Boron

Boron is a chemical element with

symbol B and atomic number 5.

Boron, an inorganic compound, is

a non-volatile metalloid that is

ubiquitous in the environment in

compounds called borates.

Common borates include boron

oxide, boric acid, and borax.

Boron properties

Boron properties

Boron properties

Boron properties

Natural weathering processes are largely

responsible for the presence of boron in seawater.

Also, boron can be found naturally in ground water,

but its presence in surface water is a consequence

of the discharge of treated sewage effluent, in

which arises from use in some detergents, to

surface waters.

Boron Sources

Anthropogenic Sources

Glass and ceramics and porcelain,

detergents and soaps , cosmetics, bleaching agents

coal burning power plants

Insecticides

High-hardness and abrasive compounds

Shielding in nuclear reactors

Pharmaceutical and biological applications

fire retardants, glazes and agricultural products

food preservatives

For humans boron can represent reproductive

dangers and has suspected teratogenetic properties.

A major limiting factor is the possible damage to plants

and crops. Excess boron also reduces fruit yield and

induces premature ripening on other species such as

kiwi

two predominant reasons for

limiting boron in water:

Intakes of more than 0.5 grams per day for 50 days cause

minor digestive and Boron toxicity symptoms in humans

include diarrhea, nausea, vomiting, lethargy, dermatitis,

poor appetite, weight loss, and decreased sexual activity

Health issues and toxicity

In seawater sources, the typical boron concentration in

the raw water is in the range of 4.6 mg/l while in confined

ocean bodies boron concentration can deviate

substantially from this average value, for example, boron

concentration in the Mediterranean Sea can be as high

as 9.6 mg/l Depending on location and seasonal effects,

the boron concentration can exceed 7 mg/l, e. g. in the

persian Gulf

Boron

Boron removal processes

Unlike most of the elements in seawater, boron is not ionized (i.e. it

has no charge)

Boron takes two forms in drinking water (or seawater):

Boric Acid: H3BO3

Borate Ion: H3BO2-

RO is much better at removing charged ions. Hence the removal of

borate ion is much better than the removal of boric acid.The dominant

form (borate or boric acid) depends on the pH:H3BO3=H++H3BO2-

Why is Boron Hard to Remove?

Boric acid is uncharged and has trigonal structure.

Therefore, boric acid is nonpolar, which causes it to

interact very differently with membrane materials relative

to charged salt ions and polar water molecules. RO

membranes that permit high boron rejection along with

high salt rejection and high water permeation might be

manufactured through careful consideration of physical

structure and chemical composition.

Why is Boron Hard to Remove?

.

RO

RO processes have beenwidely used for seawaterdesalination. Despite highremoval (>99%) of otherionic species from seawater,the removal of boron by ROhas proven challenging.Due to the recentimprovement of membraneperformance, seawater RO(SWRO) membranes, whichachieve up to 95% rejectionof boron in themanufacturer’s testingcondition, have beencommercialized.

It is difficult for a single-stage RO process to

achieve an average rejection over 90% and to

produce permeate that meets the provisional WHO

boron guideline.

RO

Generally, the rejection of boron has been lower than

90% and has been reported to be as low as 40% with

low-pressure brackish water RO membranes.

Previous studies have shown that boron rejection by

RO membranes improves as pH increases (i.e., as

major species shift toward increasingly deprotonated

forms), as operating temperature decreases, and as

transmembrane pressure increases

RO

Chemistry of boron

Boron is usually present in water

as boric acid, a weak acid which

dissociates according to:In the

usual pH operating range of

reverse osmosis elements, Eq.

(1) is the one with the highest

importance. We thus have a

presence of both dissociated and

non-dissociated boric acid

species in the water

There are several methods applied in seawaterdesalination and they can also be implemented forremoval of boron

1. Use of improved RO membranes with higher B-rejection

2. Increasing the pH of the water to be treated bycaustic soda (or other base) prior to RO membrane,and reacidifying the treated water after the membraneto bring it to the desired acidity

3. Passing the desalinated water through two extrapassages of RO treatment

4. Adding an electrodialysis stage after RO treatment

5. decreasing the tempreture

AMENDMENT

3233 BOHHBOH

Not

rejected

Well

rejected

ln practice, RO seawater desalination consist of

two or more passes with natural pH (pH 6 to 7)

at the first pass and elevated pH up to 11 at the

second pass to effectively remove boron to

acceptable levels (usually less than 0.5 mg/L)

Boron Removal

Furthermore, already at pH higher than 9, calcium

carbonate (CaCO3) and magnesium hydroxide

(Mg(OH)2) salts can crystallize on the membrane surface,

leading to fouling problems. For these reasons, several

RO desalination plants have been designed with more

stages in series operating at different pH values. In order

to obtain boron concentrations 0.4 ppm often boron

selective resins are coupled to the RO units

Solve Problem!

Single-Pass RO System

Double-Pass RO System

Single-Pass RO with Boron Specific Ion Exchange Resin

Multistage RO Systems

Configurations of different process

options for boron removal.

For the treatment of seawaters, a typical single-pass process

would operate at a recovery from 40 to 50% and a permeate

flux of 7 to 9 gfd (12 to 15 L/m2-hr).Typical feed pH for these

systems ranges from 6.0 to7.5 (acidified) or 7.8 to 8.2. Under

these conditions, the single-pass SWRO membrane unit

generally produced permeate with salinity within the potable

limits (i.e., less than 500 mg/L TDS) from the simulation data.

However, boron concentrations in the permeate were likely to

be much higher than 0.5 mg/L.

Single-Pass RO System

the rejection of boron in a single-pass configuration

can be significantly enhanced by increasing the

feed pH. Figure (b) shows a single-pass RO

process with feed pH adjustment. In this

configuration, an antiscalant must be used if the pH

is increased above 9.5.

Single-Pass RO System

The double-pass process typically consists of a leading SWRO

unit (RO1) operating at a recovery of 40 to 50% followed by a

brackish RO unit (RO2) operating at a recovery of 85 to 90%.

Since the feed to the RO2 process is the RO1 permeate (i.e.,

RO2 feed has low salinity), the RO2 unit operates at a

relatively high flux (typically 20 gfd). Therefore, the number of

elements required in the RO2 unit would be relatively small,

thereby lowering marginal capital costs.

Double-Pass RO System

Capacity: 136,000m3/d

Membrane Type

1st Pass: TM820H-400B

2nd Pass: TM720-430

Boron regulation : <0.5mg/l

Recovery Rate

1st Pass: 45%,

2nd Pass: 90%,

Pass 1

Permeate

Tank

Post

Treatment

Energy Recovery

(DWEER : Calder)

Pass 1 : SWRO

Recovery : 45%

Pass 2 : BWRO

Recovery : 90%

pH : 10.0 – 10.4

RO Feed

Water

Tank

Bypass

Scale

inhibitorNaOH

RO section Detail

Low Pressure

Pump (VFD)High Pressure

Pump

Booster

Pump (VFD)

Low Pressure

Pump

2-pass system with alkaline dosing is applied for Boron removal

High Boron Rejection Seawater Desalination Plant in Singapore

does not require a pH adjustment of the RO permeate.

Recovery rates for ion exchange systems are typically

very high (~98%). However, O&M costs of ion exchange

systems tend to be high due to the expense of specialty

resins required for removing boron and the need for resin

regeneration.. For the cost analysis, it was assumed that

the ion exchange unit treated 16% of RO permeate.

Single-Pass RO with Boron Specific Ion Exchange Resin

Dual-Pass RO with Boron Specific Ion Exchange Resin

the low recovery and scale formation potential problems

associated with double-pass systems might be effectively

avoided by multistage configurations without requiring the costly

ion exchange process. In both configurations, additional RO units

are employed to further treat the concentrate produced from the

second pass RO unit. in figure 5.1-(e), the concentrate from the

second-pass RO is treated by an ion exchange softening

process to remove divalent cations. The effluent from the

softener is further treated by RO (RO3). Since there is little

calcium and magnesium present in the effluent from the softener,

risk of scale formation in the RO3 unit is minimal, regardless of

pH. In addition, the concentrate from the RO3 unit is essentially

a pure NaCl solution.

Multistage RO Systems

In this configuration, the concentrate from the second pass RO

(RO2) is directly treated with another RO unit (RO3). To

prevent scale formation, pH of the RO2 concentrate is reduced

by acidification, prior to processing by the RO3 unit. The

permeate from RO3, which has a very low concentration of

divalent cations, is further processed with an additional RO unit

(RO4) at an elevated pH.

Multistage RO Systems

Estimated water production cost for each

configuration.

Feed water: pH, temperature, TDS

Membrane element: membrane chemistry, element

efficiency

System design and operation: average permeate

flux (APF), system recovery, concentration

polarization, cleanings

FACTORS INFLUENCING BORON REJECTION

. In the case of seawater desalination by reverse

osmosis, (RO) the boron rejection is usually

insufficient to obtain desalinated water (RO

permeate) that can meet drinking water quality

requirements.

Multi-step RO systems or RO-IE (ionic exchange)

combinations are then applied.

Conclusions

Surface analyses showed that all membranes

tested had a negative surface charge and a ridge

and valley structure. The negative charge of the

membrane played an important role in boron

removal, since charge repulsion is one of the

important mechanisms of boron rejection..

Conclusions