Boron in Produced Water: Challenges and Improvements: A Comprehensive Review
Ezerie Henry Ezechi,
Mohamed Hasnain Isa
Shamsul Rahman Bin Mohamed Kutty
Boron concentration in produced water is significantly high. Produced water is water trapped in underground formation that is brought to surface along with oil and gas during drilling. Because this water has been in contact with the hydrocarbon formation for centuries, it now contains some of the characteristics of the formation as well as the hydrocarbons itself. Concisely, boron concentration in produced water makes produced water unusable if not properly removed. The World Health Organization (WHO) regulation guidelines for discharge of water into the environment set boron concentration at 0.5 mg L-1 for potable water. Many technologies have been developed to remove boron from produced water. However, there have been series of reported limitations based on the molecular weight of boron as well as its ionic dissociation constant. The health implication of boron consumption is enormous because according to the medico-biological investigations, boron compounds belong to the second class of the toxicological danger. The purpose of this study is to make an extensive review on published literatures on boron removal technologies in general, parameters that affect the efficiency of different treatment technologies, its importance, toxicity, deficiency and dissociation constant.
December 09, 2011; Accepted: February 22, 2012;
Published: April 03, 2012
Produced water includes water trapped in underground formations and water injected
into the stratum to drive out the crude oil (Deng et
al., 2002). It is separated from crude oil above ground in an oil/water
separator (Murray-Gulde et al., 2003). 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 (Lu et al.,
2006). Produced water is the largest waste-stream of oil and gas exploration.
Global produced water production is estimated at about 250 million barrels per
day compared with about 80 million barrels per day of oil (Fakhrul-Razia
et al., 2009). The chemical composition and behavior of produced
water varies when compared with the surface waters because they are constrained
within an aquifer (Wemedo et al., 2009). Produced
water has distinctive characteristics due to the presence of organic and inorganic
matters, high salinity, BTEX, PAH, etc. which can cause toxicity to the environment.
Naturally, produced water contains various microorganisms which result in microbial
corrosion of the inner surfaces of pipes and related systems conveying the water.
Such microbial corrosion process occurs by formation of biofilms on the metal
surfaces (Puyate and Rim-Rukeh, 2008). The constituents
of produced water vary and can differ from well to well (Cakmakce
et al., 2008). The pH of produced water is about 6-8.5 (Cakmakce
et al., 2008; Veil et al., 2004) while
boron concentration in produced water is about 26-28 ppm (Cakmakce
et al., 2008). Produced water is increasingly being considered as
a way to supplement limited freshwater resources in many parts of the US as
well as other countries (Xu et al., 2008). Therefore,
effective treatment method should be employed to treat this essential water
for reuse and irrigation purposes especially in arid areas where farmers experience
Boron is a commonly known drinking water contaminant that affects the reproductability
of living organisms (Dydo et al., 2005). In nature
boron appears mostly as boric acid (H3BO3) and borax,
(Na2B4O7.10H2O). In aquatic systems,
it exists primarily as undissociated boric acid and borate ions (Bryjak
et al., 2008). The main sources of boron in surface water are urban
wastewater containing detergents and cleaning products, industrial effluents
and chemical products used in agriculture (Liu et al.,
2009a). When water with high boron concentration is used for irrigation,
boron compounds form complexes with heavy metals like Pb, Cu, Co, Ni, Cd etc.
and increase the potential toxicity of these heavy metals.
Serious health and environmental problems are caused when these complexes
pass to groundwater (Sayiner et al., 2008; Seki
et al., 2006; Seyhan et al., 2007).
Among all the elements of the periodic table, only carbon surpasses boron in
variety of applications (Melnyk et al., 2005).
The production of boron compounds has substantially increased recently due to
increasing demand for these compounds in nuclear technology, rocket engines
as fuels, production of heat resistant materials such as refractories and ceramics,
high quality steel, heat-resistant polymers, catalysts, manufacture of glass,
pharmaceuticals, corrosion inhibitors in anti-freeze formulations for motor
vehicle and other cooling systems, dyestuff production, cosmetics, flame retardants,
mild antiseptics, soaps, detergents, neutron absorber for nuclear installations,
fertilizers, disinfectants, food preservatives etc. (Yilmaz
et al., 2008a; Cengeloglu et al., 2007;
Fujita et al., 2005; Melnyk
et al., 2005; Yilmaz et al., 2005;
Magara et al., 1998).
In soil, boron is available to plants as boric acid (H3BO3),
the form in which it is absorbed by root and transported. Boron plays an important
role in plant carbohydrates metabolism, sugar translocation, hormonal action,
functioning of the apical meristem, biological membrane structure and function,
neutron capture therapy and other industrial products. Boron deficiency in plants
may result in reduced growth, yield loss and even death of plant depending on
the severity of the deficiency (Yilmaz et al., 2008b).
The range between deficiency and toxicity of boron is very narrow and influences
the total uptake of anions in plants which also affect plant growth and development
(Tariq and Mott, 2006). Boron is readily leached by
rainfall and needs to be regularly replaced in a programme commensurate with
irrigation, retentive properties of the soil and the requirements of the crops
(Gupta, 1979). For humans, boron can represent reproductive
dangers and has suspected teratogenetic properties as shown in Table
1 (Bryjak et al., 2008). Linder
et al. (1990) and Redondo et al. (2003)
reported that boron has presented impediments to male reproduction in studies
carried out in laboratories. Studies in rats, mice and rabbits have demonstrated
several developmental and teratogenetic effects.
In recent years, boron toxicity has gained an increasing interest because
of the greater demand for desalinated water, in which boron concentration may
be very high for healthy irrigation (Kaya et al.,
2009). Different authors have reported the role of salinity on toxicity
of boron to plants. Yermiyahu et al. (2008) reported
that salinity may reduce or increase boron toxic effect when both occur together
while Ferreyra et al. (1997) reported that increased
salinity decreases boron toxicity in numerous vegetables, rootstocks, wheat
and chickpeas. Very low boron concentration is required in irrigation water
for certain metabolic activities. However, if it is present in amounts higher
than required, it becomes toxic (Fujita et al., 2005;
Ozturk and Kavak, 2008). Several factors affect boron
availability to plants. Shaaban (2010) in his review,
listed soil pH, soil texture, soil temperature/moisture, soil calcium carbonate
and soil organic matter as factors affecting boron availability to plants. The
solubility and retention of boron in soil depends on the various soil components
and ions, specifically cations (K, Ca, Mg and Na) which readily combine with
boron to form metaborates. Table 2 outlines the solubility
of various boron compounds (Tariq and Mott, 2007). Therefore
care is required in the management of this essential micronutrient in plants
because the range between deficiency and toxicity is relatively narrow (Smith
et al., 2010). Plants tolerance to boron varies as outlined in Table
There is no easy method available for the removal of boron from water and wastewater
(Bick and Oron, 2005; Liu et
al., 2009b; Melnyk et al., 2005). Structural
studies have indicated that in borates, the boron atom usually combines with
either three or four oxygen atoms forming [BO3] or [BO4]
groups. Accordingly, the electronic orbitals are hybridized to a planar SP2
or a three-dimensional SP3 structure (Xue et
al., 2000). The commonly used Reverse Osmosis (RO) desalination systems
are not efficient enough in boron removal since boric acid might be transported
through RO membranes in a manner similar to water (Kabay
et al., 2008a; Oo and Song, 2009; Turek
et al., 2007a). Conventional ion exchange is also inapplicable due
to poor ionization of boron acid and requires periodical regeneration of resins
when the ion exchange capacity becomes saturated (Melnyk
et al., 2005; Park et al., 2007).
Biological treatment is inefficient because of the complex boron chemistry.
The objective of this review is to (a) Identify the factors affecting different
treatment methods in removing boron from produced water (b) Examine the importance
of integrated treatment process in removing boron from water (c) Suggest pre-treatment
and post-treatment process for boron removal.
CHEMISTRY OF BORON
Boron is a metalloid and behaves as a Lewis acid (Arias
et al., 2011). The borate monovalent anion B(OH)4- dominates
at higher pH while non-ionized boric acid B(OH)3 dominate at lower
pH (Kabay et al., 2010). The dissociation of
boric acid in water can be described as follows:
Further dissociation of boric acid in water occurs at higher temperature:
In aqueous solution, boric acid does not dissociate as a Bronsted acid but
as a Lewis acid which interacts with water molecules to form tetrahydroxyborate
ion (Arias et al., 2011):
Between pH 7 and 10 and at high concentration (>0.025 mol L-1), polyborate anions are formed:
At lower pH, the dominant boron species is boric acid while at high pH, borate compounds predominate. This property of boron has been an obstacle to many treatment methodologies.
Reverse Osmosis (RO): Reverse osmosis is a major technology in wastewater
treatment and have been used in removing different water contaminants. Newly
produced seawater reverse osmosis membranes are claimed to have boron removal
efficiency of 91-93% (Oo and Song, 2009). Conventional
RO membranes reject boron to a level of about 40-78%. Single stage RO membranes
are able to turn seawater of boron concentration (4-5 mg L-1) into
permeate water with boron concentration of about 0.9-1.8 mg L-1 (Sagiv
and Semiat, 2004). This is because boric acid can diffuse through RO membrane
in non-ionic form similar to that of carbonic acid or water. Rejection of boron
by RO is better for the borate ion due to its charge while the rejection of
non-ionized boric acid is low due to its smaller size and lack of electrical
charge. Due to the absence of ionic charges at low pH, the hydration of the
molecule results in a smaller size and less rejection of the molecule by a membrane.
The dissociated form on the other hand will be fully hydrated, resulting in
a larger radius and an enhancement of the negative charge of the ion. This results
in higher rejection both by exclusion and repulsion by the negatively charged
membrane (Redondo et al., 2003). Because boric
acid dominate at low pH, it is able to form bridges of hydrogen with the active
groups of the membrane and diffuse in a similar way to that of carbonic acid
or water (Fig. 1) (Pastor et al.,
2001). Several factors affect RO process. These factors include temperature,
operating pressure and pH. Studies have shown that characterized parameters
for selection of RO membranes should include zeta potential, contact angle,
roughness and pore size distribution. Membrane surface roughness is also important
in trapping of pollutants (Sarp et al., 2008).
Effect of pH: Boron rejection depends greatly on pH (Magara
et al., 1998). With high salinity feeds such as seawater, boron rejection
as high as 90% has been demonstrated but for low salinity waters, boron rejection
very much depends on the pH level since pH determines the form of boron in water
due to the equilibrium reaction between boric acid and borate ions (Bick
and Oron, 2005). Higher pH levels result in appreciable decrease of boron
in the permeate.
||Mechanism of H2O transport through reverse osmosis
membrane by forming bridges of hydrogen (Pastor et
||Impact of solution pH on boron rejection by A and B membranes
(batch concentration mode, pressure: 700 psi, Feed flow rate: 3.8 L min-1,
Feed temp.: 22-24 EC) (Koseoglu et al., 2008a)
This is because uncharged boron in the form of boric acid diffuses in the
membrane for pH lower than 9.5. At high pH, boric acid transforms into borate
ions (Bouguerra et al., 2008). Potential precipitation
of calcium carbonate and magnesium hydroxide at high pH in seawater reverse
osmosis applications is a problem and must be avoided (Cengeloglu
et al., 2008). Koseoglu et al. (2008a)
reported that a much higher boron rejection was obtained at pH of 10.5 (>98%)
than those at natural seawater pH of 8.2 (about 85-90%) (Fig.
2). At high pH, there is stronger repulsion between membrane active surface
and the charged borate ion which leads to the enhanced boron rejection (Bonnelye
et al., 2007). Figure 3 shows dissociation of
boric acid at various pH.
Effect of operating pressure: Increase in applied pressure increases
the net pressure and consequently the permeate flux as well as rejection of
particle across membrane (Binyam et al., 2010).
An increase in applied pressure results in an increase in water flux which leads
to higher recoveries (Bonnelye et al., 2007).
Applied pressure also increases boron and salt rejections (Bouguerra
et al., 2008; Guler et al., 2011).
The higher the applied pressure, the higher the boron rejection (Magara
et al., 1998; Koseoglu et al., 2008b).
Figure 4 shows an upward trend in boron rejection as the
applied pressure was simultaneously increased.
Electrodialysis (ED): Electrodialysis (ED) is a commonly used treatment
technology. ED is an electrochemical process in which ions migrate through ion
selective semi permeable membranes as a result of attraction to two electrically
charged electrodes. It has high rejection for total dissolved solids, ions and
colloids (Bick and Oron, 2005). Conventional ED is only
capable of removing about 42-75% of boron (Yazicigil and
Oztekin, 2006). Advantages of ED include high rejection of contaminants,
low pressure requirement (lower than RO), requires minimal supervision in a
remote setting, reject can meet effluent disposal limits and prevents scaling
while its disadvantages include high capital and operating cost, high level
of pre-treatment and frequent electrode replacement. Several factors affect
the efficiency of boron removal by ED in produced water. They include interference
of ions, pH of the sample and nature of the ED membrane. Studies have shown
that chloride and sulfate affect boron transport in ED (Bandura-Zalska
et al., 2009; Kabay et al., 2008b).
||Effect of chloride ions on boron removal (B:Cl equivalent
ratio = 1:1; D: Dilute compartment, C: Concentrated compartment) (Kabay
et al., 2008b)
Salinity affects boron removal through electrodialysis. In a study of boron
removal by electrodialysis, Kabay et al. (2008a)
observed that chloride transported more quickly than borate ions (Fig.
5). Boron transport depends on ion charges and the hydratized radius of
ions. Therefore, transport of boron becomes harder with increasing radius of
ions and ion charges (Bandura-Zalska et al., 2009).
Increase in solution pH results in increase in transfer of boron. Also, increase
in current density results in an increase in transfer of boron from the cathode
chamber to the anode chamber. A typical ED diagram is represented in Fig.
Effect of interfering ions: Transfer of boron by electrodialysis using
anion exchange membranes is influenced by salinity. It is reported that in the
presence of chloride, percent removal of boron decreases and operation time
increases while in the presence of sulfate, percent removal of boron did not
change but operation time increased. This difference in transport property of
boron in the presence of sulfate/chloride may be attributed to the hydrated
radius of several anions present in the aqueous medium (Banasiak
and Schafer, 2009; Turek et al., 2007b).
Bandura-Zalska et al. (2009) proposed an integrated
process (desalination and ED) in boron removal because of the effect of interfering
ions. Desalination is applied at low pH to remove ions at no boron transport
while ED is applied at high pH to remove boron in the second stage since borate
ions are present at high pH. The main goal of the preliminary stage is to reduce
salinity at negligible boron transport as well as maximize desalination degree
and rate at final concentrate boron level. Turek et al.
(2007a) also used desalination and ED in removing boron from wastewater.
It was observed that high initial dilute pH values do not enable reliable boron
flux through the membrane. This may be due to high mobility and relative high
content of hydroxide ions. The dissociation constant of B(OH)3 decreases
with increasing NaCl concentration. NaCl concentration affects removal of other
ions. Ions with smaller intrinsic crystal radii have higher hydration numbers,
larger hydrated radii and hold their hydration shells more strongly. The larger
the crystal ionic radius, the more diffuse the electric charge and the fewer
water molecules surround the ion (Banasiak and Schafer,
2009). Kabay et al. (2008b) in their study,
observed that more than 90% of chloride ions was removed from the solution in
18 min while only 20% of boron was removed along with chloride. After 42 min
of operation time, almost all chloride ions were removed from the solution while
about 40% of boron still exists in the solution (Fig. 7).
Adsorption: Adsorption is a process through which boron is adsorbed
unto different kinds of particles. Many adsorbents have been used in adsorption
process. They include cerium oxide (Ozturk and Kavak, 2008),
activated carbon prepared from coconut shell impregnated with calcium and barium
chlorides, citric and tartaric acids (Rajakovic and Ristic,
1996), activated alumina (Bouguerra et al., 2008),
Al2O3 based materials (siral 30 and pural) (Seki
et al., 2006), activated carbon impregnated with salicylic acid (Celik
et al., 2008), iron-rich natural clays (Seyhan
et al., 2007), activated sludge (Fujita et
al., 2005), neutralized red mud (Cengeloglu et
al., 2007) and composite magnetic particles (Liu
et al., 2009b). Several factors affect adsorption of boron. They
include loss of adsorbent, geometry of the system, the flow of water solution
through the pores of the system, the diffusion rate, adsorbent dose and the
kinetics of the surface reactions (Ozturk and Kavak, 2008).
||Effect of pH on the adsorption of boron on cotton cellulose
(C0 = 500 mg L-1, m = 0.1 g, V = 50 mL) (Liu
et al., 2007)
Sample pH is a determinant of boron removal in adsorption study (Garcia-Soto
and Camacho, 2006). Depending upon the nature of adsorbents, Irawan
et al. (2011) reported that borate concentration in the solution
and its adsorption are both pH-dependent and the optimum pH of boron adsorption
is 2 for fly ash, 6.0 for composite magnetic particles, 7 for neutralized red
mud and 9 for layered double hydroxide. Solution pH affects both boron speciation
and surface properties of sorbent (Wei et al., 2011).
The amount of removed boron increases with increasing adsorbent dose due to
the increase in the total available surface area of the adsorbent particles
(Yazicigil and Oztekin, 2006). Studies have shown that
different adsorbents have varying adsorption capacities (Bouguerra
et al., 2008; Ozturk and Kavak, 2008; Seki
et al., 2006; Yilmaz et al., 2005).
Figure 8 represents the effect of pH on boron adsorption on
cotton cellulose. Different adsorbents have different adsorption capacities.
Table 4 is a compilation of different adsorbent materials
with their adsorption capacities.
Effect of adsorbent dose: It is essential to determine the amount of
adsorbent that is required for an experiment (Garcia-Soto
and Camacho, 2006). Increase in adsorbent dose increases the surface area
of the adsorbent particles. Boron removal increased with increase in red mud
dosage as a result of the increase in total available surface area of the adsorbent
particles. Therefore, the higher the adsorbent concentration, the lower the
boron level in the permeate (Bryjak et al., 2008).
Figure 9 represents variation of boron removal with red mud
dose (Cengeloglu et al., 2007).
|| Different adsorbents and their adsorption capacities
||Variation of boron removal with red mud dose. Initial boron
concentration: 43 mg L-1, pH 7, agitation speed: 500 rpm and
temperature: 25±1°C. (Cengeloglu et al.,
Ion exchange: Ion exchange is one treatment process that has gained
wider application in boron treatment. The use of selective ion exchange resins
(Purolite and Diaion) have not only been efficient in removing boron from wastewater
but also in recovering boron from wastewater (Kabay et
al., 2004). The most commonly used resins are the Amberlite IRA-743,
otherwise known as Amberlite XE-243 and Diaion CRB 02 and Dowex XUS 43594.00
(Kabay et al., 2008c). These resins are macroporous
polystyrene based resin, with functional groups specially designed for the selective
removal of salts of boron from aqueous solutions. They are effective for solutions
over a wide range of pH values and over a wide range of boron concentrations.
These boron selective resins show great elimination performance and the sorption
kinetics of both resins fit well to pseudo-second-order kinetics model (Shpiner
et al., 2009). This complex formation is not pure ion exchange and
therefore does not require ionization of boric acid. The resin performance is
not affected by temperature variations, by pH value or by the background salinity
of the water to be treated. Selective sorption of boron by these sorbents is
as a result of formation of stable complexes like ethers or complex anions with
polyoxicompounds. These resins used in ion exchange system have a macroporous
polystyrene backbone and a very specific functional group based on N-methyl
glucamine which has a tertiary amine end and a polyol end and makes a very stable
complex with boric acid (Jacob, 2007). Boron removal
efficiency here is reported to be about 93-98% (Melnyk et
al., 2005). A typical ion exchange selective resin is shown in Fig.
Uptake of boron by this selective ion exchange resin is high. An evaluation
study with Diaion (CRB 02) and Dowex(XUS 43594.00) resins by Kabay
et al. (2006) shows that both resins have almost similar boron adsorption
capacity and are efficient in removing boron from solution as shown in Fig.
|| Chemical structure of boron selective resin containing N-methyl
glucamine (Kabay et al., 2008c)
||Boron adsorption isotherms of Diaion CRB 02 and Dowex (XUS
43594.00) resins (Kabay et al., 2006)
The advantages of these selective ion exchange resins over the conventional
ion exchange treatment are that they are not affected much by pH of solution
The mobility and hydratized radius of ions affects boron transport in an ion
exchange process (Kir et al., 2011). An increase
in pH increases removal of boron by the resin since the resin can only extract
the ionic form of boron (Fig. 12). An increase in temperature
also increases the removal of boron from brine (Yan et
Biological treatment: Biological treatment of produced water is still
in its infancy (Shpiner et al., 2009). Reported
literatures on boron removal through biological treatment are scarce. Previous
works have been mainly laboratory/pilot scale experiments of aerobic and anaerobic
treatment such as activated sludge (Tizghadam et al.,
2008; Dincer, 2004; Tellez et
al., 2002), aerated lagoons (Boiran et al.,
1996; Montalvo et al., 2010), SBR system
(Li et al., 2003; Xiao et
al., 2008), immobilized cells system (Li et al.,
2005; Lu et al., 2009; Wang
et al., 2008), Biodegradation (Rezaee et al.,
2006), dinitrogen fixation (Perona et al., 1991),
sulfate removal (Chen et al., 2009; Zhou
et al., 2011; Zhao et al., 2009),
activated sludge, sand filters and activated carbon (Al-Jlil,
2009), sulfidogenic bacteria (Agrawal et al.,
2010; Kaura et al., 2009), geochemical influence
and biokinetics (Orphan et al., 2004; Tellez
et al., 1995), Semi-continuous and continuous anaerobic treatment
(Hong et al., 2009).
Various biomass species have been used in a single and combined process to
treat produced water but there have been marked inefficiencies from literature
over their ability to feed on colloids. Single step biological treatment has
proved ineffective in removing many pollutants of wastewater. Conventional biological
treatments have not been effective enough to reduce boron to its standard limit
level for irrigation (Linares-Hernandez et al., 2010).
One of the biological treatment methods that have been used in the treatment
of wastewater pollutants is Waste Stabilization Ponds (WSPs). WSPs incorporate
the activity of phototrophic, autotrophic and heterotrophic microorganisms,
requires little or no energy and play important role in the removal of pollutants.
Oxidation ponds are characterized by a high surface area and retention times
of a few days to a few weeks. The biodiversity in the ponds may also influence
the potential of the process to degrade a wide range of petroleum-derived compounds
including the full or partial degradation of slowly degradable and recalcitrant
material. WSPs have a large volume which enables them to tolerate shock loads
and efficiently remove heavy metals (Shpiner et al.,
2009). In a study of biological treatment of boron containing wastewater
by activated sludge conducted by Dincer, 2004, it was
observed that boron concentration affected COD removal rate when compared with
the COD removal rate of a boron free wastewater. Whereas the COD concentration
of a boron free wastewater in the reactor reached a steady state level of 20
mg L-1 after 9 h of operation, resulting in a COD removal rate of
75.39 mg L-1 h-1 and a removal efficiency of 98% at a
loading rate of 76.92 mg L-1 h-1, the COD removal rate
of a boron containing wastewater for feed boron levels lower than 2500 mg L-1
has efficiencies of about 80%. However, when the feed boron level is higher
than 2500 mg L-1, the system did not reach steady state and the COD
removal rate decreased. The rate of COD removal also decreased with increasing
boron in the feed wastewater, as shown in Fig. 13. The COD
removal rate was nearly 75.39 mg L-1 h-1 with boron free
wastewater and decreased to a value of 45.39 mg L-1 h-1
for 5000 mg L-1 boron concentration, indicating approximately 40%
||Variations of total COD removal rate with H3BO3-B
concentration in feed wastewater at steady-state (Dincer,
Electrocoagulation (EC): Electrocoagulation (EC) is the process of destabilizing
suspended, emulsified or dissolved contaminants in an aqueous medium by introducing
an electric current into the medium (Emamjomeh and Sivakumar,
2009). EC is based on the valid scientific principle of water response to
strong electric field. EC is an emerging treatment technology which has been
applied with success in wastewater treatment (Linares-Hernandez
et al., 2010). Electrocoagulation involves the generation of coagulants
in situ by electrical dissolution of the sacrificial anode and involves the
following three mechanisms; electrode oxidation, gas bubble generation, flotation
and sedimentation of flocs formed (Emamjomeh and Sivakumar,
EC presents similar advantages as chemical coagulation and reduces its disadvantages
which results in higher yields and less waste sludge (Essadki
et al., 2009).The advantages of EC include high particulate removal
efficiency, compact treatment facility, relatively low cost and possibility
of complete automation (Aoudj et al., 2010; Chen,
2004). EC is characterized by a fast rate of pollutant removal, simplicity
in operation and low capital and operating costs (Chen et
al., 2000). EC has been used in reducing boron to the World Health Organization
(WHO) standard limit of 0.5 mg L-1. The efficiency of EC depends
on sample pH, current density, temperature, boron concentration, electrode spacing
and treatment time. pH is an important parameter influencing the performance
of the electrochemical process because pH determines the ionic form of boron
(borate ions) (Balasubramanian et al., 2009).
At higher inter-electrode distance, rate of aggregation of suspended particles
as well as adsorption of contaminants will be low. At minimum inter electrode
distance, the resistance for current flow in the solution medium is lower and
that facilitates the electrolytic process for enhanced removal (Ghosh
et al., 2008).
|| Changing of borate ions species depending
The current density is the current per unit surface; it is a parameter that
controls the anode dissolution speed on the one hand and that of hydrogen formation
on the other hand (Nanseu-Njiki et al., 2009).
Effect of pH and current density on EC is represented in Fig.
14 and 15.
Boron removal efficiency of 95% with electrocoagulation was reported by Yilmaz
et al. (2008b) (Fig. 16). Electrocoagulation can
be an efficient means of removing boron from wastewater in a cost effective
way when the operating parameters are favorably combined.
Integrated system: Different treatment techniques can be combined to treat produced water. Some studies have used integrated process to treat produced water.
|| The effect of temperature on boron removal (3.0 mA m-2
current density, pH 8.0 and 150 rpm stirring speed) (Yilmaz
et al., 2008b)
Ebrahimi et al. (2010) used Microfiltration
(MF) and Ultrafiltration (UF) as a pretreatment step and Nanofiltration (NF)
as the final step in the treatment of oilfield produced water. Turek
et al. (2005) studied adsorption/co-precipitation process coupled
with reverse osmosis. Li et al. (2010) investigated
the effects of the combination of anaerobic process and micro-electrolysis in
the removal of COD and biodegradables. The integrated system can be used to
remove boron from produced water. Each component of the integrated system should
be operated at its best operating condition. However, the cost of such combination
should also be taken into consideration to ascertain its feasibility. Various
membrane methods like MF, UF, NF and RO can be combined, RO and ED; Micro-electrolysis
and biological treatment; EC and biological treatment/RO; adsorption/co-precipitation/RO;
ion exchange and RO, 2 pass RO with increased pH especially in the 2nd pass
Boron is a complex compound and its behavior in water has made its removal
very difficult. One of the best ways to reduce boron problem is to reduce the
level of its pollution anthropogenically. Therefore, perborates used in detergent
products should be regulated because it also increases the quantity of boron
in surface water. The treatment technologies enumerated in this study have shown
their ample abilities in removing boron from wastewater but their efficiencies
depend on the operating conditions of each system. Reverse osmosis should be
equipped to remove boron at one stage pass. This can be achieved by manufacturing
membranes that will enable more investigation on boron removal based on transport
equation and not in terms of flux and salt rejection only. Pre-treatment or
post-treatment of wastewater through an integrated process results in effective
treatment. However, these integrated systems should be arranged in such a way
that the systems will be compatible to each other and also operate at their
optimal efficiencies. In the case of boron, produced water should be pre-treated
to recover boron based on its wide importance and applicability.
This study is supported by Universiti Teknologi PETRONAS (UTP). The authors wish to thank UTP for their overwhelming support.
Agrawal, A., K. Vanbroekhoven and B. Lal, 2010. Diversity of culturable sulfidogenic bacteria in two oil-water separation tanks in the North-Eastern oil fields of India. Anaerobe, 16: 12-18.
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