Boron (B) is a naturally occurring element found combined with oxygen in borates. Its presence in water originates from the natural leaching of rocks and soils, particularly in geothermal areas and regions with mineral-rich deposits. While boron is a trace element necessary for plant growth, elevated concentrations in drinking water or irrigation sources can pose health and environmental risks. Removing boron from water is necessary to meet increasingly stringent regulatory guidelines.
Understanding Boron Sources and Standards
Boron exists in water primarily in two forms: uncharged boric acid (\(\text{H}_3\text{BO}_3\)) and the negatively charged borate ion (\(\text{B}(\text{OH})_4^{-}\)). The ratio between these two forms is highly dependent on the water’s pH. Boric acid is the dominant species under neutral or slightly acidic conditions. Since the acid dissociation constant (\(\text{pK}_{\text{a}}\)) for boric acid is approximately 9.2, the shift to the charged borate ion only occurs significantly above a pH of 9.
Natural sources, such as the weathering of borosilicate minerals and volcanic activity, contribute to its presence. Human activities, including industrial discharges and the use of certain fertilizers, also introduce boron into water systems. The concentration of boron in groundwater can range widely, sometimes exceeding 100 milligrams per liter (\(\text{mg}/\text{L}\)) in areas with rich deposits.
Regulatory bodies establish guidelines to limit exposure due to potential health concerns, such as effects on the reproductive system observed in animal studies. The U.S. Environmental Protection Agency (EPA) has set a long-term health advisory level for children at 2.0 \(\text{mg}/\text{L}\) and for adults at 5.0 \(\text{mg}/\text{L}\). The World Health Organization (WHO) provides a guideline value for boron in drinking water of 2.4 \(\text{mg}/\text{L}\).
Boron Removal Using Reverse Osmosis
Reverse Osmosis (RO) utilizes high pressure to force water through a semi-permeable membrane, separating dissolved salts and larger molecules. While RO membranes excel at rejecting charged species, their effectiveness against boron is limited under typical operating conditions. This limitation arises because the neutral boric acid molecule is small and uncharged, allowing it to pass through the membrane material with relative ease. This results in boron rejection rates as low as 50-70% at a neutral pH.
To significantly enhance boron rejection, the water’s pH must be deliberately increased, a process known as alkalization. Raising the pH above 9.2 converts the neutral boric acid molecules into the charged borate ions. These charged ions are then effectively rejected by the negatively charged RO membrane surface through electrostatic repulsion. Implementing this pH adjustment, often using sodium hydroxide (\(\text{NaOH}\)), can boost boron rejection rates to over 99%.
This chemical conditioning introduces operational challenges, primarily increasing the risk of mineral scaling on the membrane surface. The elevated alkalinity can cause sparingly soluble salts, such as calcium carbonate and magnesium hydroxide, to precipitate. This necessitates careful system design and often requires the use of anti-scalants to protect the membrane. Industrial-scale systems frequently employ a multiple-pass RO configuration or a second-pass RO with \(\text{pH}\) adjustment to balance rejection efficiency with membrane longevity.
Removal Using Specialized Resins and Adsorption
For situations requiring very low boron concentrations or where the feed water chemistry is incompatible with high-\(\text{pH}\) RO operation, specialized removal methods utilizing chemical affinity are often implemented. Boron-selective ion exchange (IX) resins operate on the principle of adsorption and chelation rather than physical size exclusion. These resins are typically macroporous, polystyrene-based beads that contain specific functional groups chemically bound to their surface.
The most common functional group used in these specialized resins is \(\text{N}\)-methyl-D-glucamine (NMDG), which has a high affinity for borate ions. The NMDG groups contain diol structures that form a stable complex with the borate ion, effectively pulling it out of the solution. This chemical selectivity allows the resin system to achieve consistently high removal efficiencies, often exceeding 99%.
Once the resin’s capacity is exhausted, a two-stage regeneration process is necessary to restore its function. The first stage involves displacing the bound borate ions using a chemical regenerant, typically a strong acid or a concentrated salt solution. The second stage neutralizes or converts the resin back into its active form, preparing it for the next service cycle. While specialized resins offer superior performance, they involve higher operational costs and greater complexity due to the need for chemical handling and managing the concentrated regeneration effluent.
Selecting the Appropriate Removal System
Selecting an appropriate boron removal system requires comprehensive water quality testing to determine the baseline boron concentration and the water’s \(\text{pH}\). This data is crucial because the water’s existing \(\text{pH}\) dictates the ratio of boric acid to borate ions, which directly influences the feasibility and cost of an RO system. The required final water quality, such as a stringent limit for irrigation water used on sensitive crops, will also narrow the choice of technology.
Reverse Osmosis is generally the preferred solution for large-scale applications, such as municipal desalination plants, due to its high throughput and broad capability to remove total dissolved solids. However, residential or point-of-use (POU) RO systems, which lack the ability to perform \(\text{pH}\) adjustment, may not achieve satisfactory boron removal if the concentration is high. Conversely, specialized ion exchange resins are well-suited for industrial polishing applications, smaller-scale systems, or where the feed water has a low total dissolved solids concentration but requires ultra-low boron levels.
A primary consideration for both technologies is the management of the resulting waste stream. Both RO and ion exchange produce a concentrated liquid waste containing the removed boron, known as brine or regeneration effluent. This concentrated waste stream requires specialized treatment, such as evaporation or chemical precipitation, to ensure compliance with environmental regulations. Regular water re-testing after installation is necessary to confirm that the chosen system is maintaining the desired removal performance over time.