Salt water contains dissolved mineral salts, primarily sodium chloride, which make it unsuitable for human consumption or agriculture. When people seek to “neutralize” salt water, they are usually referring to the complex process of removing these dissolved solids, known as desalination. This process involves separating water molecules from the salt ions, a challenge that requires significant energy and specialized technology. Desalination is achieved through various physical, chemical, and electrical methods developed to remove salinity and produce fresh water.
Why Salinity Removal Is Not pH Neutralization
The term “neutralize” in chemistry describes balancing a solution’s acidity or alkalinity to achieve a neutral pH of 7. Seawater is naturally slightly alkaline, typically exhibiting a pH between 7.5 and 8.4, mainly due to the presence of bicarbonate and carbonate ions. Sodium chloride, the main component of sea salt, is formed from a strong acid and a strong base, meaning its dissolution in water does not significantly alter the pH level. The goal of desalination is not to change the concentration of hydrogen ions (pH), but to physically remove the much higher concentration of dissolved salt ions, which are measured as salinity. Salinity is the measure of total dissolved solids, averaging around 35 parts per thousand (ppt) in ocean water, which is far too high for human use.
Physical Desalination Methods
The most prevalent and large-scale methods for removing salt rely on physical separation, either by changing the water’s state or by forcing it through a physical barrier. Reverse osmosis (RO) is the leading technology globally, accounting for the majority of the world’s desalinated water capacity due to its relative energy efficiency. This process works by applying immense pressure—often exceeding 60 bar (870 psi) for seawater—to push the saline water across a semipermeable membrane. The membrane’s microscopic pores allow water molecules to pass through while physically rejecting the larger dissolved salt ions. The result is two streams: the purified water (permeate) and a highly concentrated salt solution (brine or reject).
Distillation, a thermal method, is the oldest form of desalination and mimics the natural water cycle of evaporation and condensation. Saline water is heated until it vaporizes into steam, leaving all the dissolved salts and minerals behind as a solid residue. The pure steam is then collected and cooled back into liquid fresh water. Modern large-scale facilities use techniques like Multi-Stage Flash (MSF) or Multi-Effect Distillation (MED) to recycle heat energy. These processes often operate under a partial vacuum, allowing the water to boil at lower temperatures, which improves overall energy efficiency.
While distillation produces exceptionally high-purity water, the process is generally more energy-intensive than modern reverse osmosis for bulk seawater desalination. Both RO and distillation create the challenge of disposing of the concentrated, hypersaline brine byproduct without causing environmental harm.
Chemical and Electrical Techniques
Desalination can also be achieved by exploiting the electrical charge of the dissolved ions. Electrodialysis (ED) utilizes an applied direct current (DC) electric field to selectively pull salt ions out of the water. The process uses an alternating arrangement of ion-exchange membranes: cation-exchange membranes allow only positive ions (cations) to pass, and anion-exchange membranes allow only negative ions (anions). When the electric field is applied, ions migrate toward the oppositely charged electrodes, and the alternating membranes trap them. This creates a fresh water stream and a concentrated brine stream. Electrodialysis is particularly energy-efficient for treating brackish water, as less energy is needed to move fewer ions than in seawater.
Ion exchange (IX) is a chemical technique that focuses on replacing undesirable ions with more suitable ones, rather than removing all dissolved solids. This method uses specialized synthetic polymer beads, called resin, which attract and hold contaminant ions from the water. The resin simultaneously releases a benign ion, such as sodium, hydrogen, or hydroxide, back into the water supply. In a typical demineralization process, cation resin exchanges positive ions for hydrogen ions, and anion resin exchanges negative ions for hydroxide ions, which then combine to form pure water molecules. Ion exchange is highly effective for pre-treating water or for polishing the output of other systems to achieve ultrapure water quality, but it is not commonly used for bulk seawater desalination.