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The effects of clay a m e n d m e n t and composting on metal speciation in digested sludge liang qiao
~) Pergamon PlI: S0043-1354(96)00290-4 Wat Res Vol 31, No 5, pp 951-964, 1997 © 1997ElsevierScienceLtd All rights reserved Printed in Great Britain 0043-1354/97$17.00+ 0.00 THE EFFECTS OF CLAY A M E N D M E N T AND COMPOSTING ON METAL SPECIATION IN DIGESTED SLUDGE LIANG 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 beginning of the composting process (Hofstede, 1994) The addition of a clay material, such as red mud, to sewage sludge and composting the mixture is expected to reduce the mobility of heavy metals along similar lines to MSW compost (Hofstede and Ho, 1992; Hofstede, 1994) Differences exist between sewage sludge and MSW because heavy metals in sewage sludge have been in contact with the organic matter in the sludge for a longer period of time With digested sludge, anaerobic digestion of the sludge means bacterial processes may have transformed readily mobile metals into more stable complexes It is desirable therefore to investigate the effect of red mud addition on heavy metal mobility during sewage sludge composting by determining the metal speciation during the composting process The results of the study are reported in this paper The red mud addition has been shown to improve the composting process of digested sludge (Qiao and Ho, 1997) In a previous experiment (Qiao et al., 1993) the effect of bauxite refining residue (red mud) on the metal speciation in sewage sludge, without composting the red mud sludge mixture, was investigated The effect of red mud is delx,ndent on the metal distribution in the sludge In general, the exchangeable fraction of metals can be effectively reduced and converted into more stable forms, by precipitation and adsorption into oxides of the red mud, either directly or indirectly through complexation The ratio of exchangeable meta.l to total metal content in sludge is therefore an important factor in assessing the role of clay addition in controlling the mobility and plant availability of metals The metals in sludge are generally in very stable insoluble forms and most of the metals are bound to the organic fraction that cannot be desorbed by red mud or extracted by DTPA The latter is usually used as a measure of ple,nt available metals The action of microorganisms in composting may make the metals more available due to metal release from the decomposed organic matter in the sludge Red mud amendment significantly reduced the mobility and plant availability of metals in municipal solid waste M T RA S N M T O S A E I LA D E H D *Author to whom correspondence should be addressed [Fax: (61) 310 z[997] Samples of sludge compost Sludge compost samples were taken from the sludge composting experimental mixtures in which 0, 10 and 20% red mud were added to the raw materials before the composting process (Qiao and Ho, 1997) To obtain 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 Only results obtained with addition of 0.5 kg of sugar are reported here, since the results obtained with drier mixtures are largely similar (Qiao, 1997) A sample was taken from each compost incubator every 10 days with the content of the incubator thoroughly mixed before the sample was taken Because drying a sample changes metal speciation, the metals in the moist sample were extracted immediately acid digestion (HNO~-HC104) to estimate leachable, plant available, and total metal content, respectively (Hofstede, 1994) Red mud neutralised with gypsum was also analysed to find out the metal speciation in the mud Samples and extractants were placed in closed centrifuge tubes shaken on a Coulter mixer for 12 h, which was enough time to reach solution equilibrium, and the residue was separated by Sorvall RC-5B ultra centrifuge at 10,000 rpm for 20 The supernatant was passed through a GF/C fibre glass filter and stored in a cool room The residue was subjected to the next step extraction Six metals (Cd, Cr, Cu, Ni, Pb, Zn) were chosen for analysis because they represent heavy metals of interest in sewage sludge The metals were analysed in duplicate on a GBC atomic absorption spectrometer All reported metal figures in this paper are based on dry weight unless otherwise specified Metal extraction Around g samples (based on dry matter) were emlSloyed for the metal extraction A sequential step extraction was carried out employing M MgC12 (exchangeable fraction); M HOAc/NaOAc at pH (carbonate fraction); 0.04 M NH2OHHCI at 96°C (reducible or bound to oxides fraction); 30% H202 at pH and 85°C/3.2 M NH4OAc (bound to organic fraction) extractions and acid digestion by concentrated HNO3, HC104 and HC1 (residue fraction) (Tessier et al., 1979) Metals bound to sulphides in this extraction scheme would be included in the organic bound fraction Two batch extractions were also conducted employing 0.01 M CaCl2 and 0.1 M DTPA followed by an RESULTS AND DISCUSSION In order to assess for each extraction the significance o f the effects o f red m u d addition and 60 650 50 ~ ~ ~ 55O 40 30 20 450 ~ 350 10 " ! • 10 I 30 " 250 ! 50 I Time (days) " I " 10 I 30 " ! 50 Time (days) 16 70 60 12 ~= E 40 ~" ~ 30 ! ! , 10 30 • 20 ! o 50 T i m e (days) • 1'o 3.0 ! so T i m e (days) 300 Red M u d Addition - A • 100 I ! ! 10 30 10% 20% I 0% 50 Time (days) Fig The total metal content in compost of digested sludge (each point represents an average of three types of measurement; each half bar represents one standard deviation) Metal 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 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 3.4 4.7 Total Zn 196 221 151 198 222 258 203 231 0.8 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 sludge composting process, a multiple analysis of variance was carried out using SPSS-X program on a V A X computer The results of statistical analysis show red mud addition and sludge composting process had statistically significant effect on all measured metal concentrations at ct < 0.05 the beginning and the end of the composting experiment (50 days) The results o f the total metal content averaged for the three measurements and their standard deviations are shown in Fig The total Cd concentration in the sludge compost was below the detection limit (0.1 mg k g - ' ) F r o m Fig 1, the total amount of metals had a slight but significant increase as the composting process progressed as a result of a reduction o f the dry matter with composting amounting to 19, 18 and 16% for the 0, 10 and 20% red mud addition, respectively (Qiao and Ho, 1997) Red mud addition diluted the metal concentration except for Cr and Pb The metal content of red mud is shown in Fig A comparison with calculated metal concentration when the red mud dilution effect was factored in is shown in Table The calculated metal concentration is derived from the metal concentrations of the components The average recovery rate of total metal from sludge compost is 97% for Cr, 87% for Cu, 81% for Ni, 85% for Zn and 111% for Pb The deviation of the recovery rate from 100% may be due to experimental error, but it appears that in general heavy metals in sludge compost become less extractable after amendment with red mud In an experiment to ascertain whether any could be bound by the silicates, the residue after the last extraction was dissolved in hydrofluoric acid (5% solution) Some additional metals were dissolved and shown in Table confirming that indeed some metals were associated with silicates Some solids residue still remained even after reaction with HF Total metal concentration To ascertain the concentration of total metal in compost as the reference for the metal distribution in the compost, three different kinds of independent measurement were carried out They are the direct measurement for moist samples, for dried and ground samples, and the sum of the metal in sequential extraction fractions for moist samples It was anticipated that the sum of the metal fractions in sequential extraction would have the largest analytical error among the three measurements due to its multiple e~tractions, analyses and the more heterogeneous (difficult to mix) nature of the moist samples The dried and ground samples were more homogeneous and should give more reliable total metal concentrations The differences in the total metal values were, however, small among the three measurements compared to the total metal content The dried and ground samples gave about 96% of the average value of the three measurements, the sum of the five fractions in the sequential extraction 104% and moist samples 107% It should bt; noted that the total metal content data obtained from the sum of the metal fractions were available for all samples, whereas for dried and ground samples and for moist samples they were determined at 15 50 S 40 10 ~,~ E ~,~ 30 20 Red mud addition -• o 10" T 10 20 Time 30 40 (days) 50 ! • | 10 20 30 40 Time w 50 (days) Fig The leachable copper and zinc in sludge compost 0% 10% 20% Liang Qiao and Goen Ho 954 5O ~ | 4o ~ D • [] Zn R"2=0.83 • Cu R^2=0.55 3o ~ 20 -~ ~0 ~,~ m, 4.5 6.5 5.5 7.5 pH Fig The relationship of leachable metal and pH in sludge compost Leachability of heavy metals (extraction with 0.01 M CaCI2) The leachable Cr, Pb, and Ni in sludge compost were below detection limits even without red mud amendment, even though the total amount of metals in the sludge compost is significant (Fig 1) The decrease of leachable Cu and Zn by red mud addition was significant, particularly the leachable Zn (Fig 2) This result agrees with the finding for red mud MSW compost (Hofstede, 1994) The leachable Cu increased during the composting process following the thermophilic stage and was likely due to the release of the organically bound Cu Red mud addition slowed down the increase of leachable Cu with the composting process About 80% of Cu in sludge compost was organically bound (see below), so it is strongly affected by organic matter degradation This is consistent with the finding of Wong (1995) and Hofstede (1994) Although the leachable Cu increased with the composting process, the magnitude of leachable Cu in sludge compost without red mud amendment after 50 days of composting was less than 2.3% of its total content indicating the low leachable metal content in sludge compost Garcia et al (1990) extracted less metals by CaCl2 during the composting of aerobic digested sludge, but the same conclusion was drawn by them that the quantities of metals extracted by CaCl2, expressed as a percentage of total metal content, were small pH of the sludge compost also affected the solubility of the metal hydroxides and carbonates, and the lower pH values increased the soluble heavy metals in the sludge compost (Fig 3) Since the initial pH of the sludge mixture with sugar addition was under 5.2 (Qiao and Ho, 1997), Cu and Zn hydroxides could not form Cu and Zn hydroxides theoretically form in pure solution at pH above 5.6 and 7.1, respectively The addition of red mud and the composting process increased pH to over in the red mud sludge compost with sugar addition, so the soluble Zn was precipitated as Zn hydroxide The increased pH also enhanced the precipitation of metal carbonates, thus reducing the exchangeable metal concentration, which can be seen from the Zn speciation described below 300 ~ 200 ,! too • I " i 10 20 " I • | 30 40 • a 50 10 Time (days) 20 30 40 50 Time (days) 12" , Red mud addition _.= so = a 20 10 20 30 T i m e (days) 40 50 10 20 30 40 50 T i m e (days) Fig The plant available heavy metals in sludge compost 0% 10% 20% Metal speciation of digested sludge 100 #168 33 20 40 955 20 # = Total metals (rag&g) 80 % [] [] [] [] [] 60 40 20 Oa Cr Ni Po Exchangeable Carbonates Iron oxides bound Organic fraction Residue No detectable Cd in red mud Zn Metals Fig The speciation of metals in red mud Plant availability of heavy metals (extraction with DTPA) The plant available Cr in sludge compost was below detection limit The reduction of plant available Cu, Ni, Pb, and Zn in the sludge compost by red mud addition was significant, especially for Zn (Fig 4) The composting process increased the plant available Cu and Ni, but the plant available Pb was dramatically decreased The finding is similar to that of Garcia et al ,(1990) who extracted more plant available metals by DTPA after composting of aerobic digested sludge except for Pb even though the total metal concentration increased due to the organic decomposition and therefore reduction of sludge mass The variation of DTPA extracted metals seems to be related to the changes in metal speciation and will be discussed below Metal speciation To determine the effect of red mud on the speciation of metals in sludge, the metal speciation 10o "18 29 499 535 17 13 in red mud needs to be known first (Fig 5) More than 60% of the metals are in residue form except Zn that was distributed more evenly into the five fractions This implies that the metals contained in red mud were mainly in very stable forms even though the Cr content in the red mud is as high as 168 mg kg-' This fact is not surprising since red mud has undergone processing (size reduction, Bayer process caustic digestion, and countercurrent washing) It has also very little organic matter associated with it After mixing with sawdust and recycled compost, the speciation of heavy metals in the sludge was shifted to more available forms (Fig 6) This may be caused by changes due to storage of the sludge and to moisture change The pH of sludge after storage dropped suggesting that some anaerobic decomposition took place A change in redox condition and solid/solution ratio therefore occurred Red mud has a high pH, cation exchange capacity, AI and Fe oxides and clay minerals and can effectively adsorb free cations from solution (Hofstede, 1994) The speciation of metals in the mixture 48 47 210 242 * - Total metal (mg/kg) 8o [] Exchangeable 40 [] [] Carbonates Oxides bound 20 [] Ill Organic fraction Residue e,O o r "-O O t- Z EL N Metals # -value calculated from the sludge and the recycled compost Fig 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 [] [] [] [] [] 40 20 m # ~ o o o o =E o o o o o =E o Exchangeable Carbonates Oxidesbound Organic fraction Residue o o o o # recycled compost Time (days) and red mud addition (%) P Fig 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) would therefore be affected by the addition of red mud Since the heavy metals have different properties and different concentrations in the red mud and compost mixtures, the speciation of metals and the effect of red mud on the speciation are quite different for each metal Cr Cr 3÷ has an electron configuration closest to a noble gas with a high spherical symmetry and its polarisability is the lowest among the six tested metals It has a valency of three and therefore it has a stronger electrostatic affinity for the sorption sites than divalent cations Consequently it forms the most stable complexes among the six metals and dominates in the residue and organic bound fractions (Fig 7) The exchangeable Cr in the initial mixture was about 10% of total Cr and the carbonate fraction was also about 10% total Cr, which means about 20% of total Cr in the mixture may become leachable or available with a changed environmental condition such as a reduction in pH Because of the high competitive nature of Cr for adsorption sites (Table 2) any released Cr will, however, displace 100 *29 21 13 29 other adsorbed metals Therefore there was no leachable and plant available Cr detected in the sludge compost even though it contained 29 mg kg -~ total Cr The composting process affected the speciation of Cr in the sludge compost though the changes were relatively small The carbonates and oxides bound Cr were converted into the organic bound fraction during the composting process perhaps as a result of the competition of Cr with other metal cations for limited humic organic ligands which were produced during composting (Fig 7) The effect of the composting process on the Cr speciation was similar to MSW composting (Qiao, 1997) This conversion would make Cr more stably fixed in mature sludge compost Factoring out the red mud dilution effect the total Cr was reduced from 29 to 13 mg kg -~ for the 20% red mud addition (Fig 8) This seems to indicate that the more red mud was added, the less Cr was recovered due most probably to the irreversible adsorption of Cr on to red mud This irreversibility 21 12 * - Total Cr (mg/kg) 80 L G) [ ] Exchangeable I~1 Carbonate [ ] Oxidesbound Organic fraction [ ] Residue 60 4O 2~ o% lO% Initial 20*/0 0% I0% 20% after 50 days Red Mud Addition Fig Effect of red mud on the Cr speciation in sludge compost 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) [] [] [] [] [] O ii - Exchangeable Carbonates Oxides bound Organic fraction Residue _ Time (days) end red mud addition (%) Fig The variation of Cu speciation in sludge composting with the red mud addition more than countered what appeared to be the conversion of the reversible part into more available forms with the addition of red mud (Fig 8) Cu Cu dominated in the organic bound fraction in both sludge and slludge compost ( > 80%) due to Cu forming very stable complexes with organic ligands (Fig 9) Furthermore Cu ion is directly bound to two or more organic functional groups mainly carboxylic, carbonyl and phenolic so that the ion is immobilised in a rigid inner-sphere complex (McBride, 1989) After mixing with the sawdust and recycled sludge compost, part of Lhe Cu was converted from organic bound into carbonates (Fig 9), though only to a minor extent The result of the Cu speciation is in agreement with the literature (Carapanella et al., 1987; Duquet and V~dy, 1991; also Table 2) Campanella et al (1987) gave the following order of stability of humic complexes: Cu >> Zn > Mn in the research of the metal speciation in urban sludge Duquet and V~dy (1991) concluded that in the case of Cu, the organic matter is the first ,;olicited phase; then comes the Fe oxides fraction and finally Mn oxides in the sludge 100 *535 451 434 515 compost; the clay plasma and the sandstone oxyhydroxides sorb little Cu The composting process reduced the organic bound Cu and transferred it into carbonate, oxides and exchangeable fractions though the amount of Cu transformed was small compared to the total Cu in the mixture The change may be caused by the decomposition of organic matter in the sludge compost releasing bound Cu Dudley et al (1987) concluded that Cu was initially associated to the greatest degree with the small molecular-size fraction of high amide content in sludge amended soils As the pH increased and soluble amide content moieties decreased, Cu remained in solution Since the pH rose slightly during the composting process (Qiao and Ho, 1997), this released Cu was precipitated as Cu carbonate, oxides bound and complexed with soluble fulvic acids increasing leachable and plant available Cu with the composting process Baham and Sposito (1994) also found the amount of Cu adsorbed decreased with an increase in the amount of dissolved organic carbon in solution due to Cu forming stable complexes in solution which had a tendency to 504 508 * - Total Cu (mg/kg) 80 60 40 [] [] 20 CJ I-I Exchangeable Ill [] Carbonates Oxides bound Organiofraction Residue o 0% 10% Initial 20% 0% 10% 20% after 50 days Red 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 Liang Qiao and Goen Ho 958 Table The sequence of complex stability for the transition metal ions Sequence of Binding materials complex stability Reference Organic matter Cu > Ni = Zn > Cd Organic substance Pb > Cu > Ni > Zn Soil organic matter Cu > Pb > Ni > Zn Soil organic matter Pb > Cu ,> Cd > Zn Red mud Cr>Pb>Cu>Cd> Ni > Zn Pb > Cu > Zn > Cd Pb > Ni > Zn Kaolinite and illite Clay material Irving-Williams order in McBride (1989) Scheffer and Schachtschabel (cited in Joregensen and Jensen, 1984) Schnitzer and Skinner at pH (1966, 1967) Elliott et al (1986 cited in Schmitt and Sticher, 1991) Hofstede (1994) Saeki et el (1993) Mitchell (cited in Jorgensen and Jensen, 1984) Amorphous AI hydroxide Cu > Pb > Zn > Ni > Cd Kinniburgh et al (1976 cited in McBride, 1989) Silanol groups of silica Pb > Cu > Zn > Ni = Cd Schindler et al (1976 cited in McBride, 1989) remain in solution The dissolved organic carbon was significantly increased with the composting of sewage sludge (Qiao, 1997) The C u complexed with soluble fuivic or h u m i c acids would be available to leaching a n d to presumably also plant F a c t o r i n g out the red m u d dilution effect the change o f C u speciation by red m u d addition was not significant, but the effect of composting process was significant (Fig 10) W i t h composting the organic b o u n d C u was converted into carbonates, oxides b o u n d a n d exchangeable fractions The red m u d addition inhibited the increase of exchangeable Cu in red m u d sludge compost t h r o u g h increasing p H to precipitate C u a n d increasing the inorganic oxides surface to a d s o r b Cu, which also reduced the metal mobility in sewage sludge when red m u d was added to it (Qiao a n d Ho, 1996) Ni The Ni cation has the stability of complexes with organic ligands just less t h a n Cu 2÷ in the 100 *27 13 16 17 20 transition metal cations according to the IrvineWilliams order (Table 2) Therefore Ni dominated in the organic bound and residue fraction in the sludge and the compost mixture (Fig 11) There was a shift in metal speciation to become more available when the sludge was mixed with the sawdust a n d recycled sludge compost, which m a y be caused by the decline o f p H from 8.3 to 5.2 after the sludge storage The composting process significantly changed the Ni speciation in the sludge compost Because the total Ni increase was more t h a n the reduction o f dry m a t t e r during the composting, the percentage o f residue Ni appeared to decrease even t h o u g h the c o n c e n t r a t i o n o f residue Ni remained the same to the 50th day o f composting Factoring out the dilution effect, the red m u d addition significantly affected the Ni speciation in sludge compost (Fig 12), particularly the conversion o f exchangeable Ni into organic b o u n d with addition 10 13 14 16 13 15 16 * - Total Ni (mg/kg) 80 # 60 i 40 [] [] [] ml [] 20 x = o o o o = Exchangeable Carbonates Oxidesbound Organicfraction Residue oooo=oogo ~- 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 100 10 "13 9.8 20 17 959 18 * - Total Ni (mg/kg) 80 IIIIII [ ] Exchangeable ~1 Carbonates [ ] Oxides bound 60 40 20 ITS Organic fraction [ ] Residue ::::::::::::::::::::: 0% 10% Initial 20% 0% 10% 20% 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 of red mud in the initial sludge mixture and into oxides bound Ni with composting The composting process humified the organic matter in the sludge releasing Ni from the bound sites on the organic matter The released Ni was likely to form complexes with the oxides surfaces in the red mud preventing the Ni to complex with the humic substance in the compost Pb Lead is also a metal cation, but it has a stronger affinity to the adsorption sites on the clay materials such as silanol groups of silica and amorphous Al hydroxide (Table 2) Therefore Pb dominated in the residue and organic fractions in the sludge, but it was more evenly distributed in the carbonates, organic and oxide:~ bound fractions in the compost mixture The composting process significantly stabilised the Pb in the mixture The 50 days composting process converted all the Pb in the exchangeable and carbonates forms into organic bound fraction Baham and Spo:sito (1994) suggested that Pb facilitated the removal of dissolved organic carbon in sewage sludge through the adsorption of positively 47 45 49 48 186 76 charged Pb-organic complexes, cation bridge, or hydrophobic interactions with the clay surface as a result of a lowering of the negative charge on the dissolved organic carbon in sewage sludge through complex formation Simeoni et al (1984) also found that the composting of sludge decreased the Pb mobility and plant availability The changes of Pb speciation in red mud sludge compost was still significant after factoring out the red mud dilution effect (Fig 14) Red mud converted the exchangeable, carbonates and oxides bound Pb into the residue and organic bound fractions in the sludge compost The composting process had the same effect on the speciation of Pb Therefore the mobility and plant availability of Pb were significantly reduced, because the leachability and plant availability of metals can be expressed as the exchangeable, carbonates and oxides bound metal species (see below) Zn Zn in the sludge and sludge compost was evenly distributed in the carbonates, oxides, organic and residue fractions as a result of the lowest value of the standard electrode potential of Zn 2+ among the 45 35 41 42 44 40 44 43 • el a [] , • • • ° • • • • Carbonates Oxides bound m [] 20 Exchangeable [] [] °JW - Total Pb (mg/kg) Organic fraction Residue • ~ oJ Time (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 * Total Pb (mg/kg) - 80 60 4O 20 • 0% 10% 20% ii 0% [] [] [] [] [] 10% Exchangeable Carbonates Oxides bound Organic fraction Residue 20% after 50 days Initial Red Mud Addition Fig 14 Effect of red mud on the Pb speciation in sludge compost after factoring out the red mud dilution effect six tested metals Based on the redox potential for the redox reaction with other metal ions, the Zn 2+ can be expected to stay in ionic form in solution Since the total concentration of Zn was high in sludge and most soluble Zn was in free ionic form (Behel et al., 1983), Zn was evenly distributed in carbonate, organic and oxides bound fractions in the sludge as result of adsorption equilibrium relationships This result agrees with the finding of Duquet and V6dy (1991) who studied a sludge compost and soil system and with other findings reported in the literature (Table 3) Duquet and V6dy (1991) found for the sludge compost that the affinity of Zn for the Fe oxide fraction was high (76%) and increased (83%) in the mature compost It should be noted that this Fe oxide fraction includes the exchangeable and carbonate fractions in our research Like Cu, Zn was not greatly fixed on sandstone oxyhydroxydes and clay (Duquet and V6dy, 1991), and similarly Zn in the residual fraction in this research was less than 5% (Fig 15) The composting process converted the organic and oxides bound Zn into exchangeable, and red mud addition inhibited this conversion Since the organic "901 229 100 matter in the mixture was oxidised during the composting process, Zn was converted from organically bound into exchangeable and tended to be more mobile as a result of the increase of the redox potential and decrease of the sulphides (Saeki, 1993) The change in redox potential more strongly affected the speciation of Zn than that of Cu and Pb The addition of red mud buffered the changes of Zn speciation during the composting, and the buffer was more effective for the more mature compost than the raw mixture, Factoring out the dilution effect, red mud addition converted the exchangeable Zn into carbonate fraction due to the increase of pH, and suppressed the changes of Zn speciation during the composting process The exchangeable Zn during the composting was precipitated as Zn hydroxide and carbonate by red mud addition through raising pH from to 7.2 Zn hydroxide dissolves under pH7.2, so the exchangeable Zn was effectively controlled by pH in the sludge compost Table gives the predominant metal species in sludges by chemical sequential extraction, and shows that there are differences between the reported 242 254 221 277 212 200 204 211 201 234 217 259 * - Total Zn (mg/kg) 80 Ze [] [] IN II1 I~ 60 I- N 40 20 Exchangeable Carbonates Oxides bound Organic fraction Residue Times (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 O0 " 233 *242 ~ ~ 260 ~ 277 ~ 233 961 319 ~ * - Total Zn (mg/kg) 80" [] 40 Exchangeable [] 60- ¢g Carbonates Oxides bound DI IT[] Organic fraction 2O [] • 0% • 10% Initial Residue ~-11 20% 0% * 10% * 20% * 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 results The differences of the metal speciation are mainly caused by the different chemical reagents and extraction scheme~,; used Although these differences make it difficult to compare results, it can be clearly seen from Table that Cu dominated in the organically bound and sulfide (oxidisable) fraction; Pb was mainly present in the organically bound and residue fractions; and Zn was distributed in the carbonates, oxides and organic bound fractions in the sludge, and the results obtained in the present research are in general agreement with these Since sludge contains a significant amount of organic matter and Cu has a high affinity to the organic matter, the extracted Cu in the exchangeable carbonates or oxid,~s fraction is likely readsorbed on the organic fraction during the procedure of extraction (Saeki et al., 1993) Similarly Pb has a high affinity to the residue fraction Therefore it may be assumed that the results for Cu, Pb and Zn speciation in the sludge mixture was governed by readsorption during extractions During the sludge composting process the organic matter in the sludge is humified causing variation in metal speciation in the sludge Duquet and V6dy (1991) reported that nearly 58% of Cu and Zn were located in the coarse fraction (=50-2000/~m) in sewage sludge, and the metals shifted into the fine fraction ( < 50 #m) as a result of humification of the coarse fraction and became more leachable with the composting Canarutto et al (1991) also reported that a correctly carried out composting process increased the humic acids with respect to the fulvic 15 " lO == 0% RM (R^2=0.82) 10% RM (R^2=0.18) • 20% RM (R^2=0.01) O o 10 20 Exchangeable ¢ 5O B 40 U ~:= = 2o 20 40 60 80 100 10%RM(FIA2=0.29) o m 0%RM (PP2=0.a4) • 30 20%RM (RA2=0.84) 120 Exchangeable Fig 17 Comparison of leachable and exchangeable fractions 962 Liang Qiao and Goen Ho 3oo 200 r~ O%RM (R~-0.92) 10%RM (RA=-0.83) 20%RM (R~-0.70) 1°° o 20 40 60 80 100 Sum of Exchangeable and Carbonate 0% RM (R^2=0.85) 10% RM (RA2=0.74) = 20% RM (RA2=0.61) Sum of Exchangeable and Carbonates 12= [] 0% RM (R^2=0.98) 10% RM (R^2=0,99) O 20% RM (R^2=0,98) = _e~ 4' =h • • • , • • • • • 10 • • • 15 Sum of Exchangeable and Carbonate 120 ° ! ,o 0%RM (R^2=0.91) 10%RM (FIA2=0.66) 20%RM (RA2=0.61) o 100 200 300 Sum 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 quantity and quality of humic substances also influenced the speciation of heavy metals Relationship of metal speciation and leachable metal Since a M MgC12 solution is a stronger extractant than a 0.01 M CaC12 solution for metals from cation sites, more metals can be extracted by the former, so the leachable metals is included in the exchangeable metal fraction (Fig 17) Differences exist between metals, however, with Cr, Ni and Pb hardly extracted by 0.01 M CaCI2, and Cu was more easily extracted than Zn from compost unamended by red mud (Fig 17) Relationship of metal speciation and plant available metal A comparison between the speciation of metals in the sludge compost and the metals extracted by DTPA is shown in Fig ! 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) DTPA could generally extract the heavy metals in the exchangeable and carbonate bound fractions in the sludge, except for Cr which was below the detectable limit in the DTPA extraction In the case of Ni and Cu the agreement would be marginally better when the oxides bound fraction is included Petruzzelli (1989) stated that the metals present in these fractions are considered to be the most available forms to plant CONCLUSIONS (i) Red mud affects the speciation of heavy metals through increasing the pH, solid-to-solution ratio, and available adsorption sites In general sludge composting with red mud addition reduces the leachability, plant availability and total metal content The effect of red mud is different for each metal with a greater effect on Cr, Pb and Zn speciation than on Cu and Ni speciation More than 80% of Cu is tightly bound to the organic fraction, and red mud addition hardly affects the Cu speciation (ii) Although the red mud increases total Cr in sludge compost, the leachable and plant available Cr are undetected in the red mud compost The Cr remains in tightly bound fractions, and thus is unlikely to be released into the environment (iii) Leachable metal as measured by 0.01 M CaC12 extraction and plant available metal as measured by 0.1 M DTPA extraction are related to the metal in exchangeable and in exchangeable/carbonates fractions, respectively, except for Cr which cannot be extracted by DTPA and CaCI2 (iv) Composting process affects the metal speciation, with Pb in particular gradually becoming insoluble in sludge compost even without red mud addition Cu, Ni anti Zn become more available with composting, but the extent of availability is reduced by red mud addition The results obtained in this paper suggest that the addition of red mud to digested sludge prior to composting should reduce metal mobility In the longer term the red mud should be able to retain heavy metals released by the decomposition of organic materials in the compost This situation is similar to compost applied to clayey soils; 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