Dow Water & Process Solutions

Industries & Applications

Boron Removal

Boron is a natural element that is prevalent in ground water from areas with volcanic geology, present in all sea water (3-5 mg/l) and is used in commercial products such as detergents. Mildly toxic to humans but frequently very toxic to plants, especially to citrus varieties, boron is regulated in most regions of the world. The World Health Organization (WHO) guideline for boron content in drinking water is 0.5 mg/l (ppm). Dow Water & Process Solutions AMBERLITE™ PWA10, a boron selective Ion Exchange (IX) resin that can help you to meet your drinking water and irrigation water requirements.

Boron present in water is always in some form of boric acid, a very weak acid similar to silicic acid with a pK value of 9.1. At a pH lower than 7, boric acid is undissociated as H3BO3 or B(OH). Boric acid is a Lewis acid, i.e. an oxidane–acceptor rather than a deuteride donor:

lewis

At a pH higher than 11.5, boron occurs as dissociated borate [B(OH)4]–. In concentrated solution, polymeric ions are formed:

borate

AMBERLITE PWA10 is used for selective boron removal in drinking water applications. This unique chelating resin uses a carbohydrate as the active group. This figure shows the resin composition and the mechanism that chelates boron.

boron-formula

Boron concentration and salt background have little effect on AMBERLITE™ PWA10 operating capacity or boron leakage. The most important parameter affecting capacity is flow rate. These figures show the operating capacity and the boron leakage under controlled laboratory conditions.

AMBERLITE PWA10 Operating Capacity

boron-op-capacity

AMBERLITE PWA10 Boron Leakage

boron-leakage

Regeneration

AMBERLITE PWA10, a weakly basic resin, regenerates in a two-step, co-flow manner. First, the borate is displaced with hydrochloric (HCl) or sulphuric acid (H2SO4), then the resin is converted back to the free base form with sodium hydroxide (NaOH). Regenerant levels are approximately stoichiometric: roughly 1 equivalent of acid per liter of resin, or about 130% of total capacity, and 0.7 eq of sodium hydroxide. Because the regeneration procedure does not depend on capacity and involves complete conversion of the active groups, the optimum service flow rate will be site specific based on local conditions.

Ste
Regeneration
Displacement 1
Neutralization
Displacement 2
Final Rinse
Solution HCI H2SO4 DL Water NaOH DL Water Water
Concentration (%) 3.5-5 3.5-5 N/A 2.5 N/A N/A
Dose (g/l) 35 50 N/A 28 N/A N/A
Volume (BV) 1 1 3 1 2 3
Rate (BV/h) 1.5-2 1.5-2 3 2-3 3 3

Boron Removal for Sea Water Desalination

Sea water contains 3-5 mg/L (ppm) of boron. Removing this boron through the Sea Water Reverse Osmosis (SWRO) desalination process can be difficult because SWRO membranes have limited boric acid rejection, unless operated at high pH (~ 10). After a first pass, SWRO permeate contains 1 to 3 mg/l of boron. IX can be used in combination with SWRO to achieve the desired levels of boron in product water during desalination. This figure shows an IX/SWRO combination. The boron leakage from AMBERLITE PWA10 is very low. If the treated water specification is higher than 0.2 mg/L as B, a part of the stream can be bypassed to reduce the size of the plant. Two parameters govern the percentage of bypass: the boron concentration in the feed and the treated water specification. Local conditions will dictate the optimum configuration of SWRO, brackish water RO, and IX for meeting boron limits and cost targets.

MDT3_fig1_r2

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