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Observations on the dispersion and aggregation of clays by humic substances, I. Dispersive effects of humic acids
Geoderma, 42 (1988) 331-337 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
331
Observations on the Dispersion and Aggregation of Clays by Humic Substances, I. Dispersive Effects of Humic Acids S.A. VISSER and M. CAILLIER
D~partement des Sols, Facult~ des Sciences de l'Agriculture et de l'Alimentation, Universit~ Laval, Qudbec, Qud. G1K 7P4 (Canada) (Received April 15, 1987; accepted after revision February 26, 1988)
ABSTRACT Visser, S.A. and Caillier, M., 1988. Observations on the dispersion and aggregation of clays by humic substances, I. Dispersive effects of humic acids. Geoderma, 42: 331-337. Although humic substances are normally considered as aggregating agents in soil formation processes, we have shown that within a certain concentration range humic substances are also able to disperse soil particles. The dispersive effect was particularly evident at humic acid concentrations of around 40 mg l - 1; at higher concentrations the action was less effective. At 40 mg l-1 humic acids were between 20 and 30% less effective than the commonly used hexametaphosphate at 0.4%. No significant differences were observed between the performance of humic acids at 40 mg l-1 and Calgon at 0.5%. When the performance of humic acid was compared with that of hexametaphosphate of equal strength (40 mg 1-1), the former appeared to be approximately 140 times more effective for the dispersion of the fine clay fraction ( < 0.6 ttm) and approximately 1.2 times more efficient in the case of the coarse clay fraction (0.6-2.0 ttm).
INTRODUCTION
On flocculation, clay minerals commonly form a cardhouse-type structure (Schofield and Samson, 1954) with the positively charged edges of the particles attached to the negative faces. Several aromatic and aliphatic polycarboxylic acids with nominal molecular weights greater than 104 have been shown to play a major role in the dispersion of kaolinite (Durgin and Chaney, 1984) by offsetting the positive charges on the edges of the clay particles and to promote, in this way, clay dispersion. Once dispersed, the clay particles can be displaced by moving water. This process would, according to Bloomfield (1954), contribute to the formation of clay-leached A horizons in podzols. In a study of the same type of environment, Guillet et al. (1979) noted that the metal ion concentrations in podzols were too small to flocculate mineral particles dis0016-7061/88/$03.50
© 1988 Elsevier Science Publishers B.V.
332
persed by organic matter. These authors also attributed the dispersive action of the latter to an increase in the negative surface charge of organo-mineral complexes. MATERIAL AND METHODS
Soil sample The soil sample for this study was taken from the surface layer (0-29 cm) of a humic gleysol belonging to the Ste-Rosalie series, collected near St-Hyacinthe, Quebec (situated at approximately 72 ° 53' W 45 ° 36' N). Part of the sample served as the source of the humic acid and another part was used in the dispersion experiments. This second part was air-dried and passed through a 2-mm sieve. Some characteristics of that part are given in Table I.
Isolation and purification of humic acids Humic acids were obtained by extracting the soil sample with 0.5 N KOH under an N2 atmosphere for 24 h at room temperature, following an initial treatment with 0.1 N HC1 for 2.5 h. The extract was centrifuged at 4500 rev. min-1 (5700 × gravity) for 30 min, after which the humic acids in the supernatant were precipitated at pH 2.0 using 1 N HC1. Humic and fulvic acids (the latter in solution) were next separated by centrifugation at 4500 rev. m i n (30 rain), after which the humic acid fraction was redissolved by the addition of 0.5 N KOH to pH 10. The resulting solution was adjusted to pH 7.0, frozen TABLE I Some characteristics of the Ste-Rosalie soil sample Origin
St-Hyacinthe, QuSbec
Chemical characteristics: pH total carbon (%) organic carbon (%) total nitrogen (%) C/N ratio CEC (meq. 100 g - l )
6.7 2.9 1.8 0.24 12.1 36.1
Granulometric characteristics: clay, 2 ttm (%) silt, fine, 2-20 ttm (%) coarse, 20-50 Bm (%) sand, >50 ttm (%)
37.4 20.9 4.4 37.3
333 TABLE II Agents used as dispersants Agent and source
Final concentration in dispersion medium
Calgon (Beecham Canada) Na-hexametaphosphate (BDH) Humic acid (extracted from soil sample)
40 mg 1 1, 0.5% 40 mg l - 1, 0.4% 1, 25, 40, 70, 100 and 400 mg l -
at - 20 ° C, allowed to thaw again, and recentrifuged. The procedures of dissolution-precipitation and of freezing-thawing were repeated until in the end all insoluble material had been removed from the humic acid fraction. The humic acids dissolved at pH 7.0 were then desalted in an ultrafiltration cell fitted with a Diaflo UMO5 ultrafiltration membrane with a nominal molecular weight cut-off of 500 (Amicon, Lexington, Mass. ) by washing with triple-distilled water until the filtrate reached a conductivity of 3 ~mhos c m - 1. The humic acids, in the form of their K-salts, were then freeze-dried and stored in the dark at -40°C.
Methods o[ soil dispersion Dispersion of the soil for granulometric analysis was achieved by either chemical or physical treatments. Chemical dispersion. 40 g of soil were shaken mechanically for 12 h with 300 ml of a solution containing one of the dispersants listed in Table II. The resulting suspension was transferred to a granulometric cylinder, completed to a volume of 1 1 and left to settle for 6 days. Physical dispersion. 50-ml batches of a suspension of 40 g soil in 300 ml H20 were subjected to ultrasonification using a Biosonik II ultrasonifier (Brownwill, Rochester, N.Y.), operating at 150 W for 30 min. In both cases, the granulometric characteristics of the soil samples were determined by means of the hydrometer method as described by McKeague (1977). All analyses were in duplicate. Variations among results were less than 2%. RESULTS
The distribution of the clay fractions in the Ste-Rosalie soil obtained by granulometric analysis in the presence of commonly used 0.4% sodium hexametaphosphate (HMP), 0.5% Calgon, or distilled water as dispersing agents is shown in Fig. 1. In the latter case dispersion was obtained by mechanical shaking (12 h) or by ultrasonification. The results indicate that 0.4% HMP is
334 25 H M P
f
CALGON
%
I
WATER
~
I [] []
Q) c v
I Cloy fraction
~
.........
Cloy fraction .6-2.0~x.m I
}: --
___
~
!ii i !~ii~i i~i:~!ili
~!~!:J~:i~:~.
i
i ill
5
o
4 0 mgL"~
0.4%
4 0 mg L-I
0.5%
Mechanical Shaking
Ultrasonification
Fig. 1. Distribution of clay fractions in Ste-Rosalie soil samples obtained by using hexametaphosphate (HMP), Calgon or pure water as dispersants. 7= 20 o
[] []
o
Clay fraction <0.6 /.Lm Clay fraction 0 . 6 - 2 0
~- J5 "5 (/) c_
} io
:));) ~
5
iI
u
I rng E I
25 mg i-I
4 0 mg E I
7 0 mg i-I
I00 mg L"l
4 0 0 mg ri
Humic acid concentration
Fig. 2. Distribution of clay fractions in Ste-Rosalie soil samples obtained by using humic acid at different concentrations as dispersant.
a much more effective dispersant for fine clay particles ( < 0.6/lm) than 0.5% Calgon or pure water. Fig. 2 and a comparison of Fig. 1 with Fig. 2 show that humic acids, when used within the concentration range of 1-400 mg l-1 (0.04%), can effectively disperse clays. Efficiency increases with concentrations of humic acid up to 40 mg 1-1 (0.004%) and then diminishes at higher concentrations. At the opti-
335
mum concentration of 40 mg 1-1, humic acids appear to be between only 20 and 30% less dispersive than HMP at 0.4% and to have an effect similar to Calgon at 0.5%. When the performances of all three dispersants are compared at 40 mg l-1, humic acids are shown to be 140 X more effective than HMP for the dispersion of the fine clay fraction and 8 X more effective than Calgon. Also at this concentration approximately 1.2 X higher values are obtained for the coarse clay fraction in the presence of humic acids than in that of HMP or Calgon. DISCUSSION
Although humic acids are normally considered as aggregating agents in soil formation processes (Visser, 1987), data in the previous section show that humic substances at a concentration of around 40 mg l-1 are also capable of dispersing clay particles. Visser (1982) observed that the minimum concentration at which maximum surface tension depression occurs in fulvic and humic acid solutions is normally between 10 and 100 mg l - 1 (Fig. 3 ). Up to this concentration range, most of the organic molecules are still present in their most 75
.E65 o
i
~
I--I
oc,,
Humic ocids
I I
I
60 g
l~ 55 o
5o
45
\ \
35 I0-2
.
lO I
.
.
.
I0 °
.
.
.
.
.
.
.
........
.
tO'
- - - -,,,1-------, , I0 2 I0 3
Humic concentration, rng L-i Fig. 3. Surface tension versus concentration curves for fulvic and humic acids (after Visser, 1982 ).
336
extended forms. At higher concentrations the molecules will start forming bridges between the clay particles, giving rise to aggregate formation. Dong et al. (1983) reported a correlation between the degree at which clay particles were dispersed and the amount of organic matter adsorbed to them. Clays rich in organic matter (3.4-4.4 % C ) showed a high degree of aggregation, whereas those accompanied by only a small amount of organic matter (0.52.5% C) were well dispersed. Our results (Fig. 2) show a similar phenomenon: efficient dispersion of clay particles took place at relatively low HA concentrations (from 25 to 100 mg l-1), whereas at higher HA concentrations flocculation occurred, the extent of which increased with the amount of HA present. The dispersive power of humic substances can be expected to affect soil structure in general and the process of podzolisation in particular (migration of mineral matter from overlying horizons, argillan formation, etc. ). In aquatic environments, especially in boreal regions where humic acid concentrations of up to 60 mg 1-1 have been reported in natural waters (Gjessing, 1976), the dispersive properties of humic substances could lead to erosion in eulittoral zones of stream beds, resulting in loss of the smaller-sized clay fractions in particular. In view of the low concentration of humic substance required (40 mg 1-1) and, because of its low cation content, no major cation exchanges will be likely to take place between clay particles and humic dispersants. This is contrary to what happens in the presence of Na-hexametaphosphate or Calgon, where high cation concentrations will lead to cation exchanges often resulting in ionic or electrical changes on clay surfaces (Robert and Tessier, 1974 ). A possible application of the dispersive power of humic substances could therefore be envisaged in their use as dispersants in chemical investigations of clays.
REFERENCES Bloomfield, C., 1954. The deflocculation of kaolin by tree leachates. Trans. 5th Congr. Soil Sci., 2: 280-283. Dong, A., Chesters, G. and Simsiman, G.V., 1983. Soil dispersibility. Soil Sci., 136: 208-212. Durgin, P.B. and Chaney, J.B., 1984. Dispersion of kaolinite by dissolved organic matter from Douglas-fir roots. Can. J. Soil Sci., 64: 445-455. Gjessing, E.T., 1976. Physical and Chemical Characteristics of Aquatic Humus. Ann Arbor Science, Ann Arbor, 120 pp. Guillet, B., Rouiller, J. and V6dy, J.C., 1979. Dispersion et migration de min6raux argileux dans les podzols. Contribution des compos6s organiques associ6s, leur r61e sur les formes et l'6tat de l'aluminium. Colloques Internationaux du C.N.R.S. No 303. Migrations organo-min6rales dans les sols temp6r6s, 49-56. McKeague, J.A. (Editor), 1977. Manuel de M6thodes d'l~chantillonnage et d'Analyse des Sols. Comit~ Canadien de P6dologie, Ottawa, Ont., 223 pp. Robert, M. and Tessier, D., 1974. M~thodes de pr6paration des argiles des sols pour des 6tudes min6ralogiques. Ann. Agron., 25: 859-882.
337 Schnitzer, M. and Khan, S.U., 1972. Humic Substances in the Environment. Marcel Dekker, New York, N.Y., 327 pp. Schofield, R.K. and Samson, H.R., 1954. Flocculation of kaolinite due to the attraction of oppositely charged crystal faces. Disc. Faraday Soc., 18: 135-145. Stevenson, F.J., 1982. Humus Chemistry. John Wiley, New York, N.Y., 443 pp. Visser, S.A., 1982. Surface active phenomena by humic substances of aquatic origin. Rev. Fr. Sci. Eau, 1: 285-295. Visser, S.A., 1987. Humic-clay interactions in soil, and in particular podzol, processes. In: F. Pagg (Editor), La Podzolisation des Sols. Cahiers Scientifiques, # 54 ACFAS, Montreal, pp. 64-90.
331
Observations on the Dispersion and Aggregation of Clays by Humic Substances, I. Dispersive Effects of Humic Acids S.A. VISSER and M. CAILLIER
D~partement des Sols, Facult~ des Sciences de l'Agriculture et de l'Alimentation, Universit~ Laval, Qudbec, Qud. G1K 7P4 (Canada) (Received April 15, 1987; accepted after revision February 26, 1988)
ABSTRACT Visser, S.A. and Caillier, M., 1988. Observations on the dispersion and aggregation of clays by humic substances, I. Dispersive effects of humic acids. Geoderma, 42: 331-337. Although humic substances are normally considered as aggregating agents in soil formation processes, we have shown that within a certain concentration range humic substances are also able to disperse soil particles. The dispersive effect was particularly evident at humic acid concentrations of around 40 mg l - 1; at higher concentrations the action was less effective. At 40 mg l-1 humic acids were between 20 and 30% less effective than the commonly used hexametaphosphate at 0.4%. No significant differences were observed between the performance of humic acids at 40 mg l-1 and Calgon at 0.5%. When the performance of humic acid was compared with that of hexametaphosphate of equal strength (40 mg 1-1), the former appeared to be approximately 140 times more effective for the dispersion of the fine clay fraction ( < 0.6 ttm) and approximately 1.2 times more efficient in the case of the coarse clay fraction (0.6-2.0 ttm).
INTRODUCTION
On flocculation, clay minerals commonly form a cardhouse-type structure (Schofield and Samson, 1954) with the positively charged edges of the particles attached to the negative faces. Several aromatic and aliphatic polycarboxylic acids with nominal molecular weights greater than 104 have been shown to play a major role in the dispersion of kaolinite (Durgin and Chaney, 1984) by offsetting the positive charges on the edges of the clay particles and to promote, in this way, clay dispersion. Once dispersed, the clay particles can be displaced by moving water. This process would, according to Bloomfield (1954), contribute to the formation of clay-leached A horizons in podzols. In a study of the same type of environment, Guillet et al. (1979) noted that the metal ion concentrations in podzols were too small to flocculate mineral particles dis0016-7061/88/$03.50
© 1988 Elsevier Science Publishers B.V.
332
persed by organic matter. These authors also attributed the dispersive action of the latter to an increase in the negative surface charge of organo-mineral complexes. MATERIAL AND METHODS
Soil sample The soil sample for this study was taken from the surface layer (0-29 cm) of a humic gleysol belonging to the Ste-Rosalie series, collected near St-Hyacinthe, Quebec (situated at approximately 72 ° 53' W 45 ° 36' N). Part of the sample served as the source of the humic acid and another part was used in the dispersion experiments. This second part was air-dried and passed through a 2-mm sieve. Some characteristics of that part are given in Table I.
Isolation and purification of humic acids Humic acids were obtained by extracting the soil sample with 0.5 N KOH under an N2 atmosphere for 24 h at room temperature, following an initial treatment with 0.1 N HC1 for 2.5 h. The extract was centrifuged at 4500 rev. min-1 (5700 × gravity) for 30 min, after which the humic acids in the supernatant were precipitated at pH 2.0 using 1 N HC1. Humic and fulvic acids (the latter in solution) were next separated by centrifugation at 4500 rev. m i n (30 rain), after which the humic acid fraction was redissolved by the addition of 0.5 N KOH to pH 10. The resulting solution was adjusted to pH 7.0, frozen TABLE I Some characteristics of the Ste-Rosalie soil sample Origin
St-Hyacinthe, QuSbec
Chemical characteristics: pH total carbon (%) organic carbon (%) total nitrogen (%) C/N ratio CEC (meq. 100 g - l )
6.7 2.9 1.8 0.24 12.1 36.1
Granulometric characteristics: clay, 2 ttm (%) silt, fine, 2-20 ttm (%) coarse, 20-50 Bm (%) sand, >50 ttm (%)
37.4 20.9 4.4 37.3
333 TABLE II Agents used as dispersants Agent and source
Final concentration in dispersion medium
Calgon (Beecham Canada) Na-hexametaphosphate (BDH) Humic acid (extracted from soil sample)
40 mg 1 1, 0.5% 40 mg l - 1, 0.4% 1, 25, 40, 70, 100 and 400 mg l -
at - 20 ° C, allowed to thaw again, and recentrifuged. The procedures of dissolution-precipitation and of freezing-thawing were repeated until in the end all insoluble material had been removed from the humic acid fraction. The humic acids dissolved at pH 7.0 were then desalted in an ultrafiltration cell fitted with a Diaflo UMO5 ultrafiltration membrane with a nominal molecular weight cut-off of 500 (Amicon, Lexington, Mass. ) by washing with triple-distilled water until the filtrate reached a conductivity of 3 ~mhos c m - 1. The humic acids, in the form of their K-salts, were then freeze-dried and stored in the dark at -40°C.
Methods o[ soil dispersion Dispersion of the soil for granulometric analysis was achieved by either chemical or physical treatments. Chemical dispersion. 40 g of soil were shaken mechanically for 12 h with 300 ml of a solution containing one of the dispersants listed in Table II. The resulting suspension was transferred to a granulometric cylinder, completed to a volume of 1 1 and left to settle for 6 days. Physical dispersion. 50-ml batches of a suspension of 40 g soil in 300 ml H20 were subjected to ultrasonification using a Biosonik II ultrasonifier (Brownwill, Rochester, N.Y.), operating at 150 W for 30 min. In both cases, the granulometric characteristics of the soil samples were determined by means of the hydrometer method as described by McKeague (1977). All analyses were in duplicate. Variations among results were less than 2%. RESULTS
The distribution of the clay fractions in the Ste-Rosalie soil obtained by granulometric analysis in the presence of commonly used 0.4% sodium hexametaphosphate (HMP), 0.5% Calgon, or distilled water as dispersing agents is shown in Fig. 1. In the latter case dispersion was obtained by mechanical shaking (12 h) or by ultrasonification. The results indicate that 0.4% HMP is
334 25 H M P
f
CALGON
%
I
WATER
~
I [] []
Q) c v
I Cloy fraction
~
.........
Cloy fraction .6-2.0~x.m I
}: --
___
~
!ii i !~ii~i i~i:~!ili
~!~!:J~:i~:~.
i
i ill
5
o
4 0 mgL"~
0.4%
4 0 mg L-I
0.5%
Mechanical Shaking
Ultrasonification
Fig. 1. Distribution of clay fractions in Ste-Rosalie soil samples obtained by using hexametaphosphate (HMP), Calgon or pure water as dispersants. 7= 20 o
[] []
o
Clay fraction <0.6 /.Lm Clay fraction 0 . 6 - 2 0
~- J5 "5 (/) c_
} io
:));) ~
5
iI
u
I rng E I
25 mg i-I
4 0 mg E I
7 0 mg i-I
I00 mg L"l
4 0 0 mg ri
Humic acid concentration
Fig. 2. Distribution of clay fractions in Ste-Rosalie soil samples obtained by using humic acid at different concentrations as dispersant.
a much more effective dispersant for fine clay particles ( < 0.6/lm) than 0.5% Calgon or pure water. Fig. 2 and a comparison of Fig. 1 with Fig. 2 show that humic acids, when used within the concentration range of 1-400 mg l-1 (0.04%), can effectively disperse clays. Efficiency increases with concentrations of humic acid up to 40 mg 1-1 (0.004%) and then diminishes at higher concentrations. At the opti-
335
mum concentration of 40 mg 1-1, humic acids appear to be between only 20 and 30% less dispersive than HMP at 0.4% and to have an effect similar to Calgon at 0.5%. When the performances of all three dispersants are compared at 40 mg l-1, humic acids are shown to be 140 X more effective than HMP for the dispersion of the fine clay fraction and 8 X more effective than Calgon. Also at this concentration approximately 1.2 X higher values are obtained for the coarse clay fraction in the presence of humic acids than in that of HMP or Calgon. DISCUSSION
Although humic acids are normally considered as aggregating agents in soil formation processes (Visser, 1987), data in the previous section show that humic substances at a concentration of around 40 mg l-1 are also capable of dispersing clay particles. Visser (1982) observed that the minimum concentration at which maximum surface tension depression occurs in fulvic and humic acid solutions is normally between 10 and 100 mg l - 1 (Fig. 3 ). Up to this concentration range, most of the organic molecules are still present in their most 75
.E65 o
i
~
I--I
oc,,
Humic ocids
I I
I
60 g
l~ 55 o
5o
45
\ \
35 I0-2
.
lO I
.
.
.
I0 °
.
.
.
.
.
.
.
........
.
tO'
- - - -,,,1-------, , I0 2 I0 3
Humic concentration, rng L-i Fig. 3. Surface tension versus concentration curves for fulvic and humic acids (after Visser, 1982 ).
336
extended forms. At higher concentrations the molecules will start forming bridges between the clay particles, giving rise to aggregate formation. Dong et al. (1983) reported a correlation between the degree at which clay particles were dispersed and the amount of organic matter adsorbed to them. Clays rich in organic matter (3.4-4.4 % C ) showed a high degree of aggregation, whereas those accompanied by only a small amount of organic matter (0.52.5% C) were well dispersed. Our results (Fig. 2) show a similar phenomenon: efficient dispersion of clay particles took place at relatively low HA concentrations (from 25 to 100 mg l-1), whereas at higher HA concentrations flocculation occurred, the extent of which increased with the amount of HA present. The dispersive power of humic substances can be expected to affect soil structure in general and the process of podzolisation in particular (migration of mineral matter from overlying horizons, argillan formation, etc. ). In aquatic environments, especially in boreal regions where humic acid concentrations of up to 60 mg 1-1 have been reported in natural waters (Gjessing, 1976), the dispersive properties of humic substances could lead to erosion in eulittoral zones of stream beds, resulting in loss of the smaller-sized clay fractions in particular. In view of the low concentration of humic substance required (40 mg 1-1) and, because of its low cation content, no major cation exchanges will be likely to take place between clay particles and humic dispersants. This is contrary to what happens in the presence of Na-hexametaphosphate or Calgon, where high cation concentrations will lead to cation exchanges often resulting in ionic or electrical changes on clay surfaces (Robert and Tessier, 1974 ). A possible application of the dispersive power of humic substances could therefore be envisaged in their use as dispersants in chemical investigations of clays.
REFERENCES Bloomfield, C., 1954. The deflocculation of kaolin by tree leachates. Trans. 5th Congr. Soil Sci., 2: 280-283. Dong, A., Chesters, G. and Simsiman, G.V., 1983. Soil dispersibility. Soil Sci., 136: 208-212. Durgin, P.B. and Chaney, J.B., 1984. Dispersion of kaolinite by dissolved organic matter from Douglas-fir roots. Can. J. Soil Sci., 64: 445-455. Gjessing, E.T., 1976. Physical and Chemical Characteristics of Aquatic Humus. Ann Arbor Science, Ann Arbor, 120 pp. Guillet, B., Rouiller, J. and V6dy, J.C., 1979. Dispersion et migration de min6raux argileux dans les podzols. Contribution des compos6s organiques associ6s, leur r61e sur les formes et l'6tat de l'aluminium. Colloques Internationaux du C.N.R.S. No 303. Migrations organo-min6rales dans les sols temp6r6s, 49-56. McKeague, J.A. (Editor), 1977. Manuel de M6thodes d'l~chantillonnage et d'Analyse des Sols. Comit~ Canadien de P6dologie, Ottawa, Ont., 223 pp. Robert, M. and Tessier, D., 1974. M~thodes de pr6paration des argiles des sols pour des 6tudes min6ralogiques. Ann. Agron., 25: 859-882.
337 Schnitzer, M. and Khan, S.U., 1972. Humic Substances in the Environment. Marcel Dekker, New York, N.Y., 327 pp. Schofield, R.K. and Samson, H.R., 1954. Flocculation of kaolinite due to the attraction of oppositely charged crystal faces. Disc. Faraday Soc., 18: 135-145. Stevenson, F.J., 1982. Humus Chemistry. John Wiley, New York, N.Y., 443 pp. Visser, S.A., 1982. Surface active phenomena by humic substances of aquatic origin. Rev. Fr. Sci. Eau, 1: 285-295. Visser, S.A., 1987. Humic-clay interactions in soil, and in particular podzol, processes. In: F. Pagg (Editor), La Podzolisation des Sols. Cahiers Scientifiques, # 54 ACFAS, Montreal, pp. 64-90.