THE EFFECTS OF CLAY A M E N D M E N T AND COMPOSTING ON METAL SPECIATION IN DIGESTED SLUDGE LIANG QIAO

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Australia, Australia (First received October 1995; accepted in revisedform September 1996) Abstract--Sewage sludge usually contains significant heavy metals that may limit its land application. Heavy metals in municipal solid waste have been shown to be less mobile by amendment with bauxite refining residue (red mud) prior to the composting process.

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Nội dung Text: THE EFFECTS OF CLAY A M E N D M E N T AND COMPOSTING ON METAL SPECIATION IN DIGESTED SLUDGE LIANG QIAO

Wat. Res. Vol. 31, No. 5, pp. 951-964, 1997 ~) Pergamon © 1997ElsevierScienceLtd. All rights reserved Printed in Great Britain P lI: S0043-1354(96)00290-4 0043-1354/97$17.00+ 0.00 T HE EFFECTS OF CLAY A M E N D M E N T AND C OMPOSTING ON METAL SPECIATION IN DIGESTED S LUDGE L IANG QIAO @ and GOEN HO *@ Institute for Environmental Science, Murdoch University, Murdoch 6150, Western Australia, Australia (First received October 1995; accepted in revisedform September 1996) Abstract--Sewage sludge usually contains significant heavy metals that may limit its land application. Heavy metals in municipal solid waste have been shown to be less mobile by amendment with bauxite refining residue (red mud) prior to the composting process. In the present research a sequential step extraction was employed to investigate metal speciation (into exchangeable, bound to carbonate, to Mn and Fe oxides, to organic matter and in residue phase) and the effect of red mud on metal speciation in compost of sewage sludge for Cr, Cu, Ni, Pb and Zn. The effect of red mud addition and composting process on metal distribution in sewage sludge compost is significant.Red mud addition generally reduces metal leachability and therefore the potential hazard of releasing metals from sludge compost through adsorption and complexation of the metals on to inorganic components to different extents for the different metals. Red mud cannot desorb, however, metals bound to organic matter in the sludge. The composting process breaks down organic matter in the sludge and may release the heavy metals. The addition of red mud prior to the sludge composting binds the released metals on to the red mud for those not strongly readsorbed by the remaining organics (Cu, Ni and Zn). © 199"7 Elsevier Science Ltd Key words--bauxite refining residue (red mud), composting, heavy metals, metal mobility, metal speciation, :~ewagesludge INTRODUCTION ( MSW) compost when the red mud was added at the b eginning of the composting process (Hofstede, I n a previous experiment (Qiao et al., 1993) the effect 1994). o f bauxite refining residue (red mud) on the metal T he addition of a clay material, such as red mud, s peciation in sewage sludge, without composting the t o sewage sludge and composting the mixture is r ed mud sludge mixture, was investigated. The effect e xpected to reduce the mobility of heavy metals along o f red mud is delx,ndent on the metal distribution in s imilar lines to MSW compost (Hofstede and Ho, t he sludge. In general, the exchangeable fraction of 1992; Hofstede, 1994). Differences exist between m etals can be effectively reduced and converted into sewage sludge and MSW because heavy metals in m ore stable forms, by precipitation and adsorption sewage sludge have been in contact with the organic i nto oxides of the red mud, either directly or m atter in the sludge for a longer period of time. With i ndirectly through complexation. The ratio of d igested sludge, anaerobic digestion of the sludge e xchangeable meta.l to total metal content in sludge m eans bacterial processes may have transformed is therefore an important factor in assessing the role r eadily mobile metals into more stable complexes. It o f clay addition in controlling the mobility and plant is desirable therefore to investigate the effect of red a vailability of metals. m ud addition on heavy metal mobility during sewage T he metals in sludge are generally in very stable s ludge composting by determining the metal specia- i nsoluble forms and most of the metals are bound to t ion during the composting process. The results of the t he organic fraction that cannot be desorbed by red s tudy are reported in this paper. The red mud m ud or extracted by DTPA. The latter is usually used a ddition has been shown to improve the composting as a measure of ple,nt available metals. The action of p rocess of digested sludge (Qiao and Ho, 1997). m icroorganisms in composting may make the metals m ore available due to metal release from the M T RA S N M T O S A E I LA D E H D d ecomposed organic matter in the sludge. Red mud Samples of sludge compost a mendment significantly reduced the mobility and Sludge compost samples were taken from the sludge p lant availability of metals in municipal solid waste composting experimental mixtures in which 0, 10 and 20% red mud were added to the raw materials before the *Author to whom correspondence should be addressed composting process (Qiao and Ho, 1997). To obtain [Fax: (61) 9 310 z[997]. successful composting and drying 0.5 kg of sugar was added 951
Liang Qiao and Goen Ho 952 as the carbon source, and also starting with drier mixtures. acid digestion (HNO~-HC104) to estimate leachable, plant Only results obtained with addition of 0.5 kg of sugar are available, and total metal content, respectively (Hofstede, r eported here, since the results obtained with drier mixtures 1994). Red mud neutralised with gypsum was also analysed are largely similar (Qiao, 1997). A sample was taken from t o find out the metal speciation in the mud. Samples and each compost incubator every 10 days with the content of e xtractants were placed in closed centrifuge tubes shaken on the incubator thoroughly mixed before the sample was a C oulter mixer for 12 h, which was enough time to reach taken. Because drying a sample changes metal speciation, s olution equilibrium, and the residue was separated by t he metals in the moist sample were extracted immediately. Sorvall RC-5B ultra centrifuge at 10,000 rpm for 20 min. T he supernatant was passed through a GF/C fibre glass filter and stored in a cool room. The residue was subjected M etal extraction t o the next step extraction. A round 1 g samples (based on dry matter) were emlSloyed Six metals (Cd, Cr, Cu, Ni, Pb, Zn) were chosen for f or the metal extraction. A sequential step extraction was analysis because they represent heavy metals of interest in c arried out employing 1 M MgC12 (exchangeable fraction); sewage sludge. The metals were analysed in duplicate on a 1 M H OAc/NaOAc at pH 5 (carbonate fraction); 0.04 M G BC atomic absorption spectrometer. All reported metal NH2OHHCI at 96°C (reducible or bound to oxides figures in this paper are based on dry weight unless fraction); 30% H202 at pH 2 and 85°C/3.2 M NH4OAc o therwise specified. ( bound to organic fraction) extractions and acid digestion b y concentrated HNO3, HC104 and HC1 (residue fraction) RESULTS AND DISCUSSION (Tessier e t al., 1979). Metals bound to sulphides in this e xtraction scheme would be included in the organic bound I n order to assess for each extraction the f raction. Two batch extractions were also conducted e mploying 0.01 M CaCl2 and 0.1 M DTPA followed by an s ignificance o f the effects o f red m u d addition and 60 650 50 ~ ~--~ 55O 40 450 30 20 ~ 350 10 250 " ! • I " ! I " I " I " ! 10 30 50 0 10 30 50 T ime (days) Time (days) 16 70 60 12 ~= 8 E 40 ~" 4 ~ 30 0 , 20 ! • ! ! • ! o 1'o 3.0 so 0 10 30 50 T i m e (days) T i m e (days) 300 R ed M u d A ddition - 0% A 10% • 2 0% 100 I ! ! I 0 10 30 50 T ime (days) Fig. 1. The total metal content in compost of digested sludge (each point represents an average of three t ypes of measurement; each half bar represents one standard deviation).
M etal speciation of digested sludge 953 Table I. The effect of factoring in the dilution by red mud addition on total metal content in sludge compost and the metal content associated with the silicates Metals (rag kg t),[ At day 0 of composting At 50th day of composting Associated with silicates RM %--, 1,9% 10%* 20% 20%* 10% 10%* 20% 20%* 0% 10% 20% Total Cr 35 38 45 51 37 37 46 51 1.7 2.6 4.8 Total Cu 397 446 314 400 490 543 430 487 0.8 2.5 2.0 Total Ni 7.3 10.6 5.2 10 12 13 14 13 2.2 3.1 3.4 Total Pb 45 43 51 42 47 47 55 46 3 3.4 4.7 Total Zn 196 221 151 198 222 258 203 231 0.8 2 2.1 Note: RM % = percentage of red mud addition; * = calculated from metal in compost and in red mud; metals in RM can be seen in Fig. 5. s ludge composting process, a multiple analysis of t he beginning and the end of the composting v ariance was carried out using SPSS-X program on e xperiment (50 days). The results o f the total metal a V A X computer. The results of statistical analysis c ontent averaged for the three measurements and their s how red mud addition and sludge composting s tandard deviations are shown in Fig. 1. p rocess had statistically significant effect on all T he total Cd concentration in the sludge compost m easured metal concentrations at ct < 0.05. w as below the detection limit (0.1 mg k g - ' ) . F r o m F ig. 1, the total amount of metals had a slight but T otal metal concentration s ignificant increase as the composting process T o ascertain the concentration of total metal in p rogressed as a result of a reduction o f the dry matter c ompost as the reference for the metal distribution in w ith composting amounting to 19, 18 and 16% for t he 0, 10 and 20% red mud addition, respectively t he compost, three different kinds of independent ( Qiao and Ho, 1997). m easurement were carried out. They are the direct m easurement for moist samples, for dried and ground R ed mud addition diluted the metal concentration e xcept for Cr and Pb. The metal content of red mud s amples, and the sum of the metal in sequential is shown in Fig. 5. e xtraction fractions for moist samples. It was a nticipated that the sum of the metal fractions A c omparison with calculated metal concentration w hen the red mud dilution effect was factored in is i n sequential extraction would have the largest s hown in Table 1. The calculated metal concentration a nalytical error among the three measurements due is derived from the metal concentrations of the t o its multiple e~tractions, analyses and the more h eterogeneous (difficult to mix) nature of the moist c omponents. The average recovery rate of total metal f rom sludge compost is 97% for Cr, 87% for Cu, s amples. The dried and ground samples were more h omogeneous and should give more reliable total 8 1% for Ni, 85% for Zn and 111% for Pb. The d eviation of the recovery rate from 100% may be due m etal concentrations. t o experimental error, but it appears that in general T he differences in the total metal values were, h eavy metals in sludge compost become less h owever, small among the three measurements e xtractable after amendment with red mud. In an c ompared to the total metal content. The dried and e xperiment to ascertain whether any could be bound g round samples gave about 96% of the average value b y the silicates, the residue after the last extraction o f the three measurements, the sum of the five fractions w as dissolved in hydrofluoric acid (5% solution). i n the sequential extraction 104% and moist samples S ome additional metals were dissolved and shown in 1 07%. It should bt; noted that the total metal content T able 1 confirming that indeed some metals were d ata obtained from the sum of the metal fractions were a ssociated with silicates. Some solids residue still a vailable for all samples, whereas for dried and ground r emained even after reaction with HF. s amples and for moist samples they were determined at 15 50 S 40 R ed mud addition 10 ~,~ 30 -- 0% E 20 • 1 0% ~,~ 10" o 2 0% T ! • | w 10 20 30 40 50 0 10 20 30 40 50 0 Time (days) Time (days) Fig. 2. The leachable copper and zinc in sludge compost.
Liang Qiao and Goen Ho 954 5O (see below), so it is strongly affected by organic | ~ 4o m atter degradation. This is consistent with the [] Zn R"2=0.83 f inding of Wong (1995) and Hofstede (1994). ~ 3o • D A lthough the leachable Cu increased with the • Cu R^2=0.55 ~ 20 c omposting process, the magnitude of leachable Cu -~ ~0 in sludge compost without red mud amendment after ~,~ m, 50 days of composting was less than 2.3% of its total 0 6 .5 7.5 4 .5 5.5 c ontent indicating the low leachable metal content in pH sludge compost. Garcia et al. (1990) extracted less m etals by CaCl2 during the composting of aerobic Fig. 3. The relationship of leachable metal and pH in sludge digested sludge, but the same conclusion was drawn compost. b y them that the quantities of metals extracted by CaCl2, expressed as a percentage of total metal c ontent, were small. Leachability of heavy metals (extraction with 0.01 M p H of the sludge compost also affected the CaCI2) s olubility of the metal hydroxides and carbonates, T he leachable Cr, Pb, and Ni in sludge compost a nd the lower pH values increased the soluble heavy were below detection limits even without red mud m etals in the sludge compost (Fig. 3). Since the initial a mendment, even though the total amount of metals p H of the sludge mixture with sugar addition was i n the sludge compost is significant (Fig. 1). The u nder 5.2 (Qiao and Ho, 1997), Cu and Zn decrease of leachable Cu and Zn by red mud addition h ydroxides could not form. Cu and Zn hydroxides w as significant, particularly the leachable Zn (Fig. 2). t heoretically form in pure solution at pH above 5.6 T his result agrees with the finding for red mud MSW a nd 7.1, respectively. The addition of red mud and c ompost (Hofstede, 1994). t he composting process increased pH to over 7 in the T he leachable Cu increased during the composting r ed mud sludge compost with sugar addition, so the p rocess following the thermophilic stage and was soluble Zn was precipitated as Zn hydroxide. The likely due to the release of the organically bound Cu. i ncreased pH also enhanced the precipitation of metal R ed mud addition slowed down the increase of c arbonates, thus reducing the exchangeable metal l eachable Cu with the composting process. About c oncentration, which can be seen from the Zn 8 0% of Cu in sludge compost was organically bound s peciation described below. 3 00 8 ~ 2 00 ,! t oo • I " i " I • | • a 10 20 30 40 50 0 10 20 30 40 50 T ime (days) Time (days) 1 2" , R ed mud addition _.= so = 0% 10% a 2 0% 20 0 10 20 30 40 50 0 10 20 30 40 50 T i m e (days) T i m e (days) Fig. 4. The plant available heavy metals in sludge compost.
Metal speciation of digested sludge 955 # 168 20 40 20 33 100 # = T otal metals ( rag&g) 80 [] Exchangeable % 60 [] Carbonates [] Iron oxides bound 40 [] Organic fraction [] Residue 20 Oa N o detectable Cd in red mud. Ni Po Zn Cr M etals Fig. 5. The speciation of metals in red mud. Plant availability of heavy metals (extraction with in red mud needs to be known first (Fig. 5). More t han 60% of the metals are in residue form except Zn D TPA) t hat was distributed more evenly into the five T he plant available Cr in sludge compost was f ractions. This implies that the metals contained in b elow detection limit. The reduction of plant r ed mud were mainly in very stable forms even a vailable Cu, Ni, Pb, and Zn in the sludge compost t hough the Cr content in the red mud is as high as b y red mud addition was significant, especially for Zn 168 mg kg-'. This fact is not surprising since red mud (Fig. 4). h as undergone processing (size reduction, Bayer T he composting process increased the plant p rocess caustic digestion, and countercurrent wash- a vailable Cu and Ni, but the plant available Pb was ing). It has also very little organic matter associated d ramatically decreased. The finding is similar to that w ith it. o f Garcia et al. ,(1990) who extracted more plant A fter mixing with sawdust and recycled compost, a vailable metals by DTPA after composting of t he speciation of heavy metals in the sludge was a erobic digested sludge except for Pb even though the shifted to more available forms (Fig. 6). This may be t otal metal concentration increased due to the c aused by changes due to storage of the sludge and o rganic decomposition and therefore reduction of t o moisture change. The pH of sludge after storage s ludge mass. The variation of DTPA extracted metals d ropped suggesting that some anaerobic decompo- seems to be related to the changes in metal speciation s ition took place. A change in redox condition and a nd will be discussed below. s olid/solution ratio therefore occurred. R ed mud has a high pH, cation exchange capacity, M etal speciation A I and Fe oxides and clay minerals and can effectively adsorb free cations from solution (Hofst- T o determine the effect of red mud on the ede, 1994). The speciation of metals in the mixture s peciation of metals in sludge, the metal speciation "18 499 535 17 13 210 242 48 47 29 10o * - Total metal ( mg/kg) 8o [] Exchangeable e,O [] Carbonates [] Oxides bound o 40 [] Organic fraction 20 I ll Residue 0 r "-- .O t- # ---value calculated Z EL N O f rom the sludge and t he recycled compost M etals Fig. 6. Comparison of the metal speciation calculated from the metal in the mixture's components and the speciation measured in the initial compost mixture.
Liang Qiao and Goen Ho 956 *53 23 29 27 29 29 33 33 31 34 38 37 37 38 100 " - ~ * - Total Cr (mg/kg) ae 60 [] Exchangeable [] Carbonates 8 40 [] Oxidesbound 20 [] Organic fraction [] Residue 0 . . . . . . . . . . . . . # ~ m o o o o =E o o o o =E o o o o o o # recycled compost Time (days) and red mud addition (%) P Fig. 7. The variation of Cr speciation in sludge composting with the red mud addition (*total metal figure is sum of metal fractions in the speciation study). w ould therefore be affected by the addition of red o ther adsorbed metals. Therefore there was no m ud. Since the heavy metals have different properties l eachable and plant available Cr detected in the a nd different concentrations in the red mud and s ludge compost even though it contained 29 mg kg -~ c ompost mixtures, the speciation of metals and the t otal Cr. effect of red mud on the speciation are quite different T he composting process affected the speciation of C r in the sludge compost though the changes were f or e ach metal. Cr. Cr 3÷ has an electron configuration closest to a r elatively small. The carbonates and oxides bound Cr n oble gas with a high spherical symmetry and its were converted into the organic bound fraction p olarisability is the lowest among the six tested d uring the composting process perhaps as a result of m etals. It has a valency of three and therefore it has t he competition of Cr with other metal cations for a s tronger electrostatic affinity for the sorption sites l imited humic organic ligands which were produced t han divalent cations. Consequently it forms the most d uring composting (Fig. 7). The effect of the s table complexes among the six metals and dominates c omposting process on the Cr speciation was similar i n the residue and organic bound fractions (Fig. 7). t o MSW composting (Qiao, 1997). This conversion T he exchangeable Cr in the initial mixture was w ould make Cr more stably fixed in mature sludge a bout 10% of total Cr and the carbonate fraction was c ompost. a lso about 10% total Cr, which means about 20% of F actoring out the red mud dilution effect the total t otal Cr in the mixture may become leachable or C r was reduced from 29 to 13 mg kg -~ for the 20% a vailable with a changed environmental condition red mud addition (Fig. 8). This seems to indicate that s uch as a reduction in pH. Because of the high t he more red mud was added, the less Cr was c ompetitive nature of Cr for adsorption sites r ecovered due most probably to the irreversible ( Table 2) any released Cr will, however, displace a dsorption of Cr on to red mud. This irreversibility *29 21 13 29 21 12 100 * - Total Cr (mg/kg) 80 [ ] Exchangeable 60 L. I~1 Carbonate G) 4O [ ] Oxidesbound Organic fraction 2~ . . . . [ ] Residue o% lO% 20*/0 0% I 0% 20% Initial after 50 days Red Mud Addition F ig. 8. Effect of red mud o n the Cr speciation in sludge c ompost after factoring out the red mud dilution effect.
Metal speciation of digested sludge 957 "1.954 635 535 528 515 515 409438 413 476 352 363 346 406 100 - * - Total Cu (mg/kg) [] Exchangeable [] Carbonates [] Oxides bound O ii [] Organic fraction [] Residue . . . . . . . . . . . . . . . . . . . - _ T ime (days) end red mud addition (%) Fig. 9. The variation of Cu speciation in sludge composting with the red mud addition. m ore than countered what appeared to be the c ompost; the clay plasma and the sandstone c onversion of the reversible part into more available o xyhydroxides sorb little Cu. f orms with the addition of red mud (Fig. 8). T he composting process reduced the organic C u. Cu dominated in the organic bound fraction in b ound Cu and transferred it into carbonate, oxides b oth sludge and slludge compost ( > 80%) due to Cu a nd exchangeable fractions though the amount of Cu f orming very stable complexes with organic ligands t ransformed was small compared to the total Cu in ( Fig. 9). Furthermore Cu ion is directly bound to two t he mixture. The change may be caused by the o r more organic functional groups mainly carboxylic, d ecomposition of organic matter in the sludge c arbonyl and phenolic so that the ion is immobilised c ompost releasing bound Cu. Dudley e t al. (1987) i n a rigid inner-sphere complex (McBride, 1989). c oncluded that Cu was initially associated to the A fter mixing with the sawdust and recycled sludge g reatest degree with the small molecular-size fraction c ompost, part of Lhe Cu was converted from organic o f high amide content in sludge amended soils. As the b ound into carbonates (Fig. 9), though only to a p H increased and soluble amide content moieties m inor extent. d ecreased, Cu remained in solution. Since the pH rose T he result of the Cu speciation is in agreement with s lightly during the composting process (Qiao and Ho, t he literature (Carapanella e t al., 1987; Duquet and 1997), this released Cu was precipitated as Cu V~dy, 1991; also Table 2). Campanella e t al. (1987) c arbonate, oxides bound and complexed with soluble g ave the following order of stability of humic f ulvic acids increasing leachable and plant available c omplexes: Cu >> Zn > Mn in the research of the C u with the composting process. Baham and Sposito m etal speciation in urban sludge. Duquet and V~dy (1994) also found the amount of Cu adsorbed (1991) concluded that in the case of Cu, the organic d ecreased with an increase in the amount of dissolved m atter is the first ,;olicited phase; then comes the Fe o rganic carbon in solution due to Cu forming stable o xides fraction and finally Mn oxides in the sludge c omplexes in solution which had a tendency to 451 434 515 504 508 *535 100 * - T otal Cu (mg/kg) 80 I -I Exchangeable 60 [] Carbonates CJ 40 [] Oxides bound I ll Organiofraction 20 [] Residue o 0% 1 0% 20% 0% 10% 20% I nitial after 50 days R ed M u d Addition Fig. 10. Effec, of red mud on the Cu speciation in sludge compost after factoring out the red mud dilution t effect.
L iang Qiao and Goen Ho 958 Table 2. The sequence of complex stability for the transition metal ions Sequence of Binding materials complex stability Reference Organic matter Cu > Ni = Zn > Cd Irving-Williams order in McBride (1989) Organic substance Pb > Cu > Ni > Zn Scheffer and Schachtschabel (cited in Joregensen and Jensen, 1984) Soil organic matter Cu > Pb > Ni > Zn Schnitzer and Skinner at pH 5 (1966, 1967) Soil organic matter Pb > Cu ,> Cd > Zn Elliott e t al. (1986 cited in Schmitt and Sticher, 1991) Hofstede (1994) Red mud Cr>Pb>Cu>Cd> Ni > Zn Kaolinite and illite Pb > Cu > Zn > Cd Saeki e t el. (1993) Clay material Pb > Ni > Zn Mitchell (cited in Jorgensen and Jensen, 1984) Amorphous AI hydroxide Cu > Pb > Zn > Ni > Cd Kinniburgh e t al. (1976 cited in McBride, 1989) Silanol groups of silica Pb > Cu > Zn > Ni = Cd Schindler e t al. (1976 cited in McBride, 1989) r emain in solution. The dissolved organic carbon was t ransition metal cations according to the Irvine- significantly increased with the composting of sewage Williams order (Table 2). Therefore Ni dominated in t he organic bound and residue fraction in the sludge s ludge (Qiao, 1997). The C u complexed with soluble a nd the compost mixture (Fig. 11). There was a shift f uivic or h u m i c acids would be available to leaching a n d to presumably also plant. i n metal speciation to become more available when F a c t o r i n g out the red m u d dilution effect the t he sludge was mixed with the sawdust a n d recycled c hange o f C u speciation by red m u d addition was not s ludge compost, which m a y be caused by the decline s ignificant, but the effect of composting process was o f p H from 8.3 to 5.2 after the sludge storage. s ignificant (Fig. 10). W i t h composting the organic T he composting process significantly changed the b o u n d C u was converted into carbonates, oxides N i speciation in the sludge compost. Because the total b o u n d a n d exchangeable fractions. The red m u d N i increase was more t h a n the reduction o f dry a ddition inhibited the increase of exchangeable Cu in m a t t e r during the composting, the percentage o f r ed m u d sludge compost t h r o u g h increasing p H to r esidue Ni appeared to decrease even t h o u g h the p recipitate C u a n d increasing the inorganic oxides c o n c e n t r a t i o n o f residue Ni remained the same to the s urface to a d s o r b Cu, which also reduced the metal 5 0th day o f composting. m obility in sewage sludge when red m u d was added F actoring out the dilution effect, the red m u d t o it (Qiao a n d Ho, 1996). a ddition significantly affected the Ni speciation in N i. The Ni cation has the stability of complexes s ludge compost (Fig. 12), particularly the conversion w ith organic ligands just less t h a n Cu 2÷ in the o f exchangeable Ni into organic b o u n d with addition *27 4 13 16 17 20 10 13 14 16 9 13 15 16 100 * - T otal Ni (mg/kg) 80 60 [] Exchangeable # [] Carbonates i 40 [] Oxidesbound ml Organicfraction 20 [] Residue 0 oooo=oogo x=oooo= ~- Time (days) end red mud addition (%) Fig. 11. The variation of Ni speciation in sludge composting with the red mud addition.
Metal speciation of digested sludge 959 10 9.8 20 17 18 "13 100 * - Total Ni (mg/kg) 80 IIIIII [ ] Exchangeable 60 ~ 1 Carbonates 40 [ ] Oxides bound ITS Organic fraction 20 [ ] Residue : :::::::::::::::::::: 0 0% 10% 20% 0% 10% 20% Initial after 50 days T i m e (days) and Red M u d addition ( %) Fig. 12. Effect of red mud on the Ni speciation in sludge compost after factoring out the red mud dilution effect. o f red mud in the initial sludge mixture and into c harged Pb-organic complexes, cation bridge, or h ydrophobic interactions with the clay surface as a oxides bound Ni with composting. The composting r esult of a lowering of the negative charge on the p rocess humified the organic matter in the sludge releasing Ni from the bound sites on the organic d issolved organic carbon in sewage sludge through m atter. The released Ni was likely to form complexes c omplex formation. Simeoni e t al. (1984) also found w ith the oxides surfaces in the red mud preventing the t hat the composting of sludge decreased the Pb N i to complex with the humic substance in the m obility and plant availability. T he changes of Pb speciation in red mud sludge c ompost. c ompost was still significant after factoring out the P b. Lead is also a metal cation, but it has a stronger r ed mud dilution effect (Fig. 14). Red mud converted affinity to the adsorption sites on the clay materials s uch as silanol groups of silica and amorphous Al t he exchangeable, carbonates and oxides bound Pb i nto the residue and organic bound fractions in the h ydroxide (Table 2). Therefore Pb dominated in the sludge compost. The composting process had the residue and organic fractions in the sludge, but it s ame effect on the speciation of Pb. Therefore the w as more evenly distributed in the carbonates, m obility and plant availability of Pb were signifi- o rganic and oxide:~ bound fractions in the compost c antly reduced, because the leachability and plant m ixture. a vailability of metals can be expressed as the T he composting process significantly stabilised the e xchangeable, carbonates and oxides bound metal P b in the mixture. The 50 days composting process species (see below). c onverted all the Pb in the exchangeable and Z n. Zn in the sludge and sludge compost was c arbonates forms into organic bound fraction. e venly distributed in the carbonates, oxides, organic B aham and Spo:sito (1994) suggested that Pb a nd residue fractions as a result of the lowest value f acilitated the removal of dissolved organic carbon in o f the standard electrode potential of Zn 2+ among the sewage sludge through the adsorption of positively 47 45 49 48 45 35 41 42 . 186 76 44 40 44 43 • - Total Pb (mg/kg) [] Exchangeable °JW [] Carbonates el a. [] Oxides bound m Organic fraction 20 [] Residue 0 . . . . . , • • • ° • • • • • ~ oJ T ime (days) and red mud addition (%) Fig. 1:3.The variation of Pb speciation in sludge composting with the red mud addition.
960 Liang Qiao and Goen Ho *47 47 50 45 45 47 100 * T otal Pb (mg/kg) - ii 80 [] Exchangeable 60 [] Carbonates 4O [] Oxides bound [] Organic fraction 20 [] Residue 0• 1 0% 20% 0% 10% 20% 0% after 50 days I nitial R ed Mud Addition Fig. 14. Effect of red mud on the Pb speciation in sludge compost after factoring out the red mud dilution effect. s ix tested metals. Based on the redox potential for the m atter in the mixture was oxidised during the r edox reaction with other metal ions, the Zn 2+ can be c omposting process, Zn was converted from organi- e xpected to stay in ionic form in solution. Since the c ally bound into exchangeable and tended to be more t otal concentration of Zn was high in sludge and most m obile as a result of the increase of the redox s oluble Zn was in free ionic form (Behel e t al., 1983), p otential and decrease of the sulphides (Saeki, 1993). Z n was evenly distributed in carbonate, organic and T he change in redox potential more strongly affected o xides bound fractions in the sludge as result of t he speciation of Zn than that of Cu and Pb. The a dsorption equilibrium relationships. a ddition of red mud buffered the changes of Zn T his result agrees with the finding of Duquet and s peciation during the composting, and the buffer was V 6dy (1991) who studied a sludge compost and soil m ore effective for the more mature compost than the s ystem and with other findings reported in the r aw mixture, l iterature (Table 3). Duquet and V6dy (1991) found F actoring out the dilution effect, red mud addition f or the sludge compost that the affinity of Zn for the c onverted the exchangeable Zn into carbonate F e oxide fraction was high (76%) and increased f raction due to the increase of pH, and suppressed the ( 83%) in the mature compost. It should be noted that c hanges of Zn speciation during the composting t his Fe oxide fraction includes the exchangeable and p rocess. The exchangeable Zn during the composting c arbonate fractions in our research. Like Cu, Zn was w as precipitated as Zn hydroxide and carbonate by n ot greatly fixed on sandstone oxyhydroxydes and r ed mud addition through raising pH from 5 to 7.2. c lay (Duquet and V6dy, 1991), and similarly Zn in the Z n hydroxide dissolves under pH7.2, so the r esidual fraction in this research was less than 5% e xchangeable Zn was effectively controlled by pH in ( Fig. 15). t he sludge compost. T he composting process converted the organic and T able 3 gives the predominant metal species in o xides bound Zn into exchangeable, and red mud s ludges by chemical sequential extraction, and shows a ddition inhibited this conversion. Since the organic t hat there are differences between the reported 242 254 221 277 212 200 204 211 201 234 217 259 " 901 229 100 * - T otal Zn (mg/kg) 80 [] Exchangeable 60 Ze [] Carbonates I- N 40 IN Oxides bound I I1 Organic fraction 20 I~ Residue 0 T imes (days) end red mud addition ( %) Fig. 15. The variation of Zn speciation in sludge composting with the red mud addition.
Metal speciation of digested sludge 961 233 260 277 233 319 *242 1 O0 " ~ ~ ~ ~ ~ * - Total Zn (mg/kg) 80" [] Exchangeable 60- ¢g [] C arbonates 40 DI Oxides bound IT[] Organic fraction 2O [] Residue 0 • . .... • ~-11. 0% 0% 1 0% 20% * 1 0% * 20% * Initial after 50 days Time (days) and Red Mud Addition ( %) Fig. 16. Effect of red mud on the Zn speciation in sludge compost after factoring out the red mud dilution effect. r esults. The differences of the metal speciation are e xtraction (Saeki e t al., 1993). Similarly Pb has a high a ffinity to the residue fraction. Therefore it may be m ainly caused by the different chemical reagents and a ssumed that the results for Cu, Pb and Zn speciation e xtraction scheme~,; used. Although these differences in the sludge mixture was governed by readsorption m ake it difficult to compare results, it can be clearly d uring extractions. s een from Table 3 that Cu dominated in the o rganically bound and sulfide (oxidisable) fraction; D uring the sludge composting process the organic m atter in the sludge is humified causing variation in P b was mainly present in the organically bound and m etal speciation in the sludge. Duquet and V6dy r esidue fractions; and Zn was distributed in the (1991) reported that nearly 58% of Cu and Zn were c arbonates, oxides and organic bound fractions in the l ocated in the coarse fraction (=50-2000/~m) in s ludge, and the results obtained in the present s ewage sludge, and the metals shifted into the fine r esearch are in general agreement with these. f raction ( < 50 #m) as a result of humification of the Since sludge contains a significant amount of o rganic matter and Cu has a high affinity to the c oarse fraction and became more leachable with the o rganic matter, the extracted Cu in the exchangeable c omposting. Canarutto e t al. (1991) also reported t hat a correctly carried out composting process c arbonates or oxid,~s fraction is likely readsorbed on i ncreased the humic acids with respect to the fulvic t he organic fraction during the procedure of 15 " == lO 0 % RM (R^2=0.82) 1 0% RM (R^2=0.18) 2 0% RM (R^2=0.01) • O o 0 10 20 E xchangeable ¢ 5O B 40 U = 0%RM (PP2=0.a4) ~:= 30 m • 1 0%RM(FIA2=0.29) 2o o 2 0%RM (RA2=0.84) 0 20 40 60 80 100 120 E xchangeable Fig. 17. Comparison of leachable and exchangeable fractions.
962 Liang Qiao and Goen Ho 3oo 2 00 O%RM (R~-0.92) 1 0%RM (RA=-0.83) r~ 1°° 2 0%RM (R~-0.70) o 0 20 40 60 80 100 S um of Exchangeable and Carbonate 0 % RM (R^2=0.85) 10% RM (RA2=0.74) .= 2 0% RM (RA2=0.61) 0 1 2 3 S um of Exchangeable and Carbonates 1 2- =. [] 0 % RM (R^2=0.98) .= 10% RM (R^2=0,99) _e~ 4' 2 0% RM (R^2=0,98) O =h 0 • • • , • • • • • • • • 5 10 15 0 S um of Exchangeable and Carbonate 120 0 %RM (R^2=0.91) ° ! 10%RM (FIA2=0.66) ,o 2 0%RM (RA2=0.61) o 0 1 00 200 300 S um of Exchangeable and Carbonates Fig. 18. Comparison of metal extracted by DTPA and the sum of exchangeable + carbonate fractions. acids in municipal solid waste. These variations in the m etals, however, with Cr, Ni and Pb hardly extracted q uantity and quality of humic substances also b y 0.01 M CaCI2, and Cu was more easily extracted i nfluenced the speciation of heavy metals. t han Zn from compost unamended by red mud ( Fig. 17). Relationship of metal speciation and leachable metal Relationship of metal speciation and plant available Since a 1 M MgC12 solution is a stronger extractant metal t han a 0.01 M CaC12 solution for metals from cation sites, more metals can be extracted by the former, so A c omparison between the speciation of metals in t he leachable metals is included in the exchangeable t he sludge compost and the metals extracted by m etal fraction (Fig. 17). Differences exist between D TPA is shown in Fig. ! 8. It can be seen that the
Metal speciation of digested sludge 963 Table3. Predominantmetalspeciesidentifiedin sludgesby meansof sequentialchemical extraction Predominantmetal species Present Stover Emmerich Legret Oake Lake Metal data et al.* et al.* et al.* et al.* et al.* Cd -- CO~ CO3 Oxid COs Org Cu Org S Org Org S COs Ni Org/Res CO3 CO3 Org COs Sol/Res Pb Org/Res COs -- Res Org Org Zn Org/Oxid/CO3 Org CO3 Oxid Org Org Note: COt--carbonate;Org--organicallybound;Oxid--oxidesphases; Res--residual; S--sulphide; Sol--soluble/exchangeable; *---citedin Lester (1987). D TPA could generally extract the heavy metals in the w ith similar properties to red mud could be used as e xchangeable and carbonate bound fractions in the a mendment for composting digested sludge. sludge, except for Cr which was below the detectable l imit in the DTPA extraction. In the case of Ni and REFERENCES C u the agreement would be marginally better when APHA (1985) Standard methods for the examination water t he oxides bound fraction is included. Petruzzelli and waste waster, 1 6th edn. American Public Health (1989) stated that the metals present in these fractions Association, Washington, DC. Baham J. and Sposito G. (1994) Adsorption of dissolved a re considered to be the most available forms to organic carbon extracted from sewage sludge on p lant. montmorillonite and kaolinite in the presence of metal ions. J . Environ. Qual. 23, 147-153. Behel D., Nelson D. W. and Sommers L. E. (1983) CONCLUSIONS Assessment of heavy metal equilibria in sewage sludge-treated soil. J. Environ. Qual. 12, 181-186. (i) Red mud affects the speciation of heavy metals Campanella L., Cardarelli E., Ferri T., Petronio B. M. and Pupelia A. (1987) Evaluation of heavy metals speciation t hrough increasing the pH, solid-to-solution ratio, in an urban sludge I. Batch method. Sci. 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