 20-50mesh (for separation of anions from cations or of ionic species from nonionic ones). Formore difficult separations, materials of smaller particle size or lower degrees of cross linking are necessary. Depending on the type of functional groups of exchanging certain ions, the ion exchangerswith strongly acidic e.g., sulphonate -SO3H, weakly acidic e.g, carboxylate -COOH, stronglybasic e.g., quaternary ammonium -N+R3 and weakly basic e.g., tertiary, and secondary amine-N+R2H and -N+RH2 should be mentioned. The strong acidic cation exchangers are welldissociated over a wide pH range and thus reaching its maximum sorption capacity.There are also amphoteric exchangers, which depending on the pH of the solution mayexchange either cations or anions. More recently these ion exchangers are called bipolarelectrolyte exchange resins (BEE) or zwitterionic ion exchangers [50].


Ion-exchange processes have been widely used to remove heavymetals from oil-field produced water and other waste waters due toadvantages such as hightreatment capacity, high removal efficiency and fast kinetics [51]. Ion-exchange resin, either synthetic or natural solidresin, has the specific ability to exchange its cations with the metalsin the wastewater; however, synthetic resins are commonly preferred as they areeffective to nearly remove the heavy metals from the solution[52].Hydrogen ions in the sulfonicgroup or carboxylic group of the resin can serve as exchangeableions with metal cations. The uptake of heavy metal ions by ion-exchange resins is ratheraffected by certain variables such as pH, temperature, initial metalconcentration and contact time [53]. Ioniccharge also plays an important role in ion-exchange process which could be incurred from the study of removal of Ce4, Fe3 and Pb2from aqueous systems by cation-exchange resin purolite C100 [54]. Similarresults for Co2, Ni2 and Cr3in an Amberlite IRN-77 cationexchangeresin were previously obtained [55].


Besides synthetic resins, natural zeolites, naturally occurringsilicate minerals, have been widely used to remove heavy metalfrom aqueous solutions due to their low cost and high abundance.Many researchers have validated that zeolitesexhibit good cation-exchangecapacities for heavy metal ionsunder different experimental conditions [56].Clinoptilolite isone of the most frequently studied natural zeolites that havereceived extensive attention due to its selectivity for heavymetals. It has been studied that the surface of clinoptiloliteloaded with amorphous Fe-oxide species would significantly improve its ion-exchange capacity[57]. Doula [57]employedclinoptilolite-Fe system to simultaneously remove Cu, Mn and Zn from drinking water and found that the system has very large metaladsorption capacity and for most of the cases the treated watersamples were suitable for human consumption or agricultural use[58].


Adsorption Method


Adsorption is now recognized as an effective and economicmethod for heavy metal wastewater treatment. The adsorptionprocess offers flexibility in design and operation along with producing high-quality treated effluent. It is also very efficient becauseadsorption is sometimes reversible, adsorbents can be regeneratedby suitable desorption process[59]. In recent years, the search for lowcostadsorbents that have metal-binding capacities has beeninvestigated [60]. The adsorbents may be of mineral, organicor biological origins, zeolites, industrial by-products,agricultural wastes, biomass, and polymeric materials [61].Various other effective adsorbents are activated carbon, carbon nanotubes, low-cost adsorbents, bioadsorbents[59].There are many adsorbents that can be used for the removalof metal ions from wastewater and, certainly, cost plays animportant role for determining which one is applicable. Activated carbon (AC) adsorbents are widely used in the removalof heavy metal contaminants due to itslarge micropore and mesopore volumes and the resulting highsurface area. A large number of researchers are studying the use ofAC for removing heavy metals [62].


Nowadays, the depleted source of commercial coal-based AC has resulted into increase in its price thereby opting other additives along with AC such as alginate, tannic acid and magnesium [63-65].Converting carbonaceous materials intoAC for heavy metals remediation have been reported. The use of AC from eucalyptusbark in the binary component sorption of Cu2 and Pb2and poultry litter to manufacture AC for treating heavy metal-contaminated water was explored [66-67].Carbon nanotubes (CNTs) discovered by Iijima [68],have been widely studied for their excellent properties and applications as new adsorbents possessing great potential for removing heavy metal ions such as lead[69],cadmium [70], chromium[71], copper andnickel[72] fromwastewater.CNTs are divided into two types: (1) single-walled CNTs(SWCNTs) and (2) multi-walled CNTs (MWCNTs) [73].The mechanisms by which the metal ions are adsorbed ontoCNTs are very complicated and appear attributable to electrostaticattraction, sorption-precipitation and chemical interactionbetween the metal ions and the surface functional groups of CNTs[74].The sorption capacities of metal ions by raw CNTs are very lowbut significantly increase after oxidized by HNO3, NaClO andKMnO4 solutions.


Despite these recently developed adsorbents the search for low cost and easily available adsorbents has become main research focus today.  Studies have been carried out for adsorption of a few heavy metals on natural and modified kaolinite and montmorillonite [75], chemically modified plantwastes [76], industrial by-products such as lignin[77], diatomite [78],clino-pyrrhotite, lignite[79],aragonite shells [80], natural zeolites [81],clay [82],kaoliniteand peat etc[83].Another report showed zinc and copperremoval from aqueous solutions using brine sediments, sawdust,and the mixture of both materials[84].The potential to remove boron and arsenic from petrochemical wastewater by using aquatic booster wasinvestigated in batch experiment process and the results were measured by inductively coupled plasma massspectrometry (ICPMS). The main parameters influencing arsenic and boron adsorption onto the aquatic boosterwere contact time, size of particle, agitation speed, and dosage. The adsorption efficiency of arsenic andboron increases with longer contact time as well as more aquatic booster dosage and higher agitation speed. The removal efficiency for boron was around 60.36% by 35g/L dosage, 80 rpm anda particle size of 0.60 mm at of 390 minutes. As for the arsenic, the condition where it gives the removal percentagearound 71.83% is that particle size of 0.6 mm, 300 minutes contact time, agitation speed of 80 rpmand dosage of 45g/L[85]. Hydrogels, which are crosslinked hydrophilic polymers, arecapable of expanding their volumes due to their high swellingin water and hence are being used in purificationof wastewater. Various hydrogels were synthesized and theiradsorption behavior for heavy metals was investigatedwhile Barakat and Sahiner(2008) prepared poly(3-acrylamidopropyl)trimethyl ammoniumchloride hydrogels for As(V) removal [86].


Biosorption of heavy metals from aqueous solutions is a relativelynew process that is effective and inexpensive and has been confirmed a very promisingprocess in the removal of heavy metal from dilute waste waters such as oil field produced water. Typical biosorbents can be derived from threesources as follows: (1) non-livingbiomass such as bark, lignin, shrimp, krill, squid, crab shell, etc(2) algal biomass; (3)microbial biomass, e.g. bacteria, fungi and yeast.Different forms of inexpensive, non-living plant material such aspotato peels[87], sawdust [88],black gram husk[89], Rice husk [90-91], coffee husks [92],eggshell[93], seedshells[94],sugar-beet pectin gels [95].and citrus peels [96],sugarcane bagasse [97], coconut husk [98], oil palm shell [99], neem bark [100], fly ash have been widely investigated as potentialbiosorbents for heavy metals.

Algae, a renewable natural biomass proliferates ubiquitouslyand abundantly in the littoral zones of world has attracted theattention of many investigators as organisms to be tested and usedas new adsorbents to adsorb metal ions. Several advantages inapplying algae as biosorbent include the wide availability, low cost,high metal sorption capacity and reasonably regular qualityResearch works on the metal biosorption using algal biomass include the biosorption of Cu2 andZn2 using dried marine green macroalgaChaetomorphalinum[101], the biosorption of Cu2, Cd2, Pb2, andZn2 using dried marine green macroalgaCaulerpalentillifera[102], the biosorption of chromium from wastewaterusing green alga Ulva lactuca[103], and thebiosorption of lead (II) from wastewater by green algae Cladophorafascicularis[104]. Microbial removal of metal ions from wastewater has beenindicated as being highly effective. Biosorption of heavy metals inaqueous solutions by bacteria includes Bacillus cereus [105], Escherichia coli [106],Pseudomonas aeruginosa [107].Fungi and yeasts are easy to grow, produce high yields ofbiomass and at the same time can be manipulated genetically andmorphologically. Fungi biosorbents include Aspergillus niger[108],Rhizopusarrhizus[109],Saccharomyces cerevisiae[110] andLentinusedodes[111].


Membrane filtration


Membrane filtration technologies with different types ofmembranes show great promise for heavy metal removal for theirhigh efficiency, easy operation and space saving. The membraneprocesses used to remove metals from the wastewater are ultrafiltration,reverse osmosis, nanofiltration and electrodialysis.


Ultrafiltration (UF) is a membrane technique working at lowtransmembrane pressures for the removal of dissolved andcolloidal material. Since the pore sizes of UF membranes are largerthan dissolved metal ions in the form of hydrated ions or as lowmolecular weight complexes, these ions would pass easily through UF membranes, hence to obtain high removal efficiency of metal ions, themicellar enhanced ultrafiltration (MEUF) and polymer enhancedultrafiltration (PEUF) was proposed.MEUF was first introduced by Scamehorn et al. in the 1980s forthe removal of dissolved organic compounds and multivalent metalions from aqueous streams [112].This separation technique isbased on the addition of surfactants to wastewater in a quantity beyond its critical micelle concentration (CMC), the surfactant molecules willaggregate into micelles that can bind metal ions to form largemetal-surfactant structures. The micelles containing metal ions canbe retained by a UF membrane with pore sizes smaller than micellesizes, whereas the untrapped species readily pass through the UFmembrane. To obtain the highest retentions, surfactants of electriccharge opposite to that of the ions to be removed must be used and metal removal efficiency by MEUF depends on the characteristicsand concentrations of the metals and surfactants, solution pH,ionic strength, and parameters related to membrane operation. PEUFuses water-soluble polymer to complex metallic ions and forma macromolecular, having a higher molecular weight than themolecular weight cut off of the membrane which will be retained when they are pumped through UF membrane.The reverse osmosis (RO) process uses a semi-permeablemembrane, allowing the fluid that is being purified to pass throughit, while rejecting the contaminants and accounts for more than 20% of the world’s desalination capacity [113].Cu2 and Ni2 ions were successfully removed by the RO processand the rejection efficiency of the two ions increased up to 99.5% byusing Na2-EDTA [114].The major drawback of RO isthe high-power consumption due to the pumping pressures, andthe restoration of the membranes. Nanofiltration (NF) is the intermediate process between UF and RO and is a promising technology for the rejection of heavy metalions such as nickel[115],chromium[116],copper[117],and arsenic [118] from wastewater. NF process benefits from ease ofoperation, reliability and comparatively low energy consumption aswell as high efficiency of pollutant removal; however reports have been published on use of NF and RO in combination for removal of copper from process waste water[119]. Electrodialysis (ED) is another membrane process for the separationof ions across charged membranes from one solution toanother using an electric field as the driving force where ion-exchange membranes are used. This process has been widely used for the production f drinking and process water from brackish water and seawater,treatment of industrial effluents, recovery of useful materials fromeffluents and salt production [120].


Flotation Process


Flotation has nowadays found extensive use in wastewatertreatment and has been employed to separate heavy metals from a liquid phase using bubble attachment originated in mineralProcessing. Dissolved air flotation (DAF), ion flotation and precipitationflotation are the main flotation processes for the removal ofmetal ions from solution.DAF is to allow micro-bubbles of air to attach to the suspendedparticles in the water, developing agglomerates with lower densitythan water, causing the flocs to rise through the water and accumulatingat the surface where they can be removed as sludge[121]. Ion flotation method is based on imparting the ionic metal species in wastewatershydrophobic by use of surfactants and subsequent removalof these hydrophobic species by air bubbles [122]. Potential of ion flotation was investigated to remove cadmium, lead and copper from dilute aqueoussolution with a plant-derived biosurfactant tea saponin[123]. Precipitate flotation process is another alternative of flotationmethod, based on the formation of precipitate and subsequentremoval by attachment to air bubbles. Depending on the concentrationof the metal solution, the precipitation may proceed by formation of metal hydroxide or as a salt with a specific anion (sulfide,carbonate, etc.) [124].


Electrochemical method


Electrochemical methods involve the plating-out of metal ionson a cathode surface and can recover metals in the elemental metalstate; however these wastewater treatment technologies involve relativelylarge capital investment and the expensive electricity supply, sothey haven’t been widely applied. Electrocoagulation (EC) involves the generation of coagulants insitu by dissolving electrically either aluminum or iron ions fromaluminum or iron electrodes where, metal ion generation takes place at the anode, and hydrogen gas is released from the cathode that helps to float the flocculated particles out of the water [125]. This technique has been used with aluminium electrodes for removing Zn2, Cu2, Ni2, Ag and Cr2O7[126].Electroflotation (EF) is a solid/liquid separation process thatfloats pollutants to the surface of a water body by tiny bubbles ofhydrogen and oxygen gases generated from water electrolysis. EFhas wide range applications in heavy metals removal from industrial wastewater and application of the optimized parameters on the separation of someheavy metal ions such as iron, nickel, copper, zinc, lead andcadmium was studied[127].TheElectrodeposition has been usually applied for the recovery ofmetals fromwastewater and is called a “clean” technology with no presenceof the permanent residues for the separation of heavy metals[128].


Photocatalysis Process


In the recent years, photocatalytic process in aqueous suspensionof semiconductor has received considerable attention inview of solar energy conversion and this photocatalytic processwas achieved for rapid and efficient destruction of environmentalpollutants. Upon illumination of semiconductor-electrolyte interface with light energy greater than thesemiconductor band gap, electron-hole pairs (e-/h+) areformed in the conduction and the valence band of the semiconductor,respectively [129].These charge carriers which migrate to the semiconductor surface, are capable ofreducing or oxidizing species in solution having suitable redoxpotential. Various semiconductors have been used: TiO2, ZnO,CeO2, CdS, ZnS, etc. also a study showed photocatalytic degradation using UV-irradiated TiO2 suspension for destroying complex cyanide with a con-current removal of copper [130].


Several studies were reported for the photocatalytic reductionof Cr(VI), which is mobile and highly toxic, compared toCr(III), which is immobile and less harmful. Heterogeneous photocatalytic oxidation of arsenite toarsenate in aqueous TiO2 suspensions has also been provedrecently to be an effective and environmentally acceptabletechnique for the remediation of arsenite contaminated water.The process was performed using an adsorbent developed byloading iron oxide and TiO2 on municipal solid waste meltedslag[131].


Biological methods


Various biological treatments, both aerobic and anaerobic can be used for heavy metal removal. A fixed activated sludge system (FAS) for treatment of wastewater containing heavy metal compounds (chromium, lead and nickel was carried out [132]. There results showed that a reduction of 84%, 75% and 80%, respectively was observed in chromium, lead and nickel on using fixed activated sludge at concentration of 1 mg/L. However, a reduction of 90%, 84% and 87%, respectively was observed by increasing concentration of them to 5 mg/L. Mechanism of activated sludge process was studied which showed that the carboxylic and amino groups are two main groups responsible for the binding properties of the biomass [133]. A novel biofiltration technique was utilized for the treatment of heavy metals mainly nickel, and their mechanism for heavy metals removal along with the kinetics of biofilters and its modeling aspects were studied [134].The success in microbial cloning technique may improve the removal efficiency and hence the reduction in treatment cost. Trickling filter was used for removal of heavy metals, whereby the indigenous bacterial populations provided a certain advantage and ensured durability under various operating conditions. They operated the system in three different ways i.e. batch, continuous and sequencing batch reactor (SBR) with recirculation [135]. The use of an attached growth system provides the necessary surface for the development of biofilm structures. Biofilms provide high biomass concentration per unit volume, while bacteria can remain in the reactor for unlimited time, thus allowing the bacteria better adjustment to the environmental conditions. The studies on the metal removal from an aerobically digested sludge by chemical treatment and microbial leaching processes in laboratory reactors were carried out by addition of ferric sulphate that resulted intoacidification of the sludge and elution of heavy metals from the sludge[136].The investigation also showed that with an increase in the amount of iron added and decrease in the sludge concentration, the pH of the sludge decreased. They also observed that the Ferric iron eluted cadmium, copper and zinc effectively than sulphuric acid. This chemical method was found to be useful for the removal of heavy metals from aerobically digested sewage sludge. Attached growth waste stabilization ponds were used for heavy metal removal where experiments were conducted to investigate the performance of AGWSP units that received Cd and Cr shock loadings. Per the investigation, the waste stabilization pond (WSP) units without attached-growth media had more concentrations of the applied heavy metals present in the effluents than waste stabilization ponds[137].




Wastewater systems containing heavy metals with otherorganic pollutants, the presence of one species usually impedesthe removal of the other. For instance, hydrometallurgy, aclassical process to recover metals, is inhibited by the presenceof organic compounds and a pre-treatment step, to remove ordestroy organics, is generally required, pyrometallurgy whichcan decontaminate systems from organic pollutantsand recover metals suffers from lack of controllability,demanding extremely high temperatures. The most promisingmethods to treat such complex systems are the photocatalyticones which consume cheap photons from the UV-near visibleregion. These photo catalysts serve as electron relays, from theorganic substrates to metal ions. Thus, they induce bothdegradation of organic pollutants and recovery of metals inone-pot systems, operable at traces of the target compounds(less than ppm). Although all above techniques can be employed for the treatmentof heavy metal wastewater, it is important to mention thatthe selection of the most suitable treatment techniques depends onthe initial metal concentration, the component of the wastewater,capital investment and operational cost, plant flexibility and reliability, environmental impact, utility of the treated water etc.7February,2017



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Suggested Citation


Citation:Rakhi NMand Dayanand S (2017)Efficient Removal of Heavy metals from Oil-field Produced Water: A Review of Currently Available Techniques.Arch Petro Environ Biotechnol 2017: APEB-105.

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