1-=t'- Behaviour of Alkaline Sodic Soils and Clays Influenced by as pH and Particle Charge Thesis submitted for the degree of Doctor of Philosophy at The University of Adelaide by Mostafa Chorom Department of Soil Science Waite Agricultural Research [rstitute Glen Osmond, South Australia 1996 Table of Contents Page Abstract v1 Statement xll .. Acknowledgments .xiii List of Figures xlv List of Tables xvi Publications from the Thesis xv111 Chapter L Introduction.............. 1, Chapter 2 Review of Literature 4 2.1 Sodic soils ......... 4 2.1,.1. Nature of sodic soils ........... 4 2.1,.2 World distribution of saline and sodic soils 6 2.1.2.1 Australia 7 2.1,.3 Classification of saline and sodic soils......... 8 2.1..3.1. Saline soils .............. 8 2.1,.3.2 Alkaline sodic soils 9 2.1,.3.3 Calcareous soils 13 2.1,.3.4 Measuring salinity and sodicity 13 2.2 Dispersion of sodic soils......... 1,4 2.2.1. Spontaneous dispersion .............. 15 2.2.2 Mechanical dispersion................. 1,6 2.2.3 Factors affecting clay dispersion 1,6 2.2.3.1, The influence of electrolyte concentration on dispersible clay ........ 1.6 2.2.3.2 Sodicity and soil pH 1.8 2.2.3.3 Clay mineralogy 21, 2.2.3.4 Effect of organic matter on clay dispersion 23 2.3 Flocculation of dispersed clays........ 24 2.3.1. Introduction............ 24 2.3.2 Critical coagulation concentration .... 25 2.3.3 Flocculation of pure clays as related to pH and SAR 25 2.4 Interactions between particles in aqueous suspensions 27 2.4.I van der Waals interaction 27 2.4.2 Electricalinteraction 28 11 2.4.2.L Electrical double layers 28 2.4.2.2 Double layer interaction 30 2.4.3 Combined interaction and colloid suspension stability. 3L 2.4.4 Hydration effects...... 32 2.4.5 Ion-ion correlation forces 33 2.4.6 Polymer bridging 34 2.5 Mechanisms of structural change in sodic soils......... 35 2.6 Particle charge and clay behaviour 38 2.6.1. Net charge and charge location effect....... 38 2.6.2 Effect of heating on particle charge 40 2.6.3 Infrared spectra of clays as related to hydration............. 42 2.7 Reclaiming alkaline sodic soils........... M 2.7.1. Introduction............ 44 2.7.2 Chemical amendment .... 45 2.7.2.1 Gypsum application 45 2.7.2.2 Gypsum dissolution 46 2.7.2.3 Gypsum requirement 47 2.7.3 Biological reclamation............... 48 2.7.3.1 Introduction 48 2.7.3.2 Green manure 48 2.7 .3.3 Microbial reclamation through glucose application..... 5L 2.8 needed Further research .......52 Chapter 3 Dispersion and Zeta Potential of Pure Clays 54 3.1 Introduction............. 54 3.2 Materials and methods .................. 56 3.2.1, Characteristics of clays..... 56 3.2.2 Clay Preparation 56 3.2.3 Flocculation of dispersed clay.......... 57 3.2.4 Determination of electrical charge on pure clays minerals ....... 57 3.2.5 Scanningelectronmicrographs 57 3.2.6 Zeta potential measurements 58 3.3 Results and discussion 59 3.3.1 Na-clays 59 3.3.1.1 Influence of pH on the dispersibility of pure clay minerals 59 3.3.1..2 Influence of pH on net particle charge and dispersibility 62 3.3.1.3 Zeta potential as a function of pH, EC, and dispersion 65 3.3.2 Ca-clays 67 3.3.2.1. Influence of pH on the dispersibility of Ca-clay minerals 67 3.3.2.2 Zetapotential as a function of pH and EC 69 3.3.3 Flocculation values of Na and Ca clays systems 73 3.4 Conclusiorìs ............... 74 Chapter 4 Dispersion of Soil Clays as Influenced by pH and Net Particle Charge ...75 75 4.2 Dispersive potential concept. 76 4.3 Materials and methods 78 4.3.1 Soil samples 78 4.3.2 Clay mineralogy...... 78 4.3.3 Othermeasurements 79 4.3.4 Experimental procedure ........ 80 4.3.5 Determination of electiical charge on soils 80 4.3.6 Soils used for correlation studies 8L 4.3.7 Statisticalanalysis 81 4.4 Results and discussion. 82 4.4.1. Effect of pH on clay dispersion from three selected soils .......... 82 4.4.2 Critical dispersion concentration for aggregates and the c1ays......... flocculation values of soil .........85 4.4.3 Dispersive potential of Australian soils in relation to their pH and CEC...... 86 4.5 Conclusions............ 90 Chapter 5 Changing particle charge and altering clay behaviour...................................91 Introduction........... 5.L ................ 9L 5.2 Effect of heating on swelling and dispersion of different cationic forms of a smectite 93 5.2.1. Materials and methods............. 93 5.2.1..1 Clay preparation. 93 5.2.1..2 Exchangeable cations and cation exchange capacity. 94 5.2.L.3 X-ray diffraction studies........ 94 5.2.1.4 Measurements of dispersible clay....... 94 1\/ 5.2.1,.5 Zeta potential measurements 95 5.2.1..6 Clay particle size 95 5.2.2 Results 95 5.2.2.1, Chemical properties 95 5.2.2.2 Physical properties 98 5.2.2.2.1, Swelling 98 5.2.2.2.2 Dispersion r02 5.2.2.2.3 Zeta potential and particle aggregation ....... 104 5.2.3 Discussion.............. ...104 5.2.4 Conclusions............. .................. 106 5.ó Slaking and dispersion of different cationic forms of illite and heating kaolinite as influenced by .....108 5.3.1 methods........... Materials and ...................108 5.3.1.1 Characteristics of clays L08 5.3.1.2 Other measurements............. 108 5.3.1.3 Slaking test 108 5.3.2 Results and discussion L09 5.3.2.1. Chemical properties L09 properties 5.3.2.2 Physical ....LLz 5.3.2.2.1. X-ray diffraction 1t2 5.3.2.2.2 Slaking 112 5.3.2.2.3 Dispersion 113 5.3.2.2.4 Zeta potential and particle aggregation ....... 1 L5 120 5.4 Behaviour of alkaline sodic soils after heating . L21. 5.4.L Introduction............ 121 5.4.2 Materials and methods........... L22 5.4.2.1. Soils r22 preparation............... 5.4.2.2 Sample ......122 5.4.2.3 Soil chemical analyses ................. 122 5.4.2.4 Slaking measurement 123 5.4.2.5 Saturated hydraulic conductivity measurements ......t23 5.4.2.5 Other measurements.............. 123 5.4.3 Results and discussion 123 5.4.3.1. Characteristics of soils used......... I23 5.4.3.2 Chemical properties 124 5.4.3.3 Physical properties 126 5.4.4 Conclusions................. 127 5.5 Infrared spectra of heated clays 128 5.5.1 Introduction ................ t28 5.5.2 Materials and methods........... L29 5.5.3 Results and discussion L29 5.5.3.1 Li-clays 1.30 5.5.3.2 Mg-clays 1.32 v 5.5.3.3 Al-clays 135 137 140 Chapter 6 Influence of Amendments on the Behaviour of an Alkaline Sodic SoiI... L41 1,41, 6.2 Materials and methods t43 1.43 6.2.2 Treatments and experimental design 1,43 6.2.3 Gypsum trial 1,43 6.2.4 Green manure treatment 1,43 6.2.5 Glucosetreatment 1,M 6.2.6 Soil chemical analyses 744 6.2.7 Physical measurements.......... 1,45 6.2.7.1, Wet aggregate stability......... t45 6.2.7.2 Soil friability........... 1.45 6.2.8 NMRanalyses..... L46 6.2.9 Biomass measurement ..... 146 6.2.10 Methane measurement..... 147 6.3 Results 1,47 6.3.1, Gypsum and green manure effects 1,48 6.3.L.1. Chemical properties 148 6.3.1..2 Physical properties L5L 6.3.2 Glucose experiment .................... 156 6.4 Discussion L58 6.5 Conclusions 1,61, Chapter 7 General Discussion..................... 762 7.1 Introduction............. 162 7.2 Dispersion, pH and particle charge 162 7.3 Altering particle charge and clay behaviour L65 7.4 Amelioration of sodic soils 1,69 7.5 Perspectives............. 170 References L73 VI Abstract Sodic soils occur in about 30% of the total land area in Australia and86"/o of these sodic soils are alkaline (pHt 8.4), particularly in dense clayey subsoils (Rengasamy and Olsson 1991). Many of these alkaline soils also contain lime (CaCO3) ranging from 10 to 2000 t ha-1 up to a metre depth. In spite of the presence of calcium compound, sodicity is highly prevalent and affects soil physical properties related to transport of air and water. High pH of these soils also interferes with nutrient availability. These constraints are considered to reduce the potential yields of crops to less than half. The objective of this thesis is to investigate the factors affecting swelling and dispersion of alkaline sodic soils contairi^g lime and the ways to manage these soils to improve their physical conditions. Studies on pure clay systems have been included to understand the fundamental process involved in swelling and dispersion of pure and soil clays. . The literature review has identified soil pH as an important factor affecting these properties. Therefore, the mechanisms by which pH controls swelling and dispersion as well as how to reduce soil pH by utitising the native CaCO3 are the foci of this thesis. The investigations of this study are described in the following chapters dealing with: 1) Dispersion and zeta potential of pure clays 2) Dispersion of soil clays as influenced by pH and net particle charge 3) Changing particle charge and alteringclay behaviour 4) Influence of amendments on the behaviour of an alkaline sodic soil vll The effect of changing pH and electrolyte concentration on the dispersion and zeta potential of Na- and Ca- forms of kaolinite, illite and smectite was investigated in relation to changes in their net negative charge. The percentage of dispersible Na-clay and the percentage increase in net negative charge was positively correlated with pH, but the slopes varied from clay to clay. In general, the net negative charge was the primary factor in clay dispersion, and the pH affected clay dispersion by changing the net charge on clay particles. Na-smectite had larger net charge at all pHs than Na-illite and Na-kaolinite, and it always had larger flocculation values. The role of electrolyte concentration could be due to its effect both on flocculation and variable charge component of the clay minerals. The zeta potential at different pHs also reflected the same trend of clay dispersion with net particle charge. In ca-clays the trends were similar to Na-clays up to pH 7.0. In more alkaline solution CaCO3 formation led to charge reduction on clay particles, resulting in flocculation and reduction of zeta potential. At similar pHs the electrophoretic mobilities of all the clays showed constant potential behaviour. However, t}ite zeta potentials of Ca-clays were always smaller than those of sodic clays because the clays were more aggregated. Net particle charge was the most important factor in controlling clay dispersion for the whole range of pH and ionic strength and for all types of cations. The effect of changing pH on the dispersion of clay from sodic soils was investigated in relation to changes in net charge on clay particles. Positive relationship was obtained between pH and the percentage of dispersible clay for each soil clay. The percentage increase in net negative charge was also positively correlated with pH. However, the slopes of these relationships varied between soil clays. In comparing the values for pure clay minerals quoted in the literature with soil clays having similar mineralogy, it was found that soil clays had higher flocculation values. This is shown to be due to higher v111 net negative charge on soil clays than the corresponding values for pure clay minerals found in the literature. The effect of soil organic matter in enhancing the net negative charge probably contributes to the higher charge on soil clays. The critical dispersion concentration (CDC) for clay dispersion from soil aggregates was lower than the flocculation values observed for the separated soil clays. The separated soil clays had high negative charge due to exposure of surfaces which were originally bonded in the aggregates. The dispersive potential of a number of Alfisols, Oxisols, Aridisols (calcareous soils) and Vertisols collected from different parts of Australia was highly correlated with soil pH. The relationship with CEC was poor because CEC was estimated at a pH different to the natural pH of the soil. This study has brought out the importance of pH in the management of dispersive soils. The effect of heat treatments on the swelling, dispersion, particle charge and particle aggregation of Li-, Na-, K-,Mg-, Ca- and Al-Wyoming bentonite was investigated. Before thermal treatment, unheated (25"C) Li-, Na- and K- clays showed increased dggl spacings on glycerol solvation and dispersed spontaneously in water. Mg-, Ca- and Al-clays did not disperse spontaneously in water, but the dggl spacing increased on glycerol solvation. After heating at 300'C or above, none of these clays dispersed spontaneously. Flowever, swelling varied with the type of cation and the temperature of heating. The results generally suggested that swelling and dispersion of homoionic Wyoming bentonite after heating at various temperatures depended on the nature of bonding between clay particles and the cations. Enhanced swelling and dispersion of clays indicate the more ionic character of the cationic bonding than in the cases where heating resulted only in swelling, with polar covalent bonding of cations to clay surfaces allowing limited hydration. It is also suggested that in case of absence of both swelling and dispersion as a 1X result of thermal treatment, a covalent bond is formed between cation and clay surface. Thermal treatment apparently affect the bonding in different ways. It appears that the smaller cations (ionic radius <0.7Å) Li, Mg and Al migrate to octahedral vacant sites and form covalent bonds after heating at 400"C; this reduces the negative charge drastically. This process for Li-clays occurred even at 200"C. The larger cations (ionic radius > 0.9Å) Na, K and Ca ions apparently did not migrate to octahedral vacant sites after heating up to 400'C; a high proportion of them being exchangeable. The data on exchangeable cation, particle charge and clay particle size were consistent with the postulated effect of the nature of cationic bonding on swelling and dispersion properties. Slaking and dispersion of Georgia kaolinite and Grundite illite were studied after heating the cations (Li, Na, K Mg and Ca) saturated samples up to 400'C. The results on spontaneous dispersion, mechanical dispersion and particle size after thermal treatment, are consistent with the hypothesis that heating alters the nature of bonding between cations and clay surfaces. Thus, in Li-, Na- and K-clays on heating up to 100'C reduces the ionicity of bonding and hence spontaneous dispersion. Heating above 200"C increases polar covalency and covalency of bonding and prevents spontaneous dispersion. Divalent cations form predominantly polar covalent bonds with clay surfaces. The freeze dried Mg- and Ca-kaolinites and illites slake in water. On heating these clays, covalency of bonding is increased and slaking is proportionally reduced. On heating CEC is reduced progressively with temperature; with increasing polar covalency and covalency of cationic bonding, cations become difficult to be exchanged. Mechanical dispersion decreases proportionate to the charge reduction (CEC decrease). The mean diameter of particles also increases