kimia air- pengotor dalam air

8/18/2019 Kimia Air- Pengotor Dalam Air

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Kimia Air- PENGOTOR DALAM AIR

The amount of adsorbent required for effective removal cannot be universally determined for all waters from asingle equation. However, for any given system, experimental data are easily plotted on a semilogarithmic graph,producing a straight line. This graph is known as the Freundlich isotherm in Fig. below. An equation unique to agiven system can be determined from the graph and then used for dosage adjustment

Freundlich isotherm showing effectiveness of SiO2 adsorption

by Mg(OH)2. The formula for the isotherm is Q = kC1/N

Freundlich isotherm showing effectiveness of Organic adsorptionby activated carbon. The formula for the isotherm is Q = kC1/N.


Complexing Agents

A complex is a species formed by the association of two or more simpler species, each capable of independentexistence. When one of these species is a metal ion, the resulting entity is known as a metal complex. Complexingagents are called ligands (or sequestrantsor chelants) that act as an electron donor in a complex reaction. The

ligand must have at least one pair of electrons to donate to the metal ion, forming a shared electron pair bond(coordinate covalent bond).

A characteristic feature of such a complex is that the metal ion occupies a central position in the matrix.

A ligand with one electron pair donor site is an undentate ligand. An example is ammonia (NH3) as shown in Fig.below. The water molecule (H2O) is shown as a ligand. A bidentatel igand has two electron pair donor sites, suchas ethylenediamine (NH2CH2CH2NH2).


Complexing Agents

The donor sites are the nitrogen atoms, when complexed with copper (Cu+2). The ethylenediamineforms a five-member ring with copper. This is a chelate ring. When this ring system forms, it adds stability to the complex. Amultidentatel igand has two or more electron pair donor sites.

Examples of common chelates used in water treatment are nitrilotriacetic acid (NTA) and ethylenediaminetetraaceticacid (EDTA). Both form multiple chelation rings with metal ions. A chelating agent is usually an organic molecule, whichis soluble in water and undergoes reactions with metal ions that maintain the complexed ions in solution.

A common chelate is the sodium salt of EDTA. Added to water, this chelant first reacts with a coordination

site on the calcium cation, which forms a coordinate covalent bond, pulling the rest of the molecule into thecoordination sphere.

These interactions prevent the formation of CaCO3 scale in water as follows:


Figure left shows the coordination complex formed with calcium.

The bonding is through the nitrogen atoms of the ethylenediamine

backbone of the chelantand through the oxygen atoms of the

four acetic acid functional groups.

Low molecular weight polyacrylate-based polymers can complex

hardness ions in water by the same mechanism as chelates. The

molecular weight of these polymers is typically in the range of 1000

to 10 000 g/mol, although other size polymers can be used. Example

reactions are:


Although at room temperature, polyacrylates form weaker complexes than EDTA and NTA, based onexperimental and field boiler results, it has been found that they form stronger complexes at elevatedtemperatures. The synthetic polyacrylate polymers are significantly less corrosive to boiler internals thanchelates.

Low molecular weight polymers are excellent dispersants, which may operate by complexation. When

compounds precipitate, they are usually deficient in one of the counter ions. This causes an imbalance of chargefor a hydrophilic particle or colloid. Iron hydroxides are an excellent example. In industrial applications, ironcorrodes to form ferrous hydroxide and ferric hydroxide. When this occurs, the system is usually deficient inhydroxide ions, resulting in an ineffectively shielded central met al ion, creating a positive surface attraction.Negatively charged polymers are attracted and adsorbed, shielding the central metal ion and increasing thesurface partial negative charge. Like charge particles repulse each other, leading to dispersion.

On the other hand, very high molecular weight polymers (molecular weight in the millions) are used toneutralize surface charge on suspended material in water, leading to coagulation and settling of particles. Thisprocess is used in pretreatment for both industrial and municipal applications, as well as waste treatmentoperations.


A number of natural organic materials in water such as humic acid, tannins, and ligninhave complexing ability. Because of their complexingabilities, some organic materialsinterfere with certain water softening processes.

Somewhat related to complex formation is the process of threshold treatment. Avariety of phosphate compounds called polyphosphates is used in this process toprevent formation of deposits. CaCO3 scale can be prevented in scale-forming water

treated with only 0.5 mg/L of polyphosphate. The amount of polyphosphate requiredfor effective scale control is far less than that required for complex formation on astoichiometric basis, hence the name threshold treatment. Polyphosphates can hold iron(Fe) and manganese (Mn) ions in solution in an environment where they wouldotherwise precipitate, for example, in the presence of oxygen or chlorine at pH over8.

Polyphosphates are produced by dehydration of one or more orthophosphate (PO4 –3)

compounds. Using different mixtures of orthophosphates can vary the kinds ofcondensed phosphates formed. Condensed phosphates form chains containing the P– O–P group, as shown in Fig. left for tripolyphosphate.


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Kimia Air- APLIKASI

Beberapa hal yang perlu diperhatikan untukkualitasair : kation-kation yang adadi dalam air; hubunganantara pH dan alkalinitas; kelarutan mineral dan konsepoksidasi dan reduksi.

Hardness dan analisisair

Hardness merupakan salah satumasalahdalam kualitasair yang menyebabkan sabuntidak menghasilkanbusadan pakaian menjadi tidakbersih.

Untukitu, banyak rumah-rumah punyapenampungan air hujan yang digunakan sebagai pencuci.

Hubungan antara hardness dengan ketergantungan terhadapsabun menjadi dasar untukmenentukantingkatkesadahanair. Secaratradisional, kesadahanadalah kemampuanmenyerapsabun dariair.

Sederhananya, kesadahanadalah jumlah kandungan kalsium danmagnesium dalam air (meskipun logam-logamberat seperti besidan mangan jugamenyerapsabun). Maka, kesadahan adalah jumlah kation kalsiumdan magnesium di dalam air, yang tidak memandangsifat-sifatdari anionnya yang adadi dalamair.

Satuankesadahan adalah mg/L CaCO3 atau mg/L atau ppm saja.

Kimia Air- APLIKASI

Komposisibeberapa air limbahtergantung padajenis darioperasionalpabrikdari air yang digunakan.

Logamberat Zn danCu ada dalam limbah pabrik plating; senyawa-senyawa organik adadalam buanganpengolahanlimbah; dan fluoridasebagaianion atau kompleks anion dalamlimbahpabrik gelas/kaca.

pH dan alkalinitas

Hubungan antara bentukalkalinitassebagaifungsipH dapat ditentukandengan persamaansederhana.Informasiini dapatmembantumendiagnosaperalatanpengukuran pH dan sebagai alatcek apakahpengukuranpH nya sesuai berdasarkan alkalinitasnya.

pH air

pH merupakansimbol dasar untukpengolahan air sehinggapemahaman tentangpengaruh pH padasistemair yang akandiolah adalah penting.

Parameter seperti CO2 terlarut, ion karbonat dan bikarbonatdapat mempengaruhi keasaman ataualkalinitasair.

Kimia Air- APLIKASI

Concentration of hydroxide (in CaCO3 equivalents) as afunction of pH. Note that the hydroxide concentration is aninsignificant part of alkalinity below pH 9.3

TabelMineral Acidity Shown as the Concentration of H+Ions versus pH

Kimia Air- APLIKASI

DAMPAK ADANYA CO2 DALAM AIR

Terlarutnyagaram-garamkarbonatdalam badan perairan, akibat adanya karbon dioksidadalam air.

Pada keadaan normal, dari 0,04% CO2 di atmosfir, < 1mg/L CO2 terlarut di dalam air hujan.

Akan tetapi, sekaliair hujanmerembeske tanah, jumlah gas CO2 akanlebih besardari yang ada diatmosfirkarena adatambahandari proses respirasiorganisme yang adadi dalam tanah(perubahanmakanan organik menjadi energidan CO2).

KetikaCO2 terlarut di dalam air, akan membentuk asamkarbonatyang teruraimenjadi ion H+ danbikarbonat.

Dalamair tanah, adanyakeasamanair tanah akanbereaksidengan logamdari limestone (dolomite) yangmenghasilkan kesadahandan alkalinitas seperti reaksi di bawahini:

Kimia Air- APLIKASI


Ada kesetimbangan antara pH dan alkalinitas seperti gambar di bawah, karena selalu ada CO2 eksesterlarut di dalamair.

Hubunganantara CO2, alkalinitasdan pH di dalam air; dimana CO2 danalkalinitas dinyatakandalam ekivalenCaCO3.

Jumlah CO2 terlarutdi dalamair tergantungnilai pH danalkalinitas. Dalagrafikmenunjukkan bahwarasiokarbondioksida terhadap alkalinitas total (M) sebagai fungsi

pH.

Air dengan1 mg/L CO2 dan10 mg/L alkalinitas mempunyaipH yang sama dengan 10 mg/L CO2 dan 100 mg/Lalkalinitas. Kedua air inimemang punya pH sama. Tetapi, airdengan konsentrasi100 mg/L alkalinitas mempunyaikemampuan buffering lebihbesar.

Bilake dalammasing2 air di atas ditambahkan 1 mg/LCO2,efek pH akan lebihbesarpada air yang rendah

konsentrasi CO2 danalkalinitasnya. Padaair dg alkalinitasyang tinggiakan moderatatau buffer setiapperubahanpH.

Kimia Air- APLIKASI


Ex. at pH 6.9, the ratio of CO2 to total (M) alkalinity is 0.3. If the total alkalinity is 150 mg/L as CaCO3, thenthe CO2 concentration is 45 mg/L as CaCO3. Dengan menggunakan faktor konversi 1,14; kitadapatmenentukankonsentrasi CO2 sebagai CO2.

Hubunganantara CO2, alkalinitasdan pH di dalam air; dimanaCO2 danalkalinitas dinyatakandalam ekivalenCaCO3.

Jadi, 45 mg/L CO2 as CaCO3 = 39,5 mg/L sebagai CO2.

Ingat!!!

Grafik initerbatasuntuk pH dan nilaialkalinitasdi manaCO2 di atas konsentrasi yang diabaikan.


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Kimia Air- APLIKASI


Kesetimbangan antara pH, alkalinitasdan CO2 tergantung suhu, tekananparsialCO2 dan total alkalinitas.

Banyakair sumur memilikikonsentrasi CO2 terlarut yang tinggi. Dalam halini, meskipun air memilikialkalinitas yang tinggi, tapi pH nya rendah. Grafikdi bawah menggambarkan bahwa rasio CO2 terhadaptotal alkalinitas naik, pH air akanturun.

Hubunganantara CO2, alkalinitasdan pH di dalam air; dimana CO2 danalkalinitas dinyatakandalam ekivalenCaCO3.

Penting untukdipahamibahwa kesetimbangan inidipengaruhi olehperubahanantaraCO2 dengan air danudara.

Dalam tekanan normal, CO2 berkesetimbangan di air,didasarkan pH dan alkalinitas air.

Kimia Air- APLIKASI

Distribusispesies alkaline di Air

Hubungan antara pH,CO2 dan alkalinitas menggambarkankondsi umumdari air. Selainitu, perlumengetahuijumlahindividuspesiesalkalinitasdi dalamair.

Reaksi :

Fraksispesieskarbonanorganik sebagaifungsipH

Kimia Air- APLIKASI


Fraksi spesieskarbonanorganiksebagaifungsipH

Gambar menunjukkanbahwa spesieskarbon anorganikantara pH 4,3 sd. 12,3.

Pada pH < 4,3, hanya CO2 yang signifikan, sementarapH>12,3 hanya ion karbonatyang signifikan.

PadapH 8,3; hanyaHCO3-dan CO2 terlarut di dalamair. Di atas pH ini, HCO3- dan CO adalah spesies yangpredominan.

Tentusaja, pH > 7 konsentrasiOH- bertambahdanberkontribusi terhadap alkalinitas.

KonsentrasiOH- harusdiketahui untukmenggambarkanalkalinitas air secara komplet.

KonsentrasiOH- tidakberefekpada alkalinitas sampai pHdi atas9,3 di mana konsentrasiOH- > 1 mg/L sebagaiCaCO3.

Kimia Air- APLIKASI


Fraksispesieskarbonanorganik sebagaifungsipH

Dengan hubungan ini, selama titrasi ada tigajenisalkalinitasy ang dapat difefinisikan:

1. Total atau M alkalinity padaend point mo (pH≈4,3)

2. P alkalinity pada end point pp(pH≈8,3)

3. Hydrate atau caustic alkalinity, yg dihitung dari point1 dan 2

Representasispesies ionikoleh masing-masingjenisalkalinitas dapatdigambarkansbb:

Kimia Air- APLIKASI

Hubungansederhana antarapH danspesies alkalinitas

Titiktransisi yang jelas padapH 8,3 di mana ion karbonatmendekatinol; dan padapH 4,3 semuaalkalinitasmendekatinol.

Akan tetapi, pada pH 10 tidakbegitu jelas penentuannya.PadaGambardi samping, meskikecil (sedikit), ionbikarbonat eksisdi pH>10 danOH- ada pada9,2


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Kimia Air- APLIKASI

LSI dan RSI

Historically, a variety of empirical and theoreticalscaling index calculations have been used topredict the probability of mineral scale formation.

Early methods of predicting the scale-formingtendency of water range from simple equilibriumsaturation indexes to empirical indexes based onexperimental or field data. The two key examplesof these are the Langelier saturation index (LSI)and the Ryznar stability index (RSI).

These first indexes were an attempt to simplify theevaluation of the effect of water chemistry onscaling and corrosion in municipal drinking watersystems.

Both are based on calculations of the solubility of calciumcarbonate, with minor corrections for temperature and ionicstrength. These scaling indexes have many inherentlimitations because of their simplified nature and do notconsider all the complex factors in natural water that affectmineral solubility.

LSI was the first index calculation to come into wide usage(1920s). It is based on the saturation pH (pHs), at whichpoint a given water is saturated with calcium carbonate. LSIis determined by the difference between the actual pH andpHs to define a saturation index:

Positive LSI indicates the water solution is oversaturatedwith respect to calcium carbonate, while negative LSIindicates the water is undersaturated.

Kimia Air- APLIKASI

Based on studies of reported conditions of scalingand corrosion in a variety of municipal systems,Ryznar created an empirical index to predictscale-forming or corrosion tendency, based on pHand pHs:

The effects of temperature and ionic strength wereeither ignored or severely limited.

The solubility of other scaling species ( besides CaCO3)was ignored.

The effects of soluble complexes (ion pairing) were notcalculated.

A limited set of conditions was examined.

Simplifying assumptions were made about thecarbonate equilibria.

A water solution is considered to be corrosivewhen the RSI exceeds approximately 6.0 andto be scale forming when the index is less than6.0.

The limitations of indexes are due to a varietyof assumptions made in the calculations,including

Oxidation and reduction reactions,also known as redox reactions, are important in waterchemistry. A few examples of redox reactions include waterdisinfection, corrosion, precipitation of iron and manganese

from water, and oxidation of sulfide for odor removal fromwater.

Kimia Air- Impuritis dalam air

Contaminants in water sources are related to rainfall, thegeologic nature of the watershed or underground aquifer,biological activity in the ground or in the water, and t heactivities of human population.

The electrical polarity of the water molecule aids dissolutionof many ionic and covalent compounds.

Substances like salts, sugars, acids, and alkalis are verysoluble in water.

Many gases, including oxygen (O2) and nitrogen (N2), aresoluble in water. Other gases that dissolve in water, likecarbon dioxide (CO2) and ammonia (NH3), react with thewater molecule to form weak acids or bases.

Polar organic materials, like alcohols and sugars, dissolve inwater to a high extent, depending on the properties of the

molecule.

The presence of polar organics in water can act asa cosolvent to dissolve small amounts of nonpolarorganic compounds.

The action of biological processes in water canincrease or decrease the solubility of inorganic

and organic materials in water.

The density and viscosity properties of water,combined with the power of flowing water, cankeep in suspension, a wide variety of materials.

These suspended particles can be inorganic solids,insoluble organic substances, or biologicalorganisms.

Kimia Air- Impuritis dalam air

Water supports all life forms known on earth. The solventproperties allow fluids like blood to carry nutrients to thecells in a body.

Water participates in many metabolic processes necessaryfor life. An example is photosynthesis, where en ergy fromthe sun is used by plants to convert CO2 and water intosugar.

The result is that water can contain many substances, bothliving and nonliving. The materials in water may be eitherinert or reactive.

The specific type of contaminant and its concentrationdepends upon the type and source of water. Understandingthe various contaminants that can be found is t he first stepin water treatment for any use, from potable water toindustrial applications.

One of the key factors that affect the quality ofwater is the geological nature of the area wherethe groundwater or surface water exists.

Surface water composition is affected by contactwith the soil or rock of the watershed that supplies

either a river or lake.

Groundwaters are dependent on the type of rockthrough which rainwater permeates when enteringthe aquifer.

In addition, CO2 created by the action of bacteriain soil, and absorbed from air by rainwater, canmake water slightly acidic. If subsurface rockcontains limestone, groundwater in that area canbe high in hardness (calcium and magnesium) andalkalinity.

Kimia Air- Sumber air yang umum digunakan

(Groundwater dan Surfacewater)

Karakteristik Groundwater1. Temperaturrelatiftetap.2. KonsentrasiTDS lebihbesar padalokasiyang hampir

sama. Umumnya hardness dan alkalinity lebih tinggi.3. Gas-gas terlarut spt H2S danCO2 dalam air sumur

ada di atasjenuhnya. Keduagas ini dapat

menyebabkankorosi. Bila air sumur inidipompakeudara, maka gas-gas iniakan hilang. KetikaCO2released , pH akan naikshg menyebabkanterbentuknyakerak. Kalau H2S yang released ??

4. Alkalinity dan pH relatif konstan. Tergantung mineralyg dikandung.

5. Ion-ion besidan mangan sering adadalamgroundwater. Ketika kontakdg udara membentuksenyawa yang sukarlarut.

Karakteristik Surfacewater1. Temperaturseringberubah.2. Konsentrasi TDS lebihrendah padalokasi

yang hampir sama.

3. Gas-gas terlarutselaluada karena airnyaselalu kontakdengan udara, tetapitidak

sampai berlebihan.4. Alkalinity dan pH tergantung musim,

endapannyadan runoff.5. Ion-ion logamdalam bentukoksidalogam

darilimbahindustriseringada dalamsurfacewater.

6. TSS dan turbidity lebihtinggi.

Kimia Air- Types and Characteristics of Water Sources


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Kimia Air- Types and Characteristics of Water Sources (Continued) Kimia Air- Pengelompokan pengotor

1. Senyawa yang dapatlarut.2. Senyawa yang tidak dapat larut(senyawa

tersuspensi).3. Kontaminan organik.4. Kontaminan biologi.5. Gas-gas terlarut.6. Senyawa-senyawa radioaktif.

Senyawa yang dapatlarut. Contohnya, natrium dankloridadi air laut (kons. tinggi);

selenium dantitanium (trace).

Di dalamsumber airnya, senyawaini terlarut

(dissolved) dan larut(soluble); tapiketikadigunakandalamproses industri, menjaditidaklarut (insoluble) yg

disebabkanolehsuhu danperubahankonsentrasinya. Kelarutanbeberapaion utamanyadipengaruhioleh

Ksp.

Contoh kelarutankarbonatdenganbeberapaionpositif lainnya (lihatGambarnya).

Contoh yang kelarutanbeberapahidroksida dansenyawaoksida (lihatGambar).

Kimia Air- Pengelompokan pengotor

Theoretical solubility of carbonate compounds in a watersystem closed to an external CO2 environment

Theoretical solubility of oxides and hydroxides in water at

25°C

TDS adalah jumlahsemua material terlarutdi dalamair.Kons. 25-5000 mg/L.

Potable water 500 mg/LKonsentrasi tinggimenyebabkanpembentukan kerak, rasadan masalahlain.TDS tinggi mempercepatkorosi.Konduktivitas air dapatdigunakan sebagaiindikatorTDS


BICARBONATE ION (Alkalinitas)adalah parameter alkalin utamadari air untukkonsumsi.Konsentrasi 5-500 mg/L sebagai CaCO3.Kontrol alkalinitas adalah penting dalam aplikasiindustrikarena dapatmembentukkalsiumkarbonat.Kontrol alkalinitas padaboiler water dan coolingwater dalam evaporative cooling systems.Makeup water darisistemtersebutharus diolahduluuntuk mereduksialkalinitasnyadgpenambahan asam.Total alkalinitas (M-alkalinity) adalah ukuran darisemuaspesiesbasa yang dinetralisasiasam.

Bilahubunganantara pH danalkalinitasdiluarnormal, maka M-alkalinity (selain bikarbonat dankarbonat) akan meliputi fosfat, silikatatauammonia.

PELAJARI SIFAT-SIFAT

KALSIUM, KLORIDA, MAGNESIUM, SILIKA,NATRIUM, SULFAT, ALUMINUM, ARSEN, BARIUM,BORAT, BROMIDA, TEMBAGA, FLUORIDA, BESI,TIMBAL, LITIUM, MANGAN, NITRAT, FOSFAT,KALIUM, STRONSIUM, ZINC


MAGNESIUMKesadahanmagnesium, hanyasepertiga darikesadahantotal, sisanyakesadahankalsium.Konsentrasi 40-200 mg/L sebagai CaCO3 atau10-50 mg/L sebagai Mg.Pada air laut, konsentrasi Mg sekitar5 kali dari

kalsium.Karena MgCO3 lebihlarut daripadaCaCO3; Mgjarang merupakankomponen utamadalamkerak, kecualikalau adasilika dlmair tersebut.Namun, Mg harus dihilangkan dengan Ca padaair lunak (soft water) untuk air boiler.

SILIKASilika adadalam hampir semuamineral dalamkonsentrasi1 – 100 mg/L sebagaiSiO2.Silika bisalarut. Tapi sering dalambentuk koloid.

Ada kesetimbangan antara silika dalam bentukkoloidal dan anion bisilikat(HSiO3

-).

Karena kompleksitasnyaini, sulituntukmemprediksi kelarutansilikapada konsentrasiairyang tinggi (pekat).Istilah silika koloid (colloidal silica) sering dipakai,tapi dapat membingungkan. Karena, rantaipolimer silika kecildapat terbentuk dalamlarutan. Gugus polimer yang paling besarmungkin tidakdapat terukur bilaanalisissilikaterlarut dengan metode“molybdate-reactive”.


Lanjutan Silika…

Silika yang tidakterdeteksi diistilahkansebagai“colloidal silica”.

Pernyataanlebih akurat, ada silika yang inert(non reactive), karenakebanyakan silika adalah

koloidal, meski dengan ukuran yang berbeda.Padakonsentrasi yang pekat dalamair untukcooling tower, silikaakan menjadi lebihberatdikarenakan ketidakpastian tentangketerbatasan kelarutannya. Dengan keberadaansilika yang menjadilebih beratdalam air untukair boiler, akanmembentuk kerakdalam boilerakibat menguap padasuhu tinggi dan akanterendapkankembalipada “turbine blades”.

Proses treatment yang dapat menghilangkansilika, diantaranya adsorpsi pada magnesiumyang terendapkan dalam air lunak kapur;

adsorpsipada ferrihidroksidadi dalam proseskoagulasimenggunakan garam-garam besi;

pertukaran anion di dalam proses demineralisasidan osmose balik.


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Kimia Air- Parameter Kimia yang mempengaruhi kualitas air

Parameter Dampak pada sistem operasi

Kalsium Membentuk kerak.

Magnesium Garam-garamnya lebih larut dar ipadakalsium, tapidapat

menjadi masalah dalam penguapan air laut. Magnesium silikat

dapat terjadi padapH alkalin bila air mengandung silika.

Bikarbonat

(alkalinitas)

Memberikan kemampuan bufer. Potensi membentuk kerak

kalsium karbonat.

K lor ida Anion ter lar ut yang sangat agres if ter hadap logamdan

penyebab korosi.

Silika Dapat membentuk kerak y ang keras. Dalamp H tinggi lebih

larut, kecuali magnesium silika yg mengendap di pH>8.

Sulfat Anion yang lar ut d an d apat menimbulkan korosi. Dapat

membentuk kalsium sulfat.


ALUMINUM

Meskipun Al3+ persentasenyatinggi dalam kerakbumi, sebagaipenyusun utamaberbagai mineraldan clay, kelarutannyadi dalam air sangatlambat sehinggatidak begitudiperhatikan.Akan tetapi, di dalam industri, sisa-sisapengendapan aluminum dari clarifiermenyebabkandeposit, khususnyadalam coolingsystem yang menggunakan fosfat sebagaipengontrol korosi.Keberadaan alumina disebabkan adanya residukoloidal darialumina (Al2O3) dari koagulasiair,bilaalum atau natrium aluminatdigunakansebagai koagulan. Bila residunya mengeras, dptdihilangkan dg memperbaiki filtrasinya

Theoretical solubility of aluminum and zinc in water 25C,

adasifatamfotirpadakeduaatom tersebut


lanjutan ALUMINUM…

Aluminum bersifat amfoter, yaitu larutdalam pH rendahmaupun tinggi.Dalamair, aluminum larut sebagai Al3+ atauhidroksivalensirendah padapH rendahdan anion aluminat [Al(OH)4

-] padapH tinggi.Dari sifat amfotirnya, pada pH rendah, Al bermuatanpositifdan padapH tinggibermuatan negatif.The effectiveness of alum in precipitating negatively chargedcolloids, such as clay particles from water, is more likelyrelated to the charge on the precipitated alumina than thecharge on the aluminum ion itself, since the aluminum ion is not

soluble in the typical coagulation pH of 5 to 7. Its strongnegative charge at pH 10.0 to 10.5 helps explain theeffectiveness of sodium aluminate in precipitating magnesiumhardness, which is positively charged at th is pH.

Variation of particle charge with particle type and

solution pH.


FOSFAT…

Phosphorus is found in many common minerals such as apatite,in the form of orthophosphate (PO4

3-).Since phosphate compounds are widely used in fertilizers anddetergents, it is common to find phosphate in silt fromagricultural runoff, with high concentrations being found inmunicipal wastewater, generally in the range of 5 to 15mg/L as PO4, but it can be as high as 15 to 30 mg/L as P O4.Since phosphate is a primary cause of excessive algalgrowths, which lead to eutrophication of lakes and streams,legislated reduction of phosphate from all sources co ntinues inmany parts of the world.

Phosphate may be present in water as HPO42− and H2PO4

−

as well as the higher pH form PO43−.

Effect of pH on the distribution of phosphate ions in

solution


FOSFAT…

The distribution as affected by pH is shown in Fig.Phosphate can be reduced to very low levels by treatmentwith alum, sodium aluminate, or ferric chloride, which causesformation of insoluble aluminum phosphate or iron phosphate.It can also be precipitated with lime at pH greater than 10 to

produce residuals less than 2 to 3 mg/L in the form o fhydroxyapatite in a hot process system, the residuals wouldbe less than 0.5 mg/L.These phosphate precipitates are often c olloidal, andfiltration is required to achieve low residuals.

Effect of pH on the distribution of phosphate ions insolution


PHOSPHATE SPECIES

The following species in the system involving Mg2+, PO43-, NH4

+

aqueous solutions can be formed, when the pH is varied,: H3PO4,H2PO4

- , HPO42- , MgOH+ , MgNH4PO4 , MgPO4

- , MgH2PO4+ and

MgHPO4. Polynuclear species can also be formed when the totalconcentration of phosphate is increased, although the kinetics of

their formation is slow at 25°C. Fig. Shows the main magnesiumand phosphate species as a function of their total concen tration inthe system Mg–NH3 –H3PO4 (1 mol/L) at 60°C, plotted against thepH.

Magnesium speciation versus pH at 20C. Total concentrationof magnesium, CT: (a) CT = 0.015 M, (b) CT = 0.100 M and(c) CT = 0.15M

F. Mijangos, M. Kamel, G. Lesmes, D.N. Muraviev, 2004, Synthesis of struvite by ion

exchange isothermal supersaturation technique, Reactive & Functional Polymers 60, 151-

161. D. Rai, B.M. Sass, D.A. Moore, 1987, Inorg. Chem. 26, 345.


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PHOSPHATE SPECIES

The speciation of phosphate species in the aqueous solution followsthe sequence H3PO4, H2PO4

- , HPO42- and PO4

3- following anincrease of pH in the system.At pH


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WATER REUSE AND RECYCLE

These problems can be made more severe by variability ofthe concentrations of various contaminants in the waterstream to be reused.The result is that some type of pretreatment or intermediatetreatment must be used to clean the water before it isreused or recycled.In other cases, additional treatment chemicals or methodsmay be required to minimize corrosion, scale, fouling, ormicrobial growth in the system that is using the recycledwater.Pretreatment methods may be as simple as filtration or asextensive as membrane separation like reverse osmosis.Finally, to achieve the desired goal, a strategy for reuseand recycle should be developed.

The simplest options should be considered first, beforeprogressing to successively more complex reuse andrecycle methods.Water conservation should be one of the first optionsconsidered. Cascading water from one process in aplant to another, that has less critical waterrequirements, is the next option. This maybe possiblewithout intermediate treatment in some cases.Reuse of another wastewater source from outside theplant or recycle of a wastewater stream within theplant is generally the last consideration.These sources generally require some type ofpretreatment before they can be used in industrialapplications. Evaluating all the options gives the bestopportunity to achieve the desired goal at the lowestcost.

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