Controlled Release Fertilizers And Nanotechnology Traces
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Published: Tue, 09 May 2017
One of the most significant problems of crop fertilization by ordinary fertilizers is nutrient loss to environment which causes lots of environmental and human health problems besides decreasing the efficiency of crop nitrification. As a solution, controlled or slow release fertilizers have been developed to overcome drawbacks of traditional fertilizers. In this review common types of CRFs and some related concepts. Also, focusing on polymer coated CRFs, different preparation and different coating application methods will be studied. In addition, some features of nanotechnology and nano-materials in preparation of controlled release fertilizers in previous works will be reviewed.
In general, plants must be supplied with nutrients during the whole vegetation period. In horticulture this is achieved by applying quickly soluble fertilizer once to twice a week, for example. This kind of fertilizer application is very labour-intensive and requires considerable specialist knowledge, so as to select the correct rate of application, appropriate time of application and correct composition for the particular plants to ensure optimum plant production. With the use of slow or controlled release fertilizers the full amount of nutrients necessary for the whole vegetation period can be applied at the time of planting or at the earliest stages of plant growth, in the form of a nutrient pool Also, about half of the applied fertilizers, depending on the method of application and soil condition, is lost to the environment, which results in the contamination of water .This kind of environmental concerns of feeding crops with traditional fertilizers has led to developing Slow Release Fertilizers (SRFs) or Controlled Release Fertilizers (CRFs). SRFs or CRFs are easy and safe to use. They reduce risk of incorrect fertilizer application. Also, they are labour saving and minimize nutrient losses by leaching or fixation.
The idea of producing SRFs was developed since 1963 by encapsulation of fertilizers by waxes. After that, these products have been commercialized. There are lots of SRF and CRF brands. Some of these products are Scotts Professional with key brands such as Osmocote, Sierrablen and Osmoform. Aglukon and SunGro Company are also producing controlled release fertilizers.
Like lots of scientific fields, agriculture industry has been over shadowed by nanotechnology. Applications of nanotechnology in agriculture includes agriculture crop improvement, nano-biotechnology analysis of gene expression and regulation soil management, plant disease diagnostics, efficient pesticides and fertilizers, water management, bioprocessing, post harvest technology, monitoring the identity and quality of agricultural produce and precision agriculture. Efficient pesticides and fertilizers are recently being developed in terms of nano-composite based slow or controlled release fertilizers.Using nanoparticles as reinforcing or cementing agent of polymer coatings and also as reservoir of fertilizers are features of nanoparticles which have been used in preparing slow release fertilizers [2-4].
Fertilizers are applied to soil to promote plant growth. They contain some beneficial nutrients including macronutrients and micronutrients. Macronutrients are nitrogen, phosphorus, and potassium which are added to soil in quantities from 0.2% to 4.0% (on a dry matter weight basis) and are more essential than micronutrients. Micronutrients are elements which are applied to soil in much smaller amounts, ranging from 5 to 200 ppm, or less than 0.02% dry weigh. These elements could be boron (B), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn).
2.2. Types of fertilizers
Fertilizers would be categorized from source of production and also from release properties points of view. All fertilizers could be organic or synthetic from production source point of view. Organic fertilizers are naturally produced including seaweed, worm casting, manure, slurry, peat, humic acid, guano and brassin. They provide slow release of nutrient as they need soil’s bacteria to be broken down to needed elements. Also, they may improve the biodiversity of soil by supplying organic matters and micronutrients for organisms. Organic fertilizers are cheaper and safer than synthetic fertilizers.
The main synthetic or mineral fertilizers which are the sources of nitrogen (N), potassium(K), phosphate (P) are urea, ammonium sulfate, ammonium phosphate, phosphate rock, potassium chloride, super phosphates, calcium ammonium nitrate, potassium sulfate. Fertilizers could be in compound form (NP, PK, NPK).The most important drawback of synthetic fertilizer is their long term sustainability. Also, they are more expensive in contrast to organic fertilizers .
In addition, fertilizers can be categorized into ordinary and controlled release fertilizers from release properties points of view.
Drawbacks of non-controlled release fertilizers
Ordinary fertilizers leach to soil very quickly and most of them are not used by the plants. According to figures about 40-70% of nitrogen, 80-90% of phosphorous, and 50-70% of the applied normal fertilizer is lost to environment and cannot be used by plants . This rapid leaching will cause burning of plants and growing in spurts. Also, the lost elements will cause some serious problems for environment. Eutrophication, blue baby syndrome, soil acidification, persistent organic pollutants, heavy metal accumulation, atmospheric effects are environmental concerns of ordinary fertilizers. Another result of quick leaching of fertilizer is repeating the application of fertilizers which will increase the cost .
Slow or controlled release fertilizers
Slow release fertilizers or controlled release fertilizers are granules coated in a substance that reduce the releasing time of nutrients and eliminates need for constant fertilization and higher efficiency rate than soluble fertilizers .
Some of advantages of controlled release fertilizers are long availability of nutrients during growing-season, reduced loss of nutrients through leaching, reduced cost and labor outlay, better storage and handling of fertilizer, reduced immobilization reactions in soil, reduction of nitrification reaction and nitrogen loss through ammonia volatilization and denitrification, elimination of seed damage because of high concentration of salts, elimination of leaf burning from high rates of applied fertilizers, better seasonal growth distribution and better acclimatization in home or display environment .
Differences between slow and controlled release fertilizers
Although there is not special difference between general function of CRF and the one for SRF, but it should be mentioned that they are manufactured by different materials and techniques with different properties. In slow release fertilizers elements are present in fertilizers in a chemical form, which is not available to plants and they will be converted by physiochemical effects or microorganisms activities into nutrient forms in the soil. But in controlled release fertilizers elements are packed in coated granules and are released through the coating over a certain period of time. Also, in SRFs nutrients available period is affected by a lot of factors such as water content of soil, pH, temperature, microorganism’s activity and aeration. However, the longevity of CRF depends mostly on coating thickness and temperature of soil. Only coating method is effective in changing pattern of nutrients release and a fixed coating thickness control nutrients release. In CRFs declared release time refers to soil temperature of 20-21oC. Higher temperatures accelerate the element release and lower temperatures make it longer .
SRFs are fertilizers with a chemical structure which are inherently slow released. Some kinds of SRFs are Urea Aldehydes (UA) and Chelated Micronutrients (CM). Common type of UAs is urea formaldehyde which is high nitrogen fertilizer. Starting release rate of UAs is high but it dies off slowly for 3 years. This kind of fertilizer depends on microorganisms to break it down for plant use. CMs are substances that hold firmly together iron, manganese, zinc, and copper. They slowly releases over a long period of time .
2.4.2. Types of CRFs
184.108.40.206. Sulfur Coated (SC)
When elemental sulfur is oxidized to its sulfate form, the product would be one of the nutrients which is essential for some plants and is normally blended with other fertilizers. Using sulfur coating is also another way to provide sulfur while making slow release properties for a core granular fertilizer. As the sulfur containing materials like polysulfides or lingosulfonate are brittle and also give a low wetting of defects, they are normally mixed with waxes or plasticizers. Many formulas are available for SCs. Their release time is generally 3-4 months and the nutrient is released from SCs by microorganism’s activity [10-14].
220.127.116.11. Wax coated (WC)
One of the methods of reducing fertilizer release rate is dispersing granular fertilizers with molten wax and then cooling the mixture below the melting point of the wax . Paraffin is one of the most used waxes as a coating for fertilizers. Paraffin wax is a white, tasteless, odorless solid, with a typical melting point between about 47 °C and 64 °. Other types of waxes are synthetic oil based, petroleum or mineral waxes. Waxes are normally used by an additive or a tackifier to make good sealing properties [17,18]
18.104.22.168. Polymer Coated (PC)
Polymer-coated fertilizers (PCF) represent the most technically advanced controlled released fertilizers. They include a water-soluble fertilizer core and one or more than one layers of polymer. There are large varieties of polymers to coat the core fertilizer also the coatings layer could be the same one or different. In PC fertilizers release of nutrients will happen by diffusion through a semi permeable polymer membrane. Water penetrates the coating and dissolves the core. Release rate can be controlled by varying the composition and thickness of the coating. In addition, pressure builds up can cause cracks to form, from which fertilizer passes into the soil .
2.5. Review of different types of polymer coated CRFs
2.5.1. Sole Polymer coated CRFs
One type of polymer coated CRFs is the one that the fertilizer core which could be N, P, K or compound fertilizer, is just coated with one or more than one layer of polymer coating. In this case the polymer could be solvent based or water-based. The application process starts with dissolving the polymer in an organic solvent or water. After dissolving, the coating will be sprayed onto the fertilizer in a coating drum or fluid bed . Polymer coated fertilizers have some. One of them is that uniform and defect free coating will surround fertilize. The other is that the coating will be very tough and durable which is resistant against mechanical breakdown. Also, polymer coatings are biologically inactive so they will not breakdown by soil microbes.
In most cases except for degradable polymers release of fertilizers will occur by diffusion through the polymer coat rather than through defects. In some other coated fertilizers like sulfur coatings there should be a flaw in coating to cause releasing of fertilizer.
There are lots of examples for polymers which have been used in the literatures as coating for fertilizers. Some of them include dicyclopentadien , urea and urethane based [21-26], epoxy based [27-29], polyvinylidene chloride-based latex  carboxyl-carrying ethylene polymers , biodegradable starch based , urea formaldehyde .
However, polymer coated fertilizers have some week points. About solvent-based coatings using large amount of organic solvents like toluene or xylene will lead to environmental concerns. These solvents are volatile and releasing them to environment makes some hazards for human health. Also, polymer coatings are more expensive than sulfur coatings because not only polymer materials are more expensive but also process and equipments which are used for production of polymer coated fertilizers are also more complex than equipments used for other coatings.
2.5.2. Sulfur-polymer coated CRFs
One of most common coated fertilizers are the ones in which core fertilizer is covered by a layer of sulfur coating and a layer of polymer. Polymer layer can be the primer or outer layer. It means that sulfur layer in some researches has been the first layer and in some others the outer layer.
Using polymers as coating is suggested method to remove drawbacks of sulfur coated CRFs. One of these drawbacks is very fast release of sulfur coated fertilizer in first few days after application. The other one is brittleness of sulfur coatings which may lead to some fractures during handling or storage and losing the fertilizer. Another problem is that sulfur coatings have a high surface tension with water and cannot provide enough wetting for a good diffusion [11, 13, 19, 28].
The most common method for applying the sulfur coating is by spraying. Molten sulfur compound will be sprayed over a pre-polymer coated fertilizer granule [28,33].
2.5.3. Wax-polymer coated CRFs
There are lots of researches focusing on making controlled release fertilizers using wax-polymer coatings. A wax layer has three major benefits. One is that they are applied over the polymer layer for decreasing the fracture probability of coating and the other one is for decreasing the amount of polymer and avoiding consuming lots of polymers to make the process cost effective. Also, they can eliminate imperfection of granules surface to make a good surface coating.
Most common waxes which have been used in state of the art are C30 alpha-olefin and paraffin. Other petroleum products like lubricants and bitumen or natural products like canola oil, soybean oil, coconut oil and palm oil, also have been used.
After melting the wax it will be applied by just mechanical mixing with polymer coated granules. Normally the polymer is thermoset to avoid any damages of polymer by the wax’s high temperature in its melting point. The wax normally should have drop melting point from 50 to 120°C. Wax is normally about 0.2% to 10 % by weight of fertilizer [17, 28, 34, 35].
2.5.4. Filler-polymer coated CRFs
As mentioned before, despite lots of advantages of polymer coating to make slow release properties when such polymers are used as a sole coating material the ultimate product would be expensive as you have to consume large amounts of polymer. Using mineral or organic fillers is one way to avoid using large amount of polymer. Also, in some researches fillers play the role of detackifier, to prevent adherence of coated granules to each other. In addition they are strengthening agent of coatings .
Fillers may be used either as a mixture with polymer to make a nano-composite polymer  or as a separate layer. The most common method is the latter in which the filler will be added by mixing with polymer coated granules before drying the granules. Most common used fillers are some very fine(less than 20 microns) inert inorganic materials like clay, diatomaceous earth, bentonite, kaolin, gypsum powdered limestone, talc, barium sulfate. Some other fillers like waste cellulosic materials also have used as filler in combination with polymer [37-41].
2.6. Techniques of applying polymer coating
According to previous studies have been done, encapsulation methods of fertilizers can be divided into three methods including in-situ, spraying and mixing.
2.6.1. In situ
This method includes formation of fluid dispersion of the soluble fertilizer in a solvent and mixing the prepared solution with monomers of a polymer coating. Polymerization will happen and depending on the method, granules or particles of fertilizers will form.
Ni et al  have developed a double-coated urea fertilizer. For preparation of poly (N-vinyl-pyrrolidone) hydrogels containing urea (PCU), the monomer and a solution of urea in N-vinyl-pyrrolidone were mixed together. The polymerization was carried out at 65-C for 3 h. The resulting samples were vacuum-dried, milled, screened and stored. After that first coating was dried, sample and some amounts of urea were mixed with sodium alginate (SA) solution. Mixed solution was then added drop wise into 5% (w/w) CaCl2 aqueous solution and stirred constantly. The drops immediately turned into granules (about 4mm in diameter) because the SA in the drop was crosslinked by Ca2+ at once. The granules were filtered and dried in oven at 70- C. Then the granules were added to ethylcellulose – ethanol solution. Multiple ethylcellulose (EC) coatings were prepared by immersion of the previously coated granules into the ethylcellulose solution repeatedly. Thus, EC-coated urea granules with different coating thickness were obtained.
Hanafi et al , have coated a compound fertilizer by polyvinyl chloride (PVC), polyacrylamide (PA), natural rubber (NR), and polylactic acid (PLA) using in situ method. For encapsulation of compound fertilizer with polyacrylamide the granules were added to the solution mixture of monomers. Then the polymerization reaction will start in existence of fertilizers. The thickness of the coating layer on the compound fertilizer granules, determined by SEM(Fig.2), gave PVC compound coated fertilizer the highest value of 3.04 lm, and the lowest was obtained by PA (2.04 µm). Variation in the characteristics of the polymers would be utilized in producing CR compound fertilizer that fit the requirements of growing plants.
Hudson et al  used epoxy to coat the fertilizer. In this research the urea granules were charged to a pan and warmed to 95°C. Then the hydrogenated tallaw amine, 2-amino ethyl peperazine and bisphenol A diglycidyl ether were mixed and were added to the granules. Meanwhile polymerization happened and prepared mixture was agitated till the fertilizer granules dried.
2.6.2. Spraying method
This method is most common method for coating application on fertilizer granules in state of the art. Usually, the solution of polymer in a suitable solvent is sprayed on the granule of fertilizer and then the granules are dried to remove the solvent through evaporation. The treatment is repeated as often as necessary until the desired coating percentage is reached.
Tomaszewska et al  have used spray technique for encapsulation of fertilizers. In order to improve the properties of coatings, the granules of previously coated fertilizer (wet method) were sprayed with a polymer solution or pure solvent (N,N dimethylformamide). Concentration of the polymer in solutions used for spraying was in the range of 13-17 wt%. Measurements of thickness, porosity of prepared coatings and microphotographic observation of the coatings were taken. Fig.3 shows the cross section of double coated fertilizer.
Ma et al  have developed a method for encapsulation of fertilizer with a self assembled coating. The fertilizer granules were heated in a rotary drum to 75°C for 10 minutes. Then the self assembling amphiphilic molecules (N,N-bisaminoethyl eleostearate) were sprayed over the fertilizer. After 20 minutes aliphatic isocyanates were sprayed over fertilizer. This process was repeated once again. The fertilizer kept for drying in the 75°C for 20 minutes.
Dai et al  also, have developed a controlled release fertilizer using a water soluble resin as a coating. The granular compound fertilizer was coated in fluidized bed.
Lan et al , prepared a double-coated slow-release NPK fertilizer with superabsorbent and water-retention properties (DSFSW), whose inner coating was chitosan (CTS), and the outer coating was crosslinked poly (acrylic acid)/diatomite-containing urea (PAADU). This prepared product not only has slow-release property but also could absorb a large amount of water and preserve the soil moisture at the same time. In addition, the outer coating (PAADU) could protect the inner coating (CTS) from mechanical damage. These were significant advantages over the normal slow release or controlled-release fertilizers, which generally have only a slow-release property. The results indicated that the DSFSW could be found an application in agriculture and horticulture, especially in drought-prone regions where the availability of water is insufficient. At first, the outer coating containing urea diatomite (PAADU) was prepared. The NPK compound fertilizer granule was placed into a rotary drum, and the chitosan powder was stuck on the granules by means of epoxy dissolved in acetone. The adhesive was applied by spraying at regular time intervals. The process was finished until compact and homogeneous coating formed on fertilizer granule. The coated granules were dried to a constant mass at room temperature for 6 h. Then the CTS-coated NPK compound fertilizer granules were obtained. CTS-coated fertilizer granules were dipped in water and then were immediately placed on PAADU powder and shaken. In this manner, PAADU could adhere to the surface of CTS-coated NPK compound fertilizer and form the outer coating. The surface of the product was crosslinked by spraying methanol solution of epoxy chloropropane and then dried in a 70 °C oven to obtain the final product which is a double-coated slow-release fertilizer with superabsorbent and water-retention properties.
Hansen et al , encapsulate the granular fertilizer with the epoxy resin using spray method. In this research not coated fertilizer was placed in a rotating drum and pre heated to 250° F. A rapid drying solution of copolymers dicyclopentadiene and a modified vegetable oil were applied over fertilizer using spray in a thin stream of resin. Simultaneously hot air was passed through drum. Next monomers of second resin including a mixture of epoxidized soybean oil and polyester curing agent were applied over prepared granules. The solvent was a mixture of xylene and Cellosolve acetate.
Hansen et al  also have used the above mentioned method for encapsulation of fertilizer with polyurethane. First of all the preheated fertilizer was coated by a synthetic drying oil. After drying the coating for a few minutes fertilizer were dusted by clay. Then urethane solution in xylene and Cellosolve acetate was applied using spray.
In this method granules are simply mixed with the coating at its melting point or with a solution of polymer in a suitable solvent.
Tomaszewska et al  used this method for coating the granular NPK fertilizer with polysulfone (PSF), cellulose acetate (CA) and polyacrylonitrile (PAN). The coating solutions were prepared by the dissolution of the solid polymer in adequate solvent. The NPK fertilizer was successively added to adequate polymer solution, and was covered by a thin layer of the solution. Subsequently, granules were dropped into water, where the gelation process takes place. The coated granules were removed from the precipitation bath and then dried to a constant mass. The multiple coatings were prepared by immersion of the single coated fertilizer into adequate polymer solution, then into water and drying.
Hon  has prepared the coated granules by mixing method. After melting the thermoplastic polymer by heating, the cellulosic additive has been added to melt resin. Then after allowing the temperature to drop, for avoiding the fertilizer damage, the granules or powder fertilizer has been mixed with the prepared mixture using a Brabender Mixer.
Markusch et al  just has mixed the fertilizer pellets with at first, a diluted polyol and then with a diluted isocyantae to make a polyurethane coating. Then the feretilizer were placed in oven for drying.
2.7. Polymers used as coating for CRFs
A broad range of polymers has been used in fertilizer coating. These polymers could be thermoset, thermoplastic or biodegradable.
Some of common thermoset polymers are urethane resin, epoxy resin, alkyd resin, unsaturated polyester resin, phenol resin, urea resin, melamine resin, phenol resin, silicon resin. Among them, urethane resin urethane is very common used [26,50].
Thermoplastic resins are not very common used in art because of some problems. As mentioned before a very preferable method of applying polymers is spraying the dissolved polymer over granules. Either some of thermoplastic resins are not soluble in a solvent or make a very viscose solution which is not suitable for spraying. Polyolefine is used in art for coating the fertilizer granules [37, 51].
Biopolymers, dispose in bioactive environments, degrade by the enzymatic action of microorganisms such as bacteria, fungi, and algae and their polymer chains may also be broken down by non enzymatic processes such as chemical hydrolysis. Non-biodegradable polymers are not environmental friendly. Due to environmental concerns there are some trends of replacing non-bio with bio degradable polymers as coating material from 1970s. But it should be mentioned that they are expensive yet and not very cost effective. Also, in production process durability of polymer should be adjusted with release time of fertilizer.
Among natural and synthetic biopolymers starch and cellulose based polymers, chitosan (a linear polysaccharide), poly lactic acid and poly(ε-caprolactone) due to low cost and abundance are some of used biopolymers in fertilizer industry[31,42,47,52-58]
Generally, polymer coatings are applied in a thickness which is suitable to make a desired controlled release property. Thickness could be related to characteristic of polymer and also it depends on existed porosity of polymer surface. If water vapor transmission rate of coating material is 0.01 to 20, coating thickness would be 1 to 100 microns. Preferred coating thickness is 1 to 50 microns. Coating process could be repeated more than one layer to get desired nutrient release .
Typical physical method for encapsulating fertilizers are spray coating, spray drying, pan coating, rotary disk atomization. Special equipments for these methods are rotary drum, pan or ribbon or paddle mixer and fluidized bed [59, 60]
2.8. Investigation of release behavior of CRFs
Release mechanism of nutrients for different coatings is different. Thick encapsulated granules like sulfur coating will allow the nutrient to release when a flaw or rupture appears on the coating surface. In this case, rupture will happen because of permeation of water into coating and inside osmotic pressure. Also, biodegradable polymers will release nutrients while destroying by soil microbe’s activity. In polymer coatings or combination of different coatings release will happen by diffusion of water through wall’s porosities. In this case release rate can be controlled by the particle size of coated granules, thickness of coating and permeability of coating surface.
Also, changing the chemical composition of fertilizer core and also the coating will change permeability of coating. This means that even basic or acidic environment will change the permeability. In some researches when the coating has been starch-vinyl, increasing size of encapsulated granules has led to slowing down the nutrient release [61,62].
There are some standard test methods for measurement of controlled release properties. According To European standard the standard release time of N during 24 h should be 15%
of total core nutrient. Also, release rate for 15 day should be 75% of total nutrients. Also American and Japanese standards say that the initial release shouldn’t be more than 40% of total nutrient.
According to European standards for measuring the release rate encapsulated fertilizer should be immersed in pure water at 25 °C (room temperature) in incubated state [63, 64]. For example Detrick et all  have investigated the release behavior of their product by immersion of 20 g of encapsulated granules for 8 h in water. After filtration of solid they dried the solid. Evaporation of water was done at 100 °C for 8 hours. Also, Ma et all  placed 14 g of granules in wire mesh holder and then placed it into a jar with 300 ml water at 23 °C and agitated it by an orbital shaker. Then, adequate water was taken for elemental analysis. Locquenghien et al  for investigation the slow release effect of fertilizer extracted some amounts of fertilizer continuously with water. For this purpose the granules were arranged in layer in a cylindrical vessel field with water. Water was passed through this layer and its nitrogen content was analyzed.
2.9. Tracing nano-technology features in fertilizer industry
Reviewing literature shows that researches which have used nano-technology features in fertilizer industry are very rare. Nano-Clay is the most common nano-particle which have been used to produce CRFs. The main benefits of nano-clay particles in these researches are using them as reservoir of urea or as filler for polymer coating.
2.9.1. Nano-clay as carrier of urea
The layered clays like montmorillonite and kaolinite are made of high aspect ratio nano layers. Large surface areas and reactivity of nanolayers is much greater than that of micrometer size materials. Also, their surfaces and interfaces provide an active substrate for physical, chemical, and biological reactions . Because of these features nanolayers could be a suitable carrier or reservoir of fertilizers.
Mechanisms which are involved in interaction between clay and organic materials depends on some factors like clay type, functional groups of organic material and physical or chemical properties of organic material. For example basic molecules bond strongly to montmorillonite but anionic molecules show much weaker interaction bands. Also, for instance benzoic acid or anionic species are adsorbed on the edge face of clay or cationic( crystal violet) are adsorbed on the basal plane.
According to table 1 which shows different interaction of organic compound with clay, interaction between clay and urea could be through cation exchange, cation bridging and hydrogen bonding .
Interactions between clay minerals and organic compounds 
Organic functional groups involved
Hydrophobic interactions (van der Waals)
Any clay with neutral sites (e.g., kaolinite, smectites)
Uncharged, non polar (e.g., aromatic, alkyl C)
Any clay with oxygen surfaces (e.g., kaolinite)
Amines, carbonyl, carboxyl, phenylhydroxyl, heterocycle N
Alumino silicate edge sites, Fe and Al oxides, allophane, imogolite
Amines, heterocycle N, carbonyl, carboxylate,
Aluminosilicate edge sites, Fe and Al oxides, allophane, imogolite
Cation exchange (permanent charge sites)
Smectite, vermiculite, illite
Amines, ring NH, heterocyclic N
pH-dependent charge sites (anion exchange usually, cation exchange rarely)
Aluminosilicate edge sites, Fe and Al oxides, allophane, imogolite
Carboxylate for anion exchange, amines, ring NH, heterocyclic N for cation exchange
Smectite, vermiculite, illite
Carboxylate, amines, carbonyl, alcoholic OH
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