Brine (orbriny water) is a high-concentrationsolution ofsalt (typicallysodium chloride orcalcium chloride) inwater. In diverse contexts,brine may refer to the salt solutions ranging from about 3.5% (a typical concentration ofseawater, on the lower end of that of solutions used forbrining foods) up to about 26% (a typicalsaturated solution, depending on temperature). Brine forms naturally due toevaporation of ground saline water but it is also generated in the mining of sodium chloride.[1] Brine is used for food processing and cooking (pickling andbrining), forde-icing of roads and other structures, and in a number of technological processes. It is also a by-product of many industrial processes, such asdesalination, so it requireswastewater treatment for proper disposal or further utilization (fresh water recovery).[2]
Brines are produced in multiple ways in nature. Modification of seawater via evaporation results in the concentration of salts in the residual fluid, a characteristic geologic deposit called anevaporite is formed as different dissolved ions reach the saturation states of minerals, typicallygypsum andhalite. Dissolution of such salt deposits into water can produce brines as well. As seawater freezes, dissolved ions tend to remain in solution resulting in a fluid termed a cryogenic brine. At the time of formation, these cryogenic brines are by definition cooler than the freezing temperature of seawater and can produce a feature called abrinicle where cool brines descend, freezing the surrounding seawater.
The brine cropping out at the surface as saltwater springs are known as "licks" or "salines".[3] The contents of dissolved solids ingroundwater vary highly from one location to another on Earth, both in terms of specific constituents (e.g.halite,anhydrite,carbonates,gypsum,fluoride-salts,organic halides, andsulfate-salts) and regarding the concentration level. Using one of several classification of groundwater based ontotal dissolved solids (TDS), brine is water containing more than 100,000 mg/L TDS.[4] Brine is commonly produced during well completion operations, particularly after thehydraulic fracturing of a well.
Iodine, essential for human health, is obtained on a commercial scale from iodide-rich brines. The purification begins by converting iodide to hydroiodic acid, which is then oxidized to iodine using chlorine. The iodine is then separated by evaporation or adsorption.[5] Bromine is also obtained from brines. Akin to the production of iodine, the process exploits the easy oxidation ofbromide intobromine, again using chlorine as the oxidant. The product bromine can be selectively collected by exploiting its volatility.[6]
Major deposits oflithium are in the form of brines.[7] Magnesium is also produced in part from waste brine from various sources, such as potash production. Crude magnesium oxides and chlorides mixtures are converted into magnesium metal byelectrolysis.[8]
Elemental chlorine can be produced byelectrolysis of brine (NaCl solution). This process also producessodium hydroxide (NaOH) andhydrogen gas (H2). The reaction equations are as follows:
Brine (primarily cheap brines based oncalcium chloride andsodium chloride[9]) is used as a secondaryfluid in large refrigeration installations to transportthermal energy. It is used because the addition of salt to water lowers the freezing temperature of the solution, significantly enhancing its heat transport efficiency at low cost. The lowest freezing point obtainable for NaCl brine (called itseutectic point) is −21.1 °C (−6.0 °F) at the concentration of 23.3% NaCl by weight.[9]
Because of their corrosive properties, salt-based brines have been replaced by organic liquids such asethylene glycol.[10]
Sodium chloride brine spray is used on some fishing vessels to freeze fish.[11] The brine temperature is generally −5 °F (−21 °C). Air blast freezing temperatures are −31 °F (−35 °C) or lower. Given the higher temperature of brine, the system efficiency over air blast freezing can be higher. High-value fish usually are frozen at much lower temperatures, below the practical temperature limit for brine.
Brine is used for regeneration ofion-exchange resins.[12] After treatment, ion-exchange resin beads saturated withcalcium andmagnesium ions from the treated water, are regenerated by soaking in brine containing 6–12% NaCl. Thesodium ions from brine replace the calcium and magnesium ions on the beads.[13][14]
Brine is a common agent in food processing and cooking. Brining is used topreserve orseason the food. Brining can be applied tovegetables,cheeses,fruit and some fish in a process known aspickling.Meat andfish are typicallysteeped in brine for shorter periods of time, as a form ofmarination, enhancing itstenderness andflavor, or to enhance shelf period.
Quenching is a heat-treatment process when forging metals such as steel. A brine solution, along with oil and other substances, is commonly used to harden steel. When brine is used, there is an enhanced uniformity of the cooling process and heat transfer.[16]
The characteristics of thedischarge depend on different factors, such as the desalinationtechnology used,salinity andquality of the water used,environmental andoceanographic characteristics, desalination process carried out, among others.[18] The discharge of desalination plants byseawater reverse osmosis (SWRO), are mainly characterized by presenting a salinity concentration that can, in the worst case, double the salinity of the seawater used, and unlike ofthermal desalination plants, have practically the sametemperature anddissolved oxygen as the seawater used.[19][20]
The discharge could containtrace chemical products used during the industrial treatments applies, such asantiscalants,[21]coagulants,flocculants which are discarded together with the discharge, and which could affect the physical-chemical quality of theeffluent. However, these are practically consumed during the process and theconcentrations in the discharge are very low, which are practicallydiluted during the discharge, without affectingmarine ecosystems.[22][23]
The discharge is generally dumped back into the sea, through an underwater outfall or coastal release, due to its lower energy and economic cost compared to other discharge methods.[23][27] Due to its increase in salinity, the discharge has a greaterdensity compared to the surrounding seawater. Therefore, when the discharge reaches the sea, it can form a saline plume that can tends to follow thebathymetric line of the bottom until it is completely diluted.[28][29][30] The distribution of the salt plume may depend on different factors, such as theproduction capacity of the plant, the discharge method, theoceanographic and environmental conditions of the discharge point, among others.[19][28][27][31]
Brine discharge might lead to an increase in salinity above certain threshold levels that has the potential to affectbenthic communities, especially those more sensitive to osmotic pressure, finally having an effect on their abundance and diversity.[32][33][34]
However, if appropriatemitigation measures are applied, the potential environmental impacts of discharges from SWRO plants can be correctly minimized.[23][31] Some examples can be found in countries such asSpain,Israel,Chile orAustralia, in which the mitigation measures adopted reduce the area affected by the discharge, guaranteeing asustainable development of the desalination process without significant impacts on marine ecosystems.[35][36][37][38][39][31][40] When noticeable effects have been detected on theenvironment surrounding discharge areas, it generally corresponds to old desalination plants in which the correctmitigation measures were not implemented.[41][35][42] Some examples can be found in Spain, Australia or Chile, where it has been shown that saline plumes do not exceed values of 5% with respect to the natural salinity of the sea in aradius less than 100 m from the point of discharge when proper measures are adopted.[37][31]
The mitigation measures that are typically employed to prevent negatively impacting sensitive marine environments are listed below:[43][44][45]
A well-designed discharge mechanism, employing the use of efficientdiffusers orpre-dilution of discharges with seawater
Anenvironmental evaluation study, which assesses the correct location of the discharge point, consideringgeomorphological and oceanographic variables, such ascurrents, bathymetry, and type of bottom, which favor a rapidmixing process of the discharges;
The implementation of an adequate environmentalsurveillance program, which guarantees the correct operation of the desalination plants during their operational phase, allowing an accurate and earlydiagnostics of potential environmental threats
Currently, in many countries, such asSpain,Israel,Chile andAustralia, the development of a rigorousenvironmental impact assessment process is required, both for the construction and operational phases.[46][47][48] During its development, the most importantlegal management tools are established within the local environmental regulation, to prevent and adopt mitigation measures that guarantee the sustainable development of desalination projects. This includes a series of administrative tools and periodic environmental monitoring, to adopt preventive, corrective and further monitoring measures of the state of the surrounding marine environment.[49][50]
Under the context of this environmental assessment process, numerous countries require compliance with anEnvironmental Monitoring Program (PVA), in order to evaluate the effectiveness of the preventive and corrective measures established during the environmental assessment process, and thus guarantee the operation of desalination plants without producing significant environmental impacts.[51][52] The PVAs establishes a series of mandatory requirements that are mainly related to the monitoring of discharge, using a series of measurements andcharacterizations based on physical-chemical and biological information.[51][52] In addition, the PVAs could also include different requirements related to monitoring the effects of seawater intake and those that may potentially be related to effects on theterrestrial environment.
Brine is a byproduct of many industrial processes, such asdesalination, power plantcooling towers,produced water from oil andnatural gas extraction,acid mine or acid rock drainage,reverse osmosis reject,chlor-alkali wastewater treatment, pulp and paper mill effluent, and waste streams from food and beverage processing. Along with diluted salts, it can contain residues of pretreatment and cleaning chemicals, their reaction byproducts and heavy metals due to corrosion.
Wastewater brine can pose a significant environmental hazard, both due to corrosive and sediment-forming effects of salts and toxicity of other chemicals diluted in it.[53]
Unpolluted brine from desalination plants and cooling towers can be returned to the ocean. From the desalination process, reject brine is produced, which proposes potential damages to the marine life and habitats.[54] To limit the environmental impact, it can be diluted with another stream of water, such as the outfall of awastewater treatment or power plant. Since brine is heavier than seawater and would accumulate on the ocean bottom, it requires methods to ensure proper diffusion, such as installing underwaterdiffusers in thesewerage.[55] Other methods include drying inevaporation ponds, injecting to deep wells, and storing and reusing the brine for irrigation, de-icing or dust control purposes.[53]
Technologies for treatment of polluted brine include: membrane filtration processes, such asreverse osmosis andforward osmosis; ion exchange processes such aselectrodialysis orweak acid cation exchange; or evaporation processes, such as thermal brine concentrators andcrystallizers employingmechanical vapour recompression and steam. New methods for membrane brine concentration, employing osmotically assisted reverse osmosis and related processes, are beginning to gain ground as part of zero liquid discharge systems (ZLD).[56]
Brine consists of concentrated solution of Na+ and Cl− ions. Other cations found in various brines include K+, Mg2+, Ca2+, and Sr2+. The latter three are problematic because they form scale and they react with soaps. Aside from chloride, brines sometimes contain Br− and I− and, most problematically, sulfateSO2− 4. Purification steps often include the addition of calcium oxide to precipitate solidmagnesium hydroxide together with gypsum (CaSO4), which can be removed by filtration. Further purification is achieved byfractional crystallization. The resulting purified salt is calledevaporated salt orvacuum salt.[1]
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