Controlled Release Fertilizers And Nanotechnology Traces

Control lead run Fertilizers And Na n amylumnology Traces whiz of the closely significant problems of crop salad dressing by intermediate fertilisers is nutrient loss to env pushment which faces gobs of environmental and human health problems besides decreasing the efficiency of crop nitrification. As a consequence, controlled or sulky muster out plant foods go for been develop to chastise drawbacks of traditional plant foods. In this review roughhewn types of CRFs and both(prenominal) related c one timepts. Also, direction on polymer c e actuallywhere CRFs, varied preparation and different finishing exertion rules forget be studied. In addition, few features of nanotechnology and nano-hooeys in preparation of controlled rout fertilisers in previous works ordain be reviewed.IntroductionIn general, corrects must be supplied with nutrients du beleaguer the whole flora intent. In horticulture this is achieved by applying pronto soluble plant food once t o twice a hebdomad, for example. This anatomy of plant food industriousness is very labour-intensive and requires considerable specialist knowl adjoin, so as to select the sort out browse of application, appropriate time of application and correct root for the particular plants to ensure optimum plant turnout. With the use of s clinical depression or controlled descriptorling fertilisers the full amount of nutrients necessary for the whole vegetation period can be applied at the time of place 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 system of application and s oil condition, is wooly to the environment, which results in the contamination of weewee 1.This kind of environmental concerns of feeding crops with traditional fertilizers has led to developing remit Release Fertilizers (SRFs) or Controlled Release Fertilizers (CRFs). SRFs or CRFs argon easy and safe to use. They qua il risk of incorrect fertilizer application. Also, they atomic number 18 labour saving and minimise nutrient losses by discaseing or fixation.The idea of producing SRFs was developed since 1963 by encapsulation of fertilizers by waxes. after(prenominal) that, these yields give been commercialized. There ar split of SRF and CRF brands. about of these products argon Scotts Professional with key brands such as Osmocote, Sierrablen and Osmoform. Aglukon and SunGro gritty society be likewise producing controlled vent fertilizers.Like oodles of scientific fields, land industry has been over shadowed by nanotechnology. Applications of nanotechnology in land includes agriculture crop remedyment, nano-biotechnology analysis of gene expression and regulation blur management, plant disease diagnostics, efficient pesticides and fertilizers, water management, bio do bying, post harvest technology, monitoring the individualism and quality of agricultural produce and precision agriculture. Efficient pesticides and fertilizers ar belatedly being developed in terms of nano-composite ground slow or controlled turf out fertilizers.victimization nanoparticles as reinforcing or cementing agent of polymer get alongings and also as beginning of fertilizers be features of nanoparticles which have been utilise in preparing slow anaesthetize fertilizers 2-4.FertilizersFertilizers are applied to soil to promote plant growth. They contain some undecomposed 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 juiceless matter weight basis) and are to a greater extent essential than micronutrients. Micronutrients are elements which are applied to soil in much sm aloneer amounts, ranging from 5 to two hundred ppm, or less than 0.02% dry weigh. These elements could be boron (B), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn)5.2.2. Types of fertilizersFertilizers would be categorized from source of production and also from pardon properties details of view. All fertilizers could be thorough or celluloid from production source point of view. Organic fertilizers are naturally produced including seaweed, insect casting, manure, slurry, peat, humic acid, guano and brassin. They append slow outpouring of nutrient as they take up soils bacteria to be broken obliterate to needed elements. Also, they may improve the biodiversity of soil by supplying innate matters and micronutrients for organisms. Organic fertilizers are cheaper and safer than semisynthetic fertilizers.The main synthetic or mineral fertilizers which are the sources of nitrogen (N), potassium(K), phosphate (P) are carbamide, ammonium sulfate, ammonium phosphate, phosphate rock, potassium chloride, super phosphates, atomic number 20 ammonium nitrate, potassium sulfate. Fertilizers could be in obscure form (NP, PK, NPK).The close important drawback of synthetic fertilizer is their long term sustainability. Also, they are more big-ticket(prenominal) in contrast to organic fertilizers 5.In addition, fertilizers can be categorized into ordinary and controlled egress fertilizers from wall plug properties points of view.Drawbacks of non-controlled release fertilizersOrdinary fertilizers leach to soil very quickly and most of them are not employ by the plants. consort 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 use by plants 6. This rapid leaching allow for cause zealous of plants and growing in spurts. Also, the lost elements exit 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 rep eating the application of fertilizers which impart increase the m unmatchabletary value 5.Slow or controlled release fertilizersSlow release fertilizers or controlled release fertilizers are granules surface in a substance that reduce the releasing time of nutrients and eliminates need for constant saturation and higher efficiency rate than soluble fertilizers 7.Some of advantages of controlled release fertilizers are long availability of nutrients during growing-season, reduced loss of nutrients by doer of leaching, reduced cost and labor outlay, better storage and handling of fertilizer, reduced immobilization receptions in soil, reduction of nitrification reaction and nitrogen loss through ammonia volatilization and denitrification, ejection of seed damage because of high concentration of salts, elimination of leaf burning from high rates of applied fertilizers, better seasonal growth diffusion and better acclimatization in home or display environment 8.Differences betwee n slow and controlled release fertilizersAlthough there is not special going a elbow room between general function of CRF and the one for SRF, besides 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 jammed in coat granules and are released through the conclusion 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, microorganisms operation and aeration. However, the longevity of CRF depends mostly on masking thickness and temperature of soil. Only close system is effective in changing pattern of nutrients release and a fixed coating th ickness control nutrients release. In CRFs declared release time refers to soil temperature of 20-21oC. Higher temperatures accelerate the element release and start temperatures trade name it longer 9.SRFs are fertilizers with a chemical structure which are inherently slow released. Some kinds of SRFs are Urea Aldehydes (UA) and Chelated Micronutrients (CM). viridity 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 hit firmly together iron, manganese, zinc, and copper. They slowly releases over a long period of time 8.2.4.2. Types of CRFs2.4.2.1. Sulfur Coated (SC)When simple treat is oxidized to its sulfate form, the product would be one of the nutrients which is essential for some plants and is normally mingle with other fertilizers. Using sulfur coating is also another way to p rovide 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 manifold 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 microorganisms activity 10-14.2.4.2.2. Wax coat (WC) peerless of the methods of reducing fertilizer release rate is dispersing granular fertilizers with break up wax and because cooling the mixture below the warming point of the wax 15. Paraffin is one of the most utilise waxes as a coating for fertilizers. Paraffinwax is a white, tasteless, odorless solid, with a true melting point between about 47 C and 64 16. Other types of waxes are synthetic oil based, rock oil or mineral waxes. Waxes are normally used by an additive or a tackifier to make good sealing properties 17,182.4.2.3. Polymer Coated (PC )Polymer- cover fertilizers (PCF) represent the most technically advanced controlled released fertilizers. They include a water-soluble fertilizer core and one or more than one points of polymer. There are large varieties of polymers to coat the core fertilizer also the coatings point could be the same one or different. In PC fertilizers release of nutrients will exit by diffusion through a semi permeable polymer membrane. Water penetrates the coating and dissolves the core. Release rate can be controlled by varying the root word and thickness of the coating. In addition, pressure builds up can cause cracks to form, from which fertilizer passes into the soil 8.2.5. Review of different types of polymer cover CRFs2.5.1. Sole Polymer coat CRFs whizz type of polymer coated CRFs is the one that the fertilizer core which could be N, P, K or increase fertilizer, is just coated with one or more than one bed of polymer coating. In this case the polymer could be dis reply based or wate r-based. The application passage starts with dissolving the polymer in an organic solvent or water. aft(prenominal) dissolving, the coating will be nebulisered onto the fertilizer in a coating drum or fluid sleep together 19. 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 goon 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 writingss as coating for fertilizers. Some of them include dicyclopentadien 20, urea and urethane based 21-26, ep oxy based 27-29, polyvinylidene chloride-based latex 19 carboxyl-carrying ethylene polymers 30, biodegradable starch based 31, urea formaldehyde 32.However, polymer coated fertilizers have some week points. About solvent-based coatings exploitation large amount of organic solvents like toluene or xylene will accept to environmental concerns. These solvents are vapourisable and releasing them to environment makes some hazards for human health. Also, polymer coatings are more dearly-won 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 CRFsOne of most frequent 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(prenominal) layer. It means that sulfur layer in some r esearches has been the first layer and in some others the outer layer.Using polymers as coating is suggested method to get rid of drawbacks of sulfur coated CRFs. One of these drawbacks is very fast release of sulfur coated fertilizer in first few geezerhood 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 roughhewn method for applying the sulfur coating is by dust. Molten sulfur enhance will be sprayed over a pre-polymer coated fertilizer granule 28,33.2.5.3. Wax-polymer coated CRFsThere are lots of researches foc using 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 opportunity 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 disk operating system 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 waxs high temperature in its melting point. The wax normally should have drop melting point from 50 to 120C. Wax is normally about 0.2% to 10 % by weight of fertilizer 17, 28, 34, 35.2.5.4. Filler-polymer coated CRFsAs mentioned before, despite lots of advantages of polymer coating to make slow release properties when such polymers a re 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 preserve adherence of coated granules to each other. In addition they are beef up agent of coatings 36.Fillers may be used either as a mixture with polymer to make a nano-composite polymer 37 or as a clear 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 mud, diatomaceous earth, bentonite, kaolin, gypsum powderizeed limestone, talc, barium sulfate. Some other fillers like macerate cellulosic materials also have used as filler in combining with polymer 37-41.2.6. Techniques of applying polymer coatingAccording to previo us studies have been done, encapsulation methods of fertilizers can be carve up into three methods including in-situ, scatter and mixing.2.6.1. In situThis method includes formation of fluid sprinkling of the soluble fertilizer in a solvent and mixing the active source with monomers of a polymer coating. Polymerization will happen and depending on the method, granules or particles of fertilizers will form.Ni et al 42 have developed a double-coated urea fertilizer. For preparation of poly (N-vinyl-pyrrolidone) hydrogels containing urea (PCU), the monomer and a effect 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 because added drop wise into 5% (w/w) CaCl2 aqueous solution and aroused constantly. The drops instantly tur ned 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 fain by assiduity of the previously coated granules into the ethylcellulose solution repeatedly. Thus, EC-coated urea granules with different coating thickness were obtained.Hanafi et al 43, have coated a raise fertilizer by polyvinyl chloride (PVC), polyacrylamide (PA), natural rubber (NR), and polylactic acid (PLA) using in situ method. For encapsulation of entangled 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 composite fertilizer granules, determined by SEM(Fig.2), gave PVC blend coated fertilizer the highest value of 3.04 lm, and the lowest was obtaine d by PA (2.04 m). chromosomal mutation in the characters of the polymers would be utilized in producing CR compound fertilizer that fit the requirements of growing plants.Hudson et al 28 used epoxy to coat the fertilizer. In this research the urea granules were charged to a pan and warmed to 95C. Then the heat contentated tallaw amine, 2-amino ethyl peperazine and biscarbolic acid A diglycidyl ether were mixed and were added to the granules. Meanwhile polymerization happened and active mixture was agitated till the fertilizer granules dried.2.6.2. Spraying methodThis method is most common method for coating application on fertilizer granules in state of the art. Usually, the solution of polymer in a able 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 coveted coating percentage is reached.Tomaszewska et al 44 have used spray technique for encapsu lation 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). denseness of the polymer in solutions used for spraying was in the regurgitate of 13-17 wt%. Measurements of thickness, porosity of on the watch coatings and microphotographic observation of the coatings were taken. Fig.3 shows the cross section of double coated fertilizer.Ma et al 45 have developed a method for encapsulation of fertilizer with a egotism assembled coating. The fertilizer granules were heated in a orbitual drum to 75C 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 75C for 20 minutes.Dai et al 46 also, have developed a controlled releas e fertilizer using a water soluble resin as a coating. The granular compound fertilizer was coated in fluidized bed.Lan et al 47, excluded(p) a double-coated slow-release NPK fertilizer with super kotowent and water-retention properties (DSFSW), whose interior(a) 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 nominate an application in agriculture and horticulture, especially in drought-prone regions where the availability of water is insufficient. At first, the oute r coating containing urea diatomite (PAADU) was prepared. The NPK compound fertilizer granule was place 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 analogous 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 rigid 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 propert ies.Hansen et al 26, 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 fine stream of resin. concurrently 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 ethanoate.Hansen et al 29 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 carcass. Then urethane solution in xylene and Cellosolve acetate was applied using spray.2.6.3. MixingIn this method granules are simply mixed with the coating at its melting point or with a solution of polymer in a commensurate solvent.Tomaszewska et al 48 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 commensurate 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 37 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 dro p, for avoiding the fertilizer damage, the granules or powder fertilizer has been mixed with the prepared mixture using a Brabender Mixer.Markusch et al 49 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 CRFsA 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 solub le 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 successor 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 linearpolysaccharide), poly lactic acid and poly(-caprolactone) due to low cost and abundance are some of used biopolymers in fertilizer industry31,42,47,52-58Generally, polymer coatings are applied in a thickness which is suitable to make a desired controlled release property. impenetrableness 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 degree centigrade microns. Preferred coating thickness is 1 to 50 microns. Coating process could be repeated more than one layer to get desired nutrient release 39.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, 602.8. Investigation of release behavior of CRFsRelease 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 th e coating surface. In this case, rupture will happen because of permeation of water into coating and interior osmotic pressure. Also, biodegradable polymers will release nutrients while destroying by soil microbes activity. In polymer coatings or combination of different coatings release will happen by diffusion of water through walls 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 stock(a) test methods for measurement of controlled release properties. According To European standard the standard release time of N during 2 4 h should be 15%of total core nutrient. Also, release rate for 15 twenty-four hour period should be 75% of total nutrients. Also American and Japanese standards separate that the initial release shouldnt 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 33 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 century C for 8 hours. Also, Ma et all 21 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 30 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 industryReviewing 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 source of urea or as filler for polymer coating.2.9.1. Nano-clay as carrier of ureaThe 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 65. Because of these features nanolayers could be a suitable carrier or reservoir of fertilizers. apparatuss 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 cohere strongly to montmorillonite but anionic molecules show much weaker interaction bands. Also, for interpreter benzoic acid or anionic species are adsorbed on the edge face of clay or cationic( crystal violet) are adsorbed on the lowly 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 66.Table 1Interactions between clay minerals and organic compounds 66MechanismMineral examplesOrganic functional groups involvedHydrophobic interactions (van der Waals)Any clay with neutral sites (e.g., kaolinite, smectites)Uncharged, non polar (e.g., aromatic, alkyl C)Hydroge n bondingAny clay with oxygen surfaces (e.g., kaolinite)Amines, carbonyl, carboxyl, phenylhydroxyl, heterocyclic compound NProtonationAlumino silicate edge sites, Fe and Al oxides, allophane, imogoliteAmines, heterocycle N, carbonyl, carboxylate,Ligand exchangeAluminosilicate edge sites, Fe and Al oxides, allophane, imogoliteCarboxylate, PhenolateCation exchange (permanent charge sites)Smectite, vermiculite, illiteAmines, ring NH, heterocyclic NpH-dependent charge sites (anion exchange usually, cation exchange rarely)Aluminosilicate edge sites, Fe and Al oxides, allophane, imogoliteCarboxylate for anion exchange, amines, ring NH, heterocyclic N for cation exchangeCation bridgingSmectite, vermiculite, illiteCarboxylate, amines, carbonyl, spirituous OHWater bridgingReviewing