Boron in appropriate, i.e. small amounts, is very useful. It is not only essential for maintaining good health, but also has a wide range of applications in various production processes. Among others, it is an important component of fertilizers and cleaning agents. Its properties are also used in the manufacture of glass and ceramics.
Boron would not pose a major challenge as a potentially hazardous component, e.g. in water – because its “natural” amount (at least in Poland) is not too high. However, a problem is posed by municipal wastewater, which may contain more boron than specified in EU and national regulations.
According to Polish standards, boron may not exceed 1 mg/dm3 in wastewater discharged to water and soil; as for industrial wastewater discharged to sewer systems, it may not exceed a maximum of 10 mg/dm3 (and this is subject to obtaining a legally required permit).
In relation to the relevant production processes, the biggest “emitters” of boron are the power industry and the glass industry. In addition, it is important not to forget about some branches of the chemical industry.
In the above-mentioned cases the amount of boron can reach up to 100 mg/dm3.
Basic methods of boron removal from wastewater
Despite the widespread use of main methods, research is still being conducted to increase the efficiency (ecological and economical) of boron removal. The methods currently in use are not among the cheapest, so various combinations of techniques are often used – including the most popular ones, i.e. reverse osmosis and ion exchange.
Depending on the individual characteristics of each order, we adapt optimal methods for boron reduction from various sources. Sometimes (especially in the case of smaller contaminants) osmosis is sufficient (however, these are rare cases).
Ion exchange is used much more frequently (although, in addition to its advantages, it also has its drawbacks; it is not a cheap technology, but depending on the bed used, certain variables can be manipulated, both in terms of investment costs and material as well as process characteristics of boron removal).
Undoubtedly, one of the main disadvantages of using ion exchange is the necessity to dispose of wastewater (from the regeneration of ionite) showing a considerable salinity level.
On the other hand, in case of electrodialysis, it is possible to rationalize the costs of the whole boron removal process to a certain extent, thanks to the production of hydrochloric acid, which can be used for technical regeneration.
The ionic method is particularly suitable for the removal of boron from solutions in which the concentration of this element is at the level of several tens of mg/dm3.
The entire process is carried out by means of ion-exchange resins. Research has shown that the best results can be achieved with a resin having a grain diameter of about 50 to 75 µm.
A frequently used material is a macroporous resin based on polystyrene with special functional groups, optimal for performing selective boron removal from aqueous solutions, including wastewater from different sources and with different boron intensity.
The resin is a fairly versatile material – it is effective over a wide pH range and for a variable range of boron intensities, and its action is effective under a variety of thermal and chemical conditions. It is also effective with alternative purification techniques.
However, it is worth noting that boron removal efficiency increases with increasing temperature.
Ion exchange would not be possible without the “element” by which the process of selective boron binding occurs – in the case of ion exchange, this is N-methylglucamine.
The size of bed also affects the efficiency of boron removal – the larger the bed (and the higher its height) the greater the boron adsorption. The bed size is also important for the success of the entire process.
Ion exchange resins require regeneration for effective operation. For this purpose, acid solutions (e.g. sulfuric acid) are used to wash out the orthoboric acid. Neutralization is also necessary, which is possible with a sodium hydroxide solution.