Disposal of wastewater treatment sludge is a pressing environmental issue that calls for

Transcription

1 Abstract Disposal of wastewater treatment sludge is a pressing environmental issue that calls for concern. Progress has been made in sewage sludge treatment, to release and recycle the inorganic metal fraction adsorbed onto the surface of the carbonaceous component of sewage sludge. A more specific attempt is made to isolate and release Aluminum metal. Two specific parameters, pyrolysis temperature and ph, are manipulated to optimize Aluminum metal release and precipitation. The effect of pyrolysis temperature on production of sludge-based adsorbents was found optimal at 800 C, while aluminum precipitation out of solution was found optimal at a ph of 6. Introduction Urban growth and the proliferation of wastewater treatment plants have immensely increased the production of municipal sludge worldwide 4,6,7-9, 11. Increasing levels of wastewater treatment sludge pose significant pressures on the environment because of the loads of organic contaminants, heavy metals, and hazardous substances in the sludge 1,6-8. In aims to protect the environment from the adverse effects of urban wastewater discharges and to avoid costly traditional disposal routes, new techniques have been developed for alternative uses of sewage sludge 4,7. One method that has shown promise is the conversion of wastewater treatment sludge into adsorbents, and the recycling of inorganic metals adsorbed onto the carbon sludge 1, 4, 7. The effective treatment and disposal of sewage sludge from a wastewater treatment plant requires that the composition of the sludge be known. Sewage sludge is defined as the residue generated from the treatment of wastewater 6,7. The main Page 1 of 15

2 constituents of sewage sludge are proteins, fats, urea, cellulose, silica, nitrogen, phosphoric acid, iron, calcium oxide, alumina, and magnesium oxide 7. Heavy metals and a wide variety of minerals are also bound or incorporated into the organic matter 1,8. Following organic decomposition, humic substances have been identified as an integral component of sewage sludge 2, 7. There are two main types of humic substances, which differ in acidity and chemical composition 7. The two classes include fulvic acid and humic acid 7. The structures of fulvic and humic acid are depicted in Figure 1. Humic substances are macromolecular, negatively charged, branched polyelectrolytes with many carboxylic acid and phenolic type functional groups. Fulvic acid s structure can be generally characterized as an assembly of organic aromatic polymers with many carboxyl (COOH) groups. Fulvic acids have a general molecular formula of C 14 H 12 O 8, and as compared to its humic substance counterparts, have the lowest carbon content, are most greatly oxidized, and are soluble in acids 2. A carbon-to-hydrogen ratio greater than 1:1 indicates minimal aromatic character, while the prevalent oxygen character of the compound indicates its acidic character. Humic acids have a general formula of C 77 H 76 O 21, and can also be generally characterized as an assembly of more compact aromatic polymers 2. The carbon-to-hydrogen ratio of 1:1 indicates the high level of Page 2 of 15

3 aromatic character. Moreover, Humic acids have relatively high carbon content, usually polymeric, and are less soluble in acids 2. As stated before, heavy metals and a wide variety of cationic minerals are found bound or incorporated into the organic matter of sewage sludge 1,8. The acid-base chemistry of humic and fulvic acids is an essential component in understanding the interactions between the carbon matter of sewage sludge with the metals. As depicted in Figure 1, humic and fulvic acids contain few to many carboxyl groups 7. Part of the pre-treatment process helps induce a negative charge on the surface of sludge carbon. Pre-treating the sludge carbon with basic solution, such as NaOH, will deprotonate the carboxylic functional groups of the humic and fulvic substances on the surface of sludge carbon 2. Upon treatment with NaOH, the adsorbent surfaces will have more negative charge sites or a greater number of metal adsorption sites and hence greater metal adsorption capacity. The adsorption of metal ions a take place by the ion-exchange mechanism represented in equation 1 12 : 2RCOO - Na + + M 2+ (RCOO) 2 M + 2Na + (1) Sludge contains a wide variety of sites capable of metal retention 1, 8, 10. Wastewater sludge retains metals through cation exchange, adsorption, chelation, and precipitation 1. Heavy metals in sewage sludge is a hazardous problem, which makes it necessary to evaluate the metals in WWTS and further find potential use for such metals 1,6,8. Selective adsorption of metal ions can be achieved by chemical treatment and pyrolysis. Pyrolysis is a process by which the bound water associated with the Page 3 of 15

4 sludge is released by heating the sludge for short periods of time 4, 7. Exposing the sludge to heat coagulates the solids, breaks down the cell structure, and reduces the hydrophilic nature of the solids 7. Pyrolysis of sewage sludge leads to three fractions: liquid, solid and gas 7. The gas fraction vaporizes while liquid fraction becomes decanted and undergoes a secondary clarification treatment 7. The solid fraction obtained can be incinerated, disposed of on a landfill, or used as an inexpensive sludge-based adsorbent (SBA) 7. A considerable inorganic faction has been found bound to the surface of developed SBAs 6,7. Raw wastewater sludge have been found to contain very high humic/fulvic acid ratios, while digested sludge contain very low ratios 2. This data suggests that upon pyrolysis of the sewage sludge, humic substances break down to fulvic acids, and greater levels of fulvic acid allow for greater retention sites of metal compounds. A good way to treat and recycle metal components from SBAs is by the use of hydrometallurgy, which involves the use of aqueous chemistry for the recovery of metals from recycled or residual materials 8, otherwise known as leaching 9. Leaching involves the use of aqueous solutions containing a lixiviant, which is brought into contact with the material containing a valuable metal 9. The lixiviant in solution is typically a reagent that may be acidic or basic 9. In the leaching process, oxidation potential, temperature, and ph of the solution are important parameters that are manipulated to optimize the releasing of the desired metal component into the aqueous solution 8,9. Page 4 of 15

5 In many hydrometallurgical processes, hydrochloric acid has been selected as the cheapest and most effective leachant 9. HCl washing has been found to be the most promising in increasing the porosity of SBAs 7 as well as releasing a considerable amount of inorganic metals, including Al 3+, Fe 3+, Ca 2+, and Mg 2+ 1,8,11. The sludge is brought down to low ph conditions under which many of the metals present as metal oxides in the sludge become solubilized 9,10. In the acid leaching process the following reactions take place: Al 2 O 3(s) + 6HCl (l) ) 2AlCl 3(aq) + 3H 2 O (g) (2) Fe 2 O 3(s) + 6HCl (l) ) 2FeCl 3(aq) + 3H 2 O (g) (3) CaO (s) + 2HCl (l) ) CaCl 2(aq) + H 2 O (g) (4) MgO (s) + 2HCl (l) MgCl 2(aq) + 3H 2 O (g) (5) After solubilization, the removal of the solubilized metals is achieved by chemical precipitation followed by a physical separation step 8. The final step, precipitation, is an important adsorptive mechanism in the removal of heavy metals 13. It depends on the solubility product (K sp ) of the metal involved, ph of the wastewater, and concentration of metal and relevant anions 13. The equilibrium expression of a sparingly soluble salt is provided in equation 6, MX 2(s) = M 2+ (aq) + 2X - (aq), (6) where the equilibrium constant, K sp = [M 2+ ] [X - ] 2. At equilibrium, the rate of the dissolution reaction, is equal to the rate of the precipitation reaction. However, when the product values of the concentration of cations and anions exceed the K sp, precipitation occurs. Page 5 of 15

6 The implementation of acid washing in SBA production, and precipitation is highly favorable 6. The inorganic fraction that is extracted is highly useful, when recycled and reused, therefore, providing an extra revenue source and reducing the production of waste 6. Of particular interest is aluminum, with a characteristically low density 12, is abundant in the Earth s crust, but is very rare in its free form (Al 3+ ) 12. Aluminum is mostly found as Aluminum hydroxide (Al 2 O 3 ), an insoluble form of Aluminum that is the main composition of soil 12. Aluminum is very difficult to refine, and therefore, the recovery of Aluminum from recycling has become an important industry 11, 12. An attempt is made to demonstrate a useful method of handling wastewater treatment sludge, with a particular focus on removing and precipitating Aluminum metal from sludge-based adsorbents (SBAs). It is found that pyrolysis temperature has a dramatic effect on concentration of metals released from sludge-based adsorbents, and controlling ph has an effect on precipitating Aluminum metal out of solution. Results and Discussion As previously discussed, pyrolysis of sewage sludge is a critical step in forming SBAs and fostering sites for metal retention on their surface. Pyrolysis temperature is an important parameter that can be manipulated to optimize metal retention, which is followed by the metals release during chemical leaching. Metal concentration leached from activated sludge carbon was quantified at varying pyrolysis temperatures (500 C C), and the results are depicted in Figure 2. The soluble residues leached from sludge carbon consisted of various metal ions: Al 3+, Fe 3+, Ca 2+, and Mg 2+, in the form of aqueous metal chlorides, AlCl 3, FeCl 3, CaCl 2, and MgCl 2. From pyrolysis temperatures Page 6 of 15

7 between 500 C-800 C, it was found that metal concentration leached from sludge carbon increased with increasing pyrolysis temperature. The optimal pyrolysis temperature was found to be at 800 C, with concentrations of AlCl 3, FeCl 3, CaCl 2, and MgCl 2 leached at 26, 32, 18, and 7 mgl -1, respectively. Beyond 800 C, there was no further increase of leaching metal concentration, with concentrations of aluminum, among other metals, decreasing past this point. As described before, raw wastewater sludge have been found to contain very high humic/fulvic acid ratios, while digested sludge contain very low ratios 2. This finding suggests that upon pyrolysis of the sewage sludge, humic acids break down into fulvic acids, and greater levels of fulvic acids allow for greater retention sites of metal compounds. Furthermore, high pyrolysis temperatures convert the inorganic faction into mineral like compounds, as well as inducing the development of porosity in the inorganic and carbon faction 11. Therefore, with the development of pores between the inorganic and carbon faction, the inorganic faction becomes encapsulated by the carbon phase. The higher concentration of ions retained during pyrolosis allows for release of greater concentrations of metal ions during leaching However, there is a critical Page 7 of 15

8 temperature (800 C), which when exceeded, will destroy the lattice of the WWTS, and then form a stable structure, preventing the formation of a carbon-inorganic matter interface 11. Generally, literature suggests that increasing the carbonization temperature causes an increase in the ph 11, 13. This increase in ph is seen at temperatures of 500 C and above, where nitrogen becomes incorporated within the carbon matrix in the form of ammonia 13. Increasing ph enhances the negative charge on the surface of sludgebased adsorbents, which allows for greater cation exchange abilities of metals in solution. At temperatures above 800 C, an additional cause for this increase may be the decomposition of inorganic compounds such as iron, copper, and aluminum sulphates 11, 13. However the positive correlation between the carbonization temperature and the ph tends to weaken beyond a threshold temperature 13. The temperature at which the ph was maximized has reported to range from 600 C-1000 C. The threshold temperature varies based on the composition of the sludge 13. There are marked differences in concentrations of Al 3+, Fe 3+, Ca 2+, and Mg 2+ concentrations leached from sludge carbon. Different metal cations bind to the activated sludge carbon at different affinities. The extent of the interaction between ions and dipoles depends on the size and charge of the ions. The effects of these two factors can be illustrated by comparing the multivalent ions such as Al 3+ and Fe 3+, which are more strongly adsorbed, as compared to lower valence cations such as Ca 2+ and Mg 2+. Multivalent ions, in this case, Al 3+ and Fe 3+, have much smaller ionic radii and are harder metals, meaning they have high charge densities and are able to form stronger Page 8 of 15

9 ion-dipole interactions at the surface of the activated sludge carbon. The lower valence ions, Ca 2+ and Mg 2+, have larger radii and are softer metals, meaning they have lower charge densities and therefore form weaker ion-dipole interactions at the surface of the activated sludge-carbon ph Precipitation Effect on Concentration and Characterization of Al(OH) 3 After the recovery of metals from sludge carbon, in the form of metal chlorides, an attempt is made to isolate and precipitate aluminum hydroxide out of solution. The total precipitation of Al(OH) 3 is dependent upon the sum of the concentrations of all Al- OH complex species, including the free Al 3+ concentration. The solubility mass balance equation is represented as: Al T (aq)= [Al 3+ ] + [Al(OH) 2+ ] + [Al(OH) + 2 ] + [Al(OH) 3 ] + [Al(OH) - 4 ] Where Al T represents the concentration of all aluminum species at various ph values. There are two major factors often influence solubility, the chemical properties of the solute and the solvent, and the conditions of the solutions such as temperature and ph. In regards to solute and solvent chemical properties, the most important rule is that solutes dissolve better in solvents of similar polarity, which is often referred to as the rule of like dissolves like. Aluminum has a high polarity and therefore is more soluble in polar solvents, such as H 2 O and NaOH. More importantly, by manipulating ph, the cations and anions in solution can form insoluble salt products, in this case, Al(OH) 3. Aluminum hydroxide, is a metal hydroxide, and when dissolved is ionized into Al 3+ and OH -. And since the hydroxide concentration, [OH - ], is a property of the solution, the solubility of the metal hydroxide depends on ph. Page 9 of 15

10 The effect of ph on Al(OH) 3 precipitation is depicted in Figure 3, which shows the calculated solubility of aluminum as a function of ph for aqueous aluminum hydroxide solution in equilibrium with microcrystalline gibbsite (Al(OH 3 )). Gibbsite, (Al(OH 3 )), is a insoluble precipitate, and therefore its concentration remains constant under various ph values. Aluminum is a strongly hydrolyzing metal and is relatively insoluble in the neutral ph range of As illustrated in Figure 3, in the ph range of 6-8, the total aluminum species in solution (Al T ) comes to equilibrium with Al(OH 3 ) (s) precipitate. In the presence of complexing Figure 3: Solubility of aluminum species and total aluminum (Al t ) in relation to ph in a system in equilibrium with gibbsite (Al(OH) 3 ). pc=-log[al] ligands such as hydroxyl groups, in acidic conditions, (ph<6) and alkaline conditions (ph>8), aluminum solubility is enhanced. At low ph values, dissolved aluminum is presently mainly in the aqueous form (Al 3+ ). As depicted in Figure 3, at a ph value of 2, [Al 3+ ] is the most dominant species with a 1x10 5 molar concentration. As ph rises, hydrolysis occurs, resulting in a series of less soluble hydroxide complexes such as Al(OH) 2+ and Al(OH) + 2. As illustrated in Figure 3, as ph increases to values of 6-8, molar concentrations of Al(OH) 2+ + and Al(OH) 2 predominate over molar concentrations of Al 3+. As stated before, aluminum solubility is at a minimum near ph 6, where total Page 10 of 15

11 aluminum species in solution (Al T ) are at equilibrium with Al(OH) 3 (s). Finally, solubility increases as the anion, Al(OH) - 4 begins to form at higher ph. This increase in solubility is depicted at a basic ph of 12, where a 1x molarity of Al(OH) 4 predominates over all other aluminum species in solution. Metal hydroxides such as Al(OH) 3 react with acids and bases, and they are called amphoteric hydroxides. In reality, Al(OH) 3 should be formulated as Al(H 2 O) 3 (OH) 3, and as explained before, this neutral substance has a very low solubility, and as depicted in Figure 3, its concentration remains constant under various ph values. Overall, the solubility of amphoteric hydroxides can be explained as a complex formation process. The solubility of aluminum at lower ph values is due to a reaction with H 3 O +, which shifts the solubility equilibrium toward dissolution. This increase in solubility seen in Figure 3, where [Al 3+ ], [Al(OH) 2+ ], [Al(OH) + 2 ] species dominate in aqueous solution at acidic ph values of 0-4. Similarly, at higher ph values, the Al(OH 4 ) - solubility is due to the formation of soluble complexes of the metal ions with OH -. The solubility of amphoteric hydroxides can be explained as a complex formation process. In acidic solution, OH - ions are scarce so Al 3+ is completely hydrated as Al(H 2 O) As the solution becomes less acidic and more basic, the OH - ions gradually react with the Al 3+ metal, and a series of reactions take place as shown in equations 7-9: Al(H 2 O) OH - Al(H 2 O) 5 OH 2+ + H 2 O(l) (7) Al(H 2 O) 5 OH 2+ + OH - Al(H 2 O) 4 (OH) H 2 O(l) (8) Al(H 2 O) 4 (OH) OH - Al(H 2 O) 3 (OH) 3 (s) + H 2 O(l) (9) Page 11 of 15

12 the neutral complex, Al(H 2 O) 3 (OH) 3, forms a precipitate, otherwise written as - Al(OH) 3 (s). As the solution becomes more basic, the formation of Al(OH) 4 results, as shown in equation 10: Al(H 2 O) 3 (OH) 3 (s) + OH - Al(H 2 O) 3 (OH) H 2 O(l) (10) The precipitate dissolves due to the formation of the soluble negative complex ion, which is seen in Figure 3, as [Al(OH) - 4 ] dominates in aqueous solution at ph values greater than 9. Typically, at a ph of approximately 6, Al(OH) 3 predominates over all the other species. As illustrated in Figure 3, the formation and isolation of Al(OH) 3 is maximized at ph 6, when all other forms of Aluminum species present in solution are minimized. Understanding the effect of ph on the solubility and precipitation of Aluminum out of aqueous solution helps determine the most optimal conditions for Aluminum hydroxide isolation. As described before, after the recovery of metals from sludge carbon, in the form of metal chlorides, an attempt is made to isolate and precipitate aluminum hydroxide out of solution. A sodium aluminate solution is formed by reacting AlCl 3 with 12M NaOH, which is then precipitated to optimize Al(OH) 3 concentrations under various ph values. The remaining inorganic element concentrations were recorded, and results are depicted in Figure 4. Figure 4: Synthesis of Al(OH) 3 The effect of ph precipitation on remaining concentration of inorganic elements left in solution. Band labeled PL represents a diluted sample of the original sodium aluminate solution. Page 12 of 15

13 The final ph of precipitation was found to have a significant effect on the Al 3+ degree of conversion to Al(OH) 3. The initial formation of Al(OH) 3 was observed to occur at ph 10, as a stable white complex. With successive addition of HCl to solution, ph dropped, and the remaining Aluminum species in solution successively decreased as well. The Al(OH) 3 precipitation reaction was found optimal at ph 6, which was the point at which lowest remaining concentration of sodium aluminate solution was recorded, and therefore the greatest amount of Al(OH) 3 complex formed It is perceived that, as the concentration of aluminum species decreased in solution, the amount of Al(OH) 3 complex formed increased. As depicted in Figure 3, at a ph of 10, Al(OH 4 ) - is the most prominent species in solution. As ph approaches a ph of 6, Al(OH) 3 precipitation is maximized. At a ph of 6, Al(OH) 3 precipitate becomes the most prevalent species in solution, while all other forms of aluminum species present in solution are minimized. Moreover, all other metal ions remained at stable concentrations throughout the Al 3+ precipitation reaction, indicating minimal to no matrix influence. Optimal precipitation of Al(OH) (s) at ph 6 can be explained by the solubility characteristics of Al(OH) 3 as a function of ph, which were depicted and explained in Figure 3. Moreover, the relatively low ph improved diffusion rate of the Al 3+ ions, increasing the probability of collisions between ions, which improves the Al(OH) 3 precipitation efficiency 11. Interactions of ph and organic matter essentially dictate the behavior of Aluminum. Aluminum species of Al 3+ and AlOH 2+ associate readily with organic material, Page 13 of 15

14 which will form Aluminum-organic complexes and therefore reduce concentrations of monomeric forms of Aluminum in solution. Conclusion Attempts have been made to find alternative uses of sewage sludge, as its growth and disposal have become an increasing problem. A method that has shown promise is the conversion of sewage sludge into adsorbents. An inorganic faction adsorbed on the surface of sludge is released, and the aluminum components of this faction has been recycled and utilized. Two specific parameters, pyrolysis temperature and ph, were manipulated to optimize aluminum metal release and precipitation. The leachability of metal ions from sludge carbon and metal solution produced by acid washing has many potential uses. When such metals are extracted, they can be reused to develop nanosized products. These nanosized products have a wide range of applications such as catalysts, electronic devices, and drinking water treatment. This current work may lead to developing environmentally friendly approaches to treating metal contaminated soils and sewage sludge, replacing currently used techniques of metal extraction using toxic agents and landfilling. Further research may look into utilizing immobilization technology to improve the adsorption process, developing commercial adsorbents that could be regenerated and reused. Page 14 of 15

15 References 1. Alvarez, E. A.; Mochon, M. C.; Jimenez Sanchez, J. C.; Ternero Rodriguez, M. Heavy metal extractable forms in sludge from wastewater treatment plants Chemosphere 2002, 47, Atalay, B. Y.; Carbonaro, F. R.; Di Toro, M. D. Distribution of Proton Dissociation Constants for Model Humic and Fulvic Acid Molecules Eviron. Sci. Tecnol. 2009, 43, Bagreev, A.; Bandosz, J. T.; Locke, C. D. Pore structure and surface chemistry of adsorbents obtained by pyrolysis of sewage sludge-derived fertilizer Carbon 2001, 39, Fytili, D.; Zabaniotou, A. Utilization of sewage sludge in EU application of old and new methods A review Elsevier 2008, 12, Li, X.; Wang, D.; Zhou, Q.; Guihua, L.; Zhihong, P. Concentration variation of aluminate ions during the seeded precipitation process of gibbsite from sodium aluminate solution Elsevier 2011, 106, Wan Ngah, W.S.; Hanafiah, M.A.K.M. Removal of heavy metal ions from wastewater by chemically modified plant wastes as adsorbents: A review Elsevier 2008, 99, Smith, K. M.; Fowler, G. D.; Graham, N. D. Sewage sludge-based adsorbents: A review of their production, properties and use in water treatment applications Elsevier. 2009, 43, Stover, R. C.; Sommers, L. E.; Silviera, D. J. Evaluation of metals in wastewater sludge Water Pollution Control Federation 1976, 48, Ukiwe, L. N.; Rita, N. O.; Nwoko, C. A. Progressive Acidification: An Aspect of Chemical Leaching of Sewage Sludge International Journal of Chemistry 2012, 4, Xu, G.; Zou, J.; Li, G. Stabilization/Solidification of Heavy Metals in Sludge Ceramsite and Leachability Affected by Oxide Substances Eviron. Sci. Tecnol. 2009, 43, Zou, J.; Ying, D.; Chungui, T.; Kai, P; Baojiang, J.; Lei, W.; Wei, Z.; Tian, G.; Xue, W.; Zipeng, X.; Fu, H. Structure and Properties of Noncrystalline Nano-Al(OH) 3 Reclaimed from Carbonized Residual Suldge Eviron. Sci. Tecnol. 2012, 46, Feifer, N. Studying the Chemical Properties of Metallic J. Chem. Educ. 1968, 46, AjayKumar, A.; Darwish, N.; Hilal, N. Study of Various Parameters in the Biosorption of Heavy Metals on Actiavted Sludge World Applied Sciences 2009, 5, Page 15 of 15