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Chpter 2 Coagulation section 1 Basic Mechanisms and Application

Chpter 2 Coagulation section 1 Basic Mechanisms and Application. Presented by Meng Li. Backgrounds For Aliminum Chemistry. Several Al coagulants are used widely for removing contaminants in the raw water: AlCl 3 , Al 2 (SO 4 ) 3 .nH 2 O and PACl.

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Chpter 2 Coagulation section 1 Basic Mechanisms and Application

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  1. Chpter 2 Coagulationsection 1Basic Mechanisms and Application Presented by Meng Li

  2. Backgrounds For Aliminum Chemistry • Several Al coagulants are used widely for removing contaminants in the raw water: AlCl3, Al2(SO4)3.nH2O and PACl. • The classes of Al species can be divided into four categories: Monomers and small polymers; such as Al and Al(OH)4-; medium polymers, such as Al7(OH)174+; large polymers, such as Al13(OH)347+ and Al(OH)3 Solids precipitate. And the solubility of Aluminum in equilibrium with solid Al(OH)3 will be greatly depend on the surrounding PH and the concentration of Aluminum.

  3. Background for humic Substances(1) • Humic substances are acidic, hetero-disperse, polymeric complex macromolecules. • Humic substances contain humic acids which will precipitate at PH<2 in waters, and fulvic acids which will stay in solution and absorb onto a non-polar resin. The lower molecular weight fulvic acids have a higher carboxylic and phenolic acidity and hence a higher charge density. As a consequence, fulvic acids will normally be more difficult to coagulate than the humic acids which have higher molecular weight and less charged.

  4. Backgrounds for Humic Substances(2) • Carboxyl and phenolic groups provide most of the negative charge that adds to the mobility of FA in the aquatic environment. • According to the previous research, HA is more easily to be removed by coagulation than FA, which also reflect different impacts of different functional groups associated with those chemicals in the suspensions. • Apparently water quality problems related to these humic substances are color, taste and odor, corrosion and biological activity in water distribution systems, mobilization of toxic elements and micro-pollutants, and the formation of disinfection by-products such as THMs. Humic substances have a great tendency to adsorb to particles and increase their charges and stability.

  5. Fulvic acid-Aluminum interactions • Two important mechanisms for FA removal are: • (1) Charge neutralization-precipitation(CNP). The CNP consists of the reaction between soluble Al cationic polymers and soluble anionic HS, that is to say, adsorption of positively charged coagulant species to sites on the negatively charged fulvic acid to obtain charge neutralization and formation of insoluble complexes. Obviously there exists a chemistry stoichimetry between coagulant dosage and FA and re-stabilization region resulting from charge reversal should be possible upon overdosing. This mechanisms is generally more dominant during the low PH value. • (2) Adsorption and simultaneous precipitate( or sweep coagulation), that is to say, adsorption of FA to, or enmeshment in, Alum hydroxide precipitates. The bonds that form can include van der walls interactions, H-bonding, hydrophobic bonding and ligand exchange. And simultaneous precipitation can be considered to be the simultaneous reaction of FA particles with both soluble Al polymers and Al(OH)3. This mechanism is more dominant at higher PH value.

  6. Experimental protocol and method • Two sources of FA were selected for study: the New Jersey Pinelinds area and Dismal Reservior in Maryland. The main reason for choosing these sources is that the TOC concentration is high, and the majority of humic substances is FA. • The samples were rapidly stirred with magnetic stirring bars, followed by the simultaneous addition of coagulant and PH titrant. • After 2 min of rapid mixing, 30 min of slow stirring was provided, followed by 1 hr of quiescent setting. After settling was completed, the TOC remaining in solution was determined using UV absorbance at a wavelength of 254 nm. Similar residual FA data were collected after the supernatant had been filtered through a 0.22-μm filter.

  7. Aluminum speciation diagram for AlCl3 • The data cover a PH range from 4.75 to 8.0. At a low PH, monomeric and small polymeric dominate. Then, as the PH increases, the ,medium polymers become dominant at PH 6-6.5. Concurrently, Al(OH)3 increases while the concentration of predominant Al species is Al(OH)3. As PH 8 is approached, an increase in monomers is observed along with a decrease in the Al(OH)3 due to the presence of Al(OH)3 at higher PH.

  8. Aluminum speciation diagram for Alum Sulfate • In terms of speciation diagram for alum sulfate, the Al species distribution is distinctly different from that for AlCl3. Monomer and small polymers concentration decrease sharply at PH 5. As PH increases to 4.9, the formation of Al(OH)3 increases rapidly. The rapid formation of Al(OH)3 is attributed to the presence of sulfate. Sulfate causes Al(OH)3 to be formed at lower PH values.

  9. Aluminum speciation diagram for PACl The monomer and small polymer are dominant at low PH and medium polymer are predominant at PH5.0-6.6.The increase in the medium polymers.Beyond PH6.6 an Al(OH)3 precipitates forms and then the concentration of Al(OH)3 exceeds that of the medium polymers. The distribution of Al species between PH 5.5 and 6.5 is representative of the initial concentrated Al solution.at lower PH,some small and medium polymers seem to be dissolved to form monomers.

  10. Effect of sulfate • For AlCl3 with sulfate adding, Al speciation more closely resembles that for Alum than that for AlCl3. The Al(OH)3 is the dominant species at PH 4.6. When using PACL with the same sulfate-to-Al ratio also shows a radical change in the distribution of Al species. It is concluded the sulfate causes the rapid formation of Al(OH)3 at low PH and can improve the FA removal by the precipitation of Al(OH)3.

  11. Effect of Sulfate (continued) • From our study we can observe that in the presence of sulfate, less alkali is necessary to produce a visible precipitate than when solutions containing chloride or nitrate salts are titrated. Sulfate tends to link OH-Al polymers together. Therefore, most basic salts containing sulfate are amporphous. It is presumed that a screening effect occurs that accelerates the formation of polymers and assists in the linking of planar complexes to form the solid lattice. • Because of the catalytic effect of sulfate, Al solutions containing sulfate can form precipitates at low OH-to-Al ratios (in which r is defined as the molar ratio of added base to Aluminum), typically corresponding to a PH of about 4.0.

  12. Coagulation of FA (1) • Coagulation of FA with 1.78×10-4 M AlCl3 as a function of PH, with and without calcium adding. (TOC—3.5mg/L, Ca—10M). • We can find that two distinct regions of filtered removal are evident, separated by an area of particle re-stabilization at PH 5.3.

  13. Coagulation of FA (2) • Coagulation of FA with 1.78×10-5 M AlCl as a function of PH, with and without calcium. (TOC—3.5mg/L, Ca—10M). • We can find that at a lower Al dosage like this only one zone of FA removal occurs.

  14. Discussion 1 Coagulation with AlCl3 • The zones of removal observed are also related to the Al speciation diagrams. At a lower PH, the dominant removal mechanism is CNP, and over the PH range 6.0-6.5, it is likely that simultaneous competitive reactions occur between FA and medium polymers and between FA and Al(OH)3. • The dominant mechanisms and actual regions of FA removal may change at different Al dosages of the same coagulant. At the low Al concentrations in this study, higher percentage of monomers and polymers would be present over the entire PH range and too little Al(OH)3 be formed in the solution. Therefore, the CNP mechanism appears to cause FA removal under lower Al concentration. This is the reason why only one region of filtered removal at the 1.78×10-5 M Al dosage is found.

  15. Coagulation of FA (3) • Coagulation of FA with 1.7810M Alum sulfate as a function of PH. (TOC—3.5mg/L). • We can find that at this dosage, both settled and filtered removal efficiencies are high over a wider PH range. Settled removals reach 80% at PH 6.0, and filtered removals are high from PH 4.5 to 8.2.

  16. Discussion 2 Coagulation with Alum sulfate • From the above experiment we can draw a conclusion that the CNP mechanism is the only one for FA removal at low PH and low Al when alum sulfate is used and the sulfate indeed causes the rapid formation of Al(OH)3. • As the PH begins to increase above 4.75, a combination of the two mechanisms( CNP and ASP) is likely, with adsorption on Al(OH)3 predominating as the Al dosage and PH value increase. • Because of the catalytic effect of sulfate, it can improve the removal efficiency of FA under such unfavorable operational conditions (low PH and AlT).

  17. Coagulation of FA (4) • Coagulation of FA with 1.78×10-4 M AlCl3 and sulfate as a function of PH. (TOC—3.5mg/L, sulfate-to Al mole ratio—1.5:1.0) • We can find both of the filtered and settled removal closely resembles those of Alum. Increased FA removal when sulfate is added to AlCl3 may be attributed solely to the presence of sulfate. As we mentioned before, the sulfate causes the precipitation of Al(OH)3 from solution at a much lower PH value.

  18. Coagulation of FA (5) • Coagulation of FA with 1.78×10-4 M PACl as a function of PH. (TOC—3.5mg/L)

  19. Discussion 3 Coagulation with PACl • From this diagram, it is observed that for settled removals, PACl exhibits stoichiometry and re-stabilization at both PH 4.75 and PH 5.5. When an increase in the FA concentration occurs, an increase in the Al coagulant dosage is required, the zone of removal broadens, and the extent of FA removal increases. • These phenomena are consist with the CNP mechanism and provide further evidence that interaction between soluble polymers and FA at least up to PH 5.5.

  20. Coagulation of FA (6) • Adsorption of FA on 1.78×10-4 M Al(OH)3 with and without Calcium. The solution of PH is kept constant at 6.5. (FA adsorption amount verse FA concentration) • We can find a significant increase of the FA removal with the adding of Calcium.

  21. Discussion 4 Effect of Calcium • 1 Basically the complexetion of Ca with the FA particles reduces the charge on the FA, which makes the FA molecule more hydrophobic and thus allows higher removal efficiency. In addition, Ca may also reduce the electrostatic repulsion between molecules on the surface of metal hydroxide. • 2 When the Al concentration is 1.78×10-5 M, a significant improvement in FA removal could be found with the addition of Ca. The coagulant is considered relatively under-dosed compared with the FA concentration before Ca is added. Therefore there exists a lot of FA which have no chance to participate the reactions with Al. So the FA removal efficiency is low. When Ca is added, radical changes occur in the aquatic environment around FA particles. The over-dosed Ca complexes with FA in solution and occupies a portion of the reactive sites on the surface of FA. So because some of the FA sits are taken by Ca, there is additional Al available to react with remarkably increases.

  22. Effect of Calcium (continued) • 3 While Al concentration is 1.78×10-4 M, we can find the FA removal is pretty high even with a lower PH value. Because the Al coagulant dosage is relatively high, at low PH (4.0-5.0), there is sufficient Al in solution as mentioned before. So many monomers and polymers have already existed in solution. • 4 With the addition of Ca, a similar reaction between Ca and FA occurs. But because of the excess Al in solution, there is no apparent influence of Ca on the removal of FA. • 5 At the higher PH value(>6.5), under which condition Al(OH)3 is present in solution, Ca enhances the FA removal slightly because of the fact that Ca can cause the FA particles to be more hydrophobic.

  23. Effect of FA source • Coagulation of P1 FA with 1.78× 10-5 M alum as a function of PH. (TOC—3.5mg/L) • It is evident that under the same experimental conditions, better FA removals over a wider PH range are achieved for this sort of FA particles. • A review of the molecular weight and ash content data indicates that this sort of FA had larger molecular weight and more poly-disperse than the former ones.

  24. Discussion 6 Comparison of Coagulants • 1 Under the experimental conditions of this research, AlCl3 does not provide better removal of FA than does Alum sulfate or PACl. • 2 With alum sulfate, the highest removals are achieved between PH 5.0-5.5, with gradually decreasing removal efficiencies as the PH increases to 8.0. As the total concentration of Alum sulfate is decreased, the region of 80% removal also decreases. Basically, the majority of the region of 80% percent FA removal lies within the region where Al(OH)3 precipitates is formed. • Domains of 80% filtered removal of FA by aluminum sulfate

  25. Discussion 6 (continued) • 3 The use of PACl for FA removal results in a continuous band of 80 percent removal. However, the region begins at a lower PH (3.75), with a significant portion lying in the region where Al polymers are predominant (PH<4.5), and extends to the same extent as aluminum sulfate through the area where Al(OH)3 precipitates is formed. Generally speaking, PACl achieves 80% removal over a larger range of PH than does alum. • Domains of 80% filtered removal of FA by PACl.

  26. Conclusion • 1 The mechanisms of FA removal are directly related to the formation of Al speciation in solution. Charge neutralization-precipitation occurs in the presence of Al monomers and polymers. Adsorption takes place when Al(OH)3 has precipitated from solution. At higher PH value at which both Al(OH)3 and Al polymers coexist, concurrent reactions by both mechanisms appear to cause simultaneous precipitation. • 2 Different Al coagulants have been shown to have different Al species distributions over a wide range of PH, which cause different mechanisms of FA removal under the same experimental conditions. So analyzing Al species is useful by providing insights into the reactions mechanisms between FA and chemicals in the suspensions.

  27. Conclusion • 3 The use of PACl has been shown to be beneficial at low FA concentrations and low PH, producing better removals of FA than the other kinds of coagulants. • 4 The addition of Ca during water treatment may be a cost-effective means of improving FA removal. • 5 For FA particles from different sources, there does exist different removal effects when conducted under the same experimental conditions. The influencing factors may include: FA molecular weight, charge density and components and structures of FA.

  28. Acknowledgment • This research work was conducted under the instructions of professor Charles R. O’Melia, department of geo & Environmental engineering, Johns Hopkins University, Baltimore, MD. • I am also grateful for helps provided by my colleagues such as professor becker, Dr Jin, Dr Huang, and Dr Adrin.

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