11 Ion exchange and Filtration

Babita Khosla

Introduction

Waste-water treatment is becoming very essential for the sustainability due to diminishing water resources, increasing waste-water disposal costs, and stringent discharge regulations that have lowered permissible contaminant levels in waste streams. Absolutely pure water is rarely found in nature and the impurities occur in three progressively finer states – suspended, colloidal and dissolved matter. Therefore different methods of treatment and new innovative technologies are adopted for the removal or reduction of the contaminants to acceptable limits.

12.1 Ion exchange

Ion exchange is the process used for the removal of undesirable anions and cations from the waste water. It allows the separation of dissolved ionic species through their transfer from the liquid phase to a solid exchange material, where they replace other ions of the same charge that, in turn, pass to the liquid phase. In this reversible process, chemical transformations do not take place in the ionic species involved or in the exchanger material, enabling their recovery after the ion exchange. The soils and sands were the first known ion exchangers used and as the universality of ion exchangers grew from the last few years many synthetic exchangers have been evolved. Earlier ion exchangers used were natural zeolites, but now there are synthetic zeolites and polymeric ion exchangers are used. The interest for the applications of the ion exchange lies precisely in the possibility of reusing the exchanger material again and again. In order to do so, the material must previously undergo a regeneration process before recovering its initial conditions.

Ion exchange resins are typically presented in the form of spherical particles and consist of an organic or inorganic network structure with attached functional groups that contains soluble and mobile exchangeable ions. Most of the ion exchange resins are synthetic and made up by the polymerization of organic compounds into a three dimensional structure and the degree of crosslinking between organic chains determines the pore size. In aqueous solution and depending on their selectivity towards the ions contained in the solution, resin functional groups carry out the exchange process by replacing their counter ions with the ions of interest to be removed from the solution. The exchange process between the resin and the aqueous solution comprises phases of diffusion, adsorption, electrostatic attraction and acid-base balance. The process is entirely reversible and under the appropriate acid or base conditions, the equilibrium can be moved in

opposite  direction,  resulting  in  the  original  chemical  form  of  the  resin.  This

property allows ion exchange resins to be used through many load and regeneration cycles. The cost effectiveness of treatment processes based on ion exchange is due precisely to the number of regeneration cycles that can be obtained with a specific resin under certain operating conditions and constitutes a major design factor.

When the resin comes in contact with water, swelling of the resin results into the decrease of cross linking density and the functional groups are exposed. The exchangeable ion dissociates from the resin and becomes mobile, thus exchange of ions in the aqueous phase by the exchanger takes place and overall charge neutrality is maintained, otherwise the resin will attract or repel ions to maintain the charge balance.

Cations, such as calcium, magnesium, barium, strontium and radium, can be separated from an aqueous solution by using a cation-exchange resin and anions like fluoride, nitrate, arsenate, selenate, chromate, as well as humic and fulvic acids can be separated by means of an anion-exchange resin. Cations are exchanged for hydrogen or sodium and anions for hydroxyl ions. Exchange capacity is determined by the number of functional groups per unit mass of the resin.

All ion exchange resins will establish ion selectivity based on the type of resin and resin structure, removing either cations or anions. Some resins have been traced to target removal of specific chemicals and some are designed to perform in particular conditions.

The selectivity of an ion exchange media can be described by the selectivity coefficient described by the following equation

=                      /

Whereas, Cj = concentration of target ion in solution, eq/L qi = concentration of counter-ion on resin, eq/L Ci = concentration of counter-ion in solution, eq/L qj = concentration of target ion on resin, eq/L

n = valence of the exchanging ion.

12.1.1 Types of ion exchangers:

Natural Ion Exchanger: Naturally occurring ion exchange material functions much the same as synthetic resin but with a different structural makeup. Some of the most popular and widely available natural ion exchangers are zeolites, which consist of an aluminosilicate molecular structure with weak cationic bonding sites (Guisnet and Gilson, 2002). Natural zeolites have been avoided in high purity processes or where consistency is vital because of irregularities and impurities of the material. However, zeolites have been used in many applications where uniformity is less critical, such as treating of waste streams and metals separation.

Synthetic Ion exchangers:

The first synthetic ion exchangers were prepared in the mid 1930’s based on coal and phenolic resins for industrial use. A few years later resins consisting of polystyrene with sulphonate groups to form cation exchangers or amine groups to form anion exchangers were developed. There are many possible combinations of polymers, cross-linking, and functional groups. Most ion exchange materials currently used in industrial applications are synthetic compounds  traditionally known as resins. Exchange resins are obtained by styrene and vinylbenzene copolymerization, or from acrylic materials. One of the most common polymers used for ion exchange resins is polystyrene cross-linked with divinylbenzene.

On the basis of their porosity, the resins can be classified in gel-type or microporous resins, and macroporous type resins, of less packed lattice. The porosity of gel-type resins is in the order of ionic sizes, whereas macroporous resins have a pipe network in their structural matrix, known as macropores, supporting adsorption and desorption of the larger molecular size substances, such as organic compounds. The higher degree of cross-linking of gel-type resins confers them a higher resistance to chemical degradation and better mechanical properties.

Figure 2: Polystyrene based ion exchange resins

Ion exchange resins are further classified according to their chemical structure and the acidic and basic properties shown by their functional groups. Accordingly, four resin categories are defined:

Cation-exchange resins: They are characterized by sulphonic groups -SO3- (strong) or –COO-(weak) as functional groups. Sulphonic groups behave like strong acids, entirely hydrolyzed in aqueous solution and attract positively charged ions exchanged for protons or

sodium, depending on their presentation under acidic or sodium salt form. While as weak cation exchanger resins balance other weak acids in the solution, like bicarbonates, but they cannot exchange ions that are in balance with strong acid anions.

The reactions that occur depend upon chemical equilibrium situation in which one ion will selectively replace another on the ionized exchange site. Cation exchange on the sodium/hydrogen cycle can be illustrated by the following reaction:

Na2.R+Ca2+  ↔ Ca.R+2Na+

H2.R+Ca2+  ↔ Ca.R+2H+

Where R represents the exchange resin.

When all the exchange sites have been substantially replaced with calcium, the resin can be regenerated by passing a concentrated solution of sodium ions (5-10% brine solution) in case of sodium and 2-10% sulphuric acid in case of hydrogen ions through the bed.

Ca.R+2Na+ ↔ Na2.R+Ca2+

Ca.R+2H+ ↔ H2.R+Ca2+

Anion-exchange resins: They contain quaternary ammonium ions –R3N+OH- or amino groups – NH2 or –RNH in their structure as functional groups acting as strong and weak anion exchangers respectively. They bound to the dissolved anions, releasing alkalinity to the solution.

Anion exchange resins work by replacing anions with hydroxyl group. The regeneration with 5-10% sodium hydroxide will renew the exchange sites.

R.(OH)2 +SO42-  ↔R. SO4 +2OH-

12.1.2 Kinetics of the ion exchange process

The rate at which the ionic species exchange process takes place between the solid and liquid phases is regulated by both the differences of concentration between the two phases and the neutral electric balance that must be maintained between them.

One major factor controlling ion exchange rate is the diffusion time necessary in order to achieve the ionic balance between liquid and solid phases. Ion transfer between both phases is controlled by diffusion, under a 3-step classical scheme:

   1.      Transfer from the solution to the stationary boundary layer surrounding the resin sphere. This process is independent of the resin sphere size.

2.      Transfer through the boundary layer to the sphere surface. The rate of this process is inversely proportional to the sphere radius.

3.      Ion transfer into the resin sphere. The rate of this phase is inversely proportional to 1/r2. During operation phase, step one in the resin is faster than step two, which becomes limiting of the process. Use of smaller-sized resin spheres increases surface area, and thus the transfer through the boundary layer.

During resin regeneration phase, a high ion concentration in the liquid phase increases the rate of step two, and thus intra particle diffusion in step three becomes limiting. Smaller-sized spheres present the advantage of a smaller inside path, which is beneficial for step three. The result of choosing a smaller size of particle therefore favors the resin diffusion phases during the operation and regeneration periods.

12.1.3 Applications of ion exchange process :

Ion exchange resins has a wide range of applications of the technology in water and wastewater treatment (Gu et al., 1999; Velizarov et al., 2008; Wang and Peng, 2010). Mostly the aim of ion exchange is to remove a particular ionic species from a liquid solution, resulting in their specific separation and concentration in the solid phase.

1. Industrial applications are recovery of metal cations in solution, separation of ion mixtures, purification of liquids, controlled release of a chemical species, salts recovery, etc.

2.  In Water treatment, the most common uses of ion exchange are: Water softening Water demineralization Alkalinity removal Removal of heavy metal cations Removal of anions from strong acids, such as nitrate (NO3 -) and sulfate (SO4-)

Ammonia removal at low temperatures Production of high purity water

Advantages of Ion Exchange in Wastewater Treatment Processes

Capability of handling and separating components from dilute wastes Possibility of concentrating pollutants Capability of handling hazardous wastes

Possibility of recovery expensive materials from waste (e.g., precious metals) Possibility of regenerating ion exchanger Possibility of recycling components present in the waste and/or regenerating chemicals.

Disadvantages of Ion Exchange in Wastewater Treatment Processes

Limitation on the concentration in the effluent to be treated In general, lack of selectivity against specific target ions

Susceptibility to fouling by organic substances present in the wastewater Generation of waste as a result of ion exchanger regeneration Down time for regeneration

12.2 Filtration

Filtration is another ancient and widely used technology that removes particles and some of the microorganisms from water. It is the process of removing material, often but not always a solid, from a substrate in which it is suspended. Filtration is accomplished by passing the mixture to be processed through one of the many available sieves called filter media. These are of two kinds:

Surface filters: The filtration using surface filters is essentially an exclusion process. The particles having size larger than the filter’s pore size are retained on the surface of the filter called retentate and rest all other matter passes through the filter called permeate or filtrate. The filter papers, membranes, mesh sieves etc. are frequently used filters to separate the solids from the filtrate.

Depth filters: The depth filters in contrast to surface filters retain particles both on their surface and throughout their thickness. They are more likely to be used in industrial processes to clarify liquids for purification.

A variety of filter media and filtration processes are available for treatment of water but, the effectiveness, ease of use, availability, and affordability of these filtration media and methods vary widely and often depend on local factors.

12.2.1 Filters and Filtration Media:

Porous granular media including sand, anthracite, crushed sandstone or other soft rock and charcoal are used for filtration. It is the most widely used physical method for water treatment at the community level, and it has been used extensively for on-site treatment of both community and household water since ancient times.

Sand filters: From the early 19th century slow sand filtration of drinking water has been practiced and various scales of slow sand filters have been widely used to treat water at the community and sometimes local or household level (Cairncross and Feachem, 1986; Chaudhuri and Sattar, 1990; Droste and McJunken, 1982; Logsdon, 1990). Sand filtration is a process where the suspended particles and microorganisms are removed due the slime layer that develops within the top few centimeters of sand. The enteric pathogens and microbial indicators are relatively removed in the range of 99% or more, depending on the type of microbe. However, slow sand filters often do not achieve high microbial removals in practice, especially when used at the household level due to inadequacies in construction, operation and maintenance.

Fiber, fabric and membrane filters: The main objective is to carry out the filtration as rapidly as possible while retaining the precipitate on the filter with a minimum loss. So, the proper filter must be selected with regard to porosity and residue (or ash). If filter used is too coarse, very small crystals may pass through, while use of too fine filters will make filtration unduly slow.

Filters composed of compressed cellulose fibers, spun or woven thread have been used to filter water since ancient times. These filters are simply placed over the opening of a water vessel through which particulate-laden water is poured. The particles are removed and collected on the filter media as the water percolates through it. These filters can also be used in the form of porous cartridges through which water is poured, or alternatively are partially submerged in water so that filtered water passes to the inside and accumulates within. More advanced  applications employ filter holders in the form of porous plates and other supports to retain the filter medium as water flows through it.

Porous ceramic filters: Porous ceramic filters made of clay, porous stone, diatomaceous earth, glass and other fine particles in the form of hollow cylindrical “candles” have been used to filter water. The water to be filtered generally passes from the exterior of the hollow porous ceramic cylinder to the inside, although some porous clay filters are designed to filter water from inside to outside. This is a simplest and commonly used practice to filter drinking water in households. With the passage of time the pores get clogged so, all porous ceramic filters require regular cleaning to restore normal flow rate and prevent biofilm formation on the filter surface. Many commercially produced ceramic filters are impregnated with silver to act as a bacteriostatic agent. Ceramic filters were found to reduce turbidity by 90% and bacteria by 60%.

Diatomaceous earth filters: Diatomaceous earth (DE) and other fine granular media also can be used to remove particulates and microbial contaminants from water. Such filters have achieved high removal efficiencies of a wide range of waterborne microbial contaminants without chemical pre-treatment of the water (Cleasby, 1990). A thin layer of the fine powdery filter medium is coated onto a permeable material held by a porous, rigid support to comprise the filter. As water passes through the filter, particulates are removed and the system maintains target flow rates while achieving high efficient particulate removal.

Filter Aids: During filtration certain gummy, gelatinous, flocculent, semicolloidal, or extremely fine particulates often quickly clog the pores of a filter paper. Filter aids consist of diatomaceous earth and are sold under the trade names of Celite or Filter Aid. They are pure and inert powder like materials that form a porous film or cake on the filter medium. In use, they are slurried or mixed with the solvent to form a thin paste and then filtered through the paper. An alternative procedure involves the addition of the filter aid directly to the problem slurry with thorough mixing. Filter aids cannot be used when the object of the filtration is to collect a solid product because the precipitate collected also contains the filter aid.

Types of filtration

The various types of filtration method used depends on the type of solid (suspended or dissolved) to be separated from the solution. The most basic form of filtration is using earth’s gravitational pull to filter the particulate matter from the water. The water to be filtered is simply poured over  the filter medium, the liquid flows below it due to gravitational pull and the solids are left on the filter. The process is time consuming as the clogging of the pores with fine particles hinders the overall process.

Vaccum Filtration

Vacuum filtration is the process in which pressure gradient is maintained by creating vaccum below the filter medium. The vacuum is normally provided by a water aspirator, although a vacuum pump, protected by suitable traps, can also be used. Because of the inherent dangers of flask collapse from the reduced pressure, thick-walled filter flasks should be used Hardened papers are designed for use in vacuum filtrations on Büchner funnels. These papers possess great wet strength and hard lint less surfaces, and will withstand wet handling and removal of precipitates by scraping. They offer high chemical resistance to strong acids and alkalies.

Vacuum filtration is advantageous when the particles to be separated are crystalline. It should not be employed for gelatinous particles, as clogging will occur. The solutions of very volatile liquids and hot solutions are not filtered conveniently with suction. The suction may cause excessive evaporation of the solvent, which cools the solution enough to cause precipitation of the solute.

Membrane Filtration

Membrane filtration has received considerable attention for the purification of water in the 1960s with the development of high performance synthetic membranes. It is a separation process in which pressure driven force allows the solution to pass through a semipermeable membrane that allows the passage of solvent but not for suspended substances. The presence of pressure gradient is the driving force for the process and is capable of removing suspended solid, organic compounds and inorganic contaminants from the water. Depending on the size of the particle that can be retained, various types of membrane filtration such as ultrafiltration, nanofiltration and reverse osmosis can be employed for wastewater.

Types of membranes: The water treatment process employs following type of membranes made up of cellulose triacetate or polysulfone based upon their pore size range.

1. Microfiltration
2. Ultrafiltration
3. Nanofiltration
4. Reverse osmosis

Microfiltration (MF) have the largest pore size and separate large particles and microrganisms suspended in water. The molecular weight cutoff is >105 Dalton.

Ultrafiltration (UF) utilizes membrane with pore size (0.1-0.01µm) to separate suspended substances/solids, macromolecules from solution. The molecular weight cutoff is 103-105 Da. Nanofiltration employs “loose” reverse osmosis membranes with the pore size and molecular weight cutoff 102-104 Da between ultrafiltration and reverse osmosis. It separates dissolved organic carbon and larger ions from the water.

Reverse osmosis (RO) The membranes (molecule weight cutoff 102 Da) used for

reverse osmosis effectively non-porous and, therefore, exclude particles and even many low molar mass species such as salt ions, organics, etc. Reverse osmosis involves a diffusive mechanism, so that separation efficiency is dependent on solute concentration, pressure, and water flux rate.

References:

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