Boron

Boron (B)

1. Basic Information

2. Physical and Chemical Properties

• Boron is a metalloid element with three allotropic forms: amorphous, crystalline, and metallic.

• Has a melting point of 2076 ° C and a boiling point of 3927 ° C.

• Elemental boron is very hard, has good thermal conductivity, and is a semi-conductor

• Reacts with oxygen to form boron oxide (_B2O3)

• Reacts with nitrogen to form boron nitride (BN)

3. Presence in Water and Health Effects

Boron is naturally present in groundwater especially in volcanic areas. Other sources include detergents and industrial products. In general, concentrations range from 0.1-0.5 mg/L, but can be higher in some places. At high levels, boron can cause gastrointestinal irritation, organ damage, and reproductive disorders. WHO sets a limit of 0.5 mg/L boron in drinking water.

4. Water Treatment Applications and Separation Methods

Boron is difficult to remove from water due to its properties similar to silica and its weak affinity for common ion exchange resins. The main methods of boron removal include:

• Reverse osmosis (RO): RO membranes can reject boron with high efficiency. Using a high pH (10-11) can enhance boron removal.

• Specialized ion exchange resins: Boron-selective anion exchange resins such as sorbitol-N-methylglucamine can be used effectively. Regenerate the resin using acidic and alkaline solutions.

• Adsorption: Oxide-based adsorbents such as activated alumina, zirconium oxide, and modified iron oxide are capable of adsorbing boron.

• Hybrid RO-IX: Combining RO and a specialized ion exchanger (usually in a lead-lag-polish scheme) provides maximum boron removal.

5. Industrial Uses in Water Treatment

In the power generation industry, flue gas desulfurization (FGD) blow-down water often contains high levels of boron (200-500 mg/L). Special anion exchange resins can be used to reduce boron levels to meet discharge limits. In the purification of magnesium chloride solution (raw material of magnesia), boron selective resin can reduce the level from about 100 ppm to <10 ppm at 12% MgCl2 concentration.

6. Case Study and Real Application Example

• Drinking Water Treatment in Rural Turkey

In Karakoçan Village, Turkey, boron concentration in groundwater reached 2-12 mg/L. A small-scale RO system (7 m3/h) was installed to provide drinking water. The system consisted of a multimedia pretreatment unit of filter and softener, and 2 stage RO with a pH of 10.5. The system successfully reduced boron levels to <0.5 mg/L and produced water of excellent quality.

• Boron Monitoring Program in California

The California Drinking Water Agency (CDPH) has required regular monitoring of boron levels in drinking water sources since 2002. Of the >7000 water sources monitored, 8% contained boron >1 mg/L and 1% >5 mg/L. CDPH set a standard of 1 mg/L boron for health protection. Various technologies such as RO, ion exchange, and adsorption have been applied to achieve this target.

7. Regulatory Guidelines and Standards

• WHO: 0.5 mg/L (provisional, 1998)

• US EPA: not yet established

• European Union: 1.0 mg/L (parameter value, 1998)

• Canada: 5.0 mg/L (MAC, 2020)

• Australia: 4.0 mg/L (aesthetics, 2011)

• Indonesia: 0.5 mg/L (drinking water quality standard, 2010)

8. Environmental Impact and Sustainability Considerations

Boron compounds are generally not considered highly toxic chemicals and do not accumulate in the food chain. However, high levels of boron have the potential to damage sensitive crops such as citrus. The boron threshold for irrigation in most countries is 0.5-1 mg/L. The regeneration waste of boron ion exchange resin must be properly handled to prevent environmental pollution. Research needs to be developed to recycle and recover boron from RO rejection and ion exchange regeneration effluents.

9. Future Trends and Research

Some of the research and development areas related to boron in water treatment include:

• Development of boron-selective ion exchange resins and adsorbents with higher capacity and kinetics

• Optimization of hybrid/integrated systems for deep boron removal

• Energy-efficient boron separation technologies such as electrodialysis, liquid membrane, etc.

• Utilization and recovery of value-added boron compounds from wastewater/concentrate

• Toxicity studies and establishment of more comprehensive boron regulations

10. Interesting Facts

• In small doses (<0.5 mg/L), boron is actually essential for human and plant health. Boron is involved in bone metabolism, brain function and calcium absorption.

• The main sources of human exposure to boron are food (fruits, vegetables, nuts) and health supplements, not drinking water.

• The semiconductor industry requires purified water with very low boron levels (<50 ppt) to prevent chip damage due to boron contamination.