Continental solids
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Chemistry of Continental Solids- Introduction to Environmental Chemistry
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Continental solids
- 2. 4.1- Terrestrial environment, crust & material cycling - Terrestrial environments consists of solid, liquid & biological components - Continental crust mostly made up of igneous & metamorphic rocks - Diagenesis is the process where physical & biological processes & chemical rxns convert sediment to sedimentary rocks
- 4. 4.2- The structure of silicate minerals
- 5. 4.2- The structure of silicate minerals
- 6. 4.2- The structure of silicate minerals
- 7. 4.2- The structure of silicate minerals
- 8. 4.2- The structure of silicate minerals
- 9. 4.2- The structure of silicate minerals
- 11. 4.4- Mechanisms of chemical weathering
- 15. 4.6- Formation of Soils Soil- the upper layer of the Earth’s continental crust in which plants grow, usually consisting of disintegrated rock with admixture of organic remains Soil formation is influenced by key factors such as parent material, climate, relief, vegetation, & influence of organism
- 17. 4.6- Formation of Soils
- 18. 4.7- Wider Controls on Soil and Clay Mineral Formation > In upper crustal granodiorite, feldspars mainly weathers to form clay minerals > Alteration is most likely in the interlayer areas (damaged crystal edges) > Topography > Wet tropical environments promote dis- solution & transport of a number of soil constituents
- 19. 4.8- Ion Exchange & Soil pH • The CEC of a soil depends on the pH • Acidic soils have high concentrations of Al3+ & Fe3+ ions • At high pH, the insoluble hydroxides are produced (the of Al3+ & Fe3+ ions are no longer available to plants • Phosphate ions are available in weakly acidic soil • Calcium & magnesium are unavailable at high pH • Copper & zinc are only available at intermediate pH
- 20. 4.9- Soil structure & classification
- 21. 4.10- Contaminated Land Sources of land contamination Herbicides & insecticides from farms Byproducts of combustion in car engines Feedstock chemicals for synthetic processes Contaminant- substance is present in the environment at a concentration above natural background levels Pollutant- when a contaminant can be shown to have a deleterious effect on the environment
- 22. 4.10- Contaminated Land Organic contaminants - compounds with carbon skeleton - once in the soil environment, organic contaminants may move in, or interact with soil atmosphere, soil water, mineral fractions & organic matter (dissipate or persists) - organic contaminants interact mainly with either the mineral or organic components of soils - degradation of organic contaminants in soils occur by either chemical or microbiological pathways - Biodegradation of organic contaminants can be carried out by a single microbial species in pure cultures but in nature, mixture of microbes are usually required
Editor's Notes
- Materials in the terrestrial environment were formed mainly by weathering Diagenesis to orogenesis (mountain building)
- Silicates- are rock forming minerals & most rocks are composed of silicates
- Single chain- oxygen atoms of each tetrahedron is attached to the tetrahedron above & below
- Single chains forming next to each other need a cation forming an ionic bond Ionic bonds are weaker type of bonds so breaks happen in the mineral along the weaker ionic bonds Examples of single-chain silicates are pyroxenes, augite
- Example of double-chain silicate- amphiboles (hornblende) Sheet silicates- double chain silicates are connected in all directions in a 2-dimensional sheet Sheet silicates- mica and clay Framework silicates- all 4 oxygens are connected to other oxygens of adjacent tetrahedrons so no need for a cation - very strong mineral - feldspar which is the most common mineral is a framework silicate
- Types of weathering: mechanical, chemical and biological
- Oxidation of organic matter- catalyzed by heterotrophic microorganisms. Bacterially mediated oxidation of organic matter to CO2 is important because it generates acidity. Acid hydrolysis- reaction between a mineral and acidic weathering agent (CO2+H2O>>>>H2CO3+CaSiO3 (calcium metasilicate)>>>>CaCO3+SiO2+H20
- Clay evolved primarily from chemical weathering of certain rocks. Clay minerals are very small particles and very active electrochemically Tetrahedral sheet- layers of SiO4 tetrahedral w/c share 3 oxygens w/ neighboring tetrahedral. Octahedral sheet- composed of cations (Al, Mg), arranged equidistant from 6 oxygens. Ideal octahedral sheet has the composition of the aluminum hydroxide mineral, gibbsite (Al (OH)3). Combination of these 2 sheets gives the basic clay mineral structure. Montmorillonite- the interlayer bonding is by Van de Waals forces and by cations which balance charge deficiencies; water & cations exist between unit layers causing the minerals to swell Van der Waals- residual attractive or repulsive forces between molecules or atomic groups that do not arise from covalent nor ionic bonds, weak & short-range forces of attraction
- The combination allows the apical oxygen of the tetrahedral sheet & the OH groups lodged in the center of the hexagonal holes of the basal tetrahedral sheet to be shared with the octahedral sheet
- If we are to look at soil formation as a whole, the 5 factors dictate the processing side Climate- weathering rates are enhanced if temperature and precipitation are high - mean annual temperature & mean annual precipitation characterize the major vegetational systems of the world (Whittaker, 1970) Organisms- animals have tremendous impact as they keep mixing and moving soils (manure and nutrients), plants also contribute organic matter to the ground
- Relief or Topography- contribute on the profile of the soil Parent Material- dictates what kind of soil will be produced, the rate of rock weathering is strongly dependent on the solubility & stability of the constituent minerals - soil forming on limestone will have a high Ca content Time- soils as they mature will take on more and more horizons because of more weathering over time
- Feldspars weather to form clay minerals. Since feldspars are framework silicates, the formation of clay minerals (sheet silicates) must involve an intermediate step. It is proposed that fulvic acids may react with Al to form soluble aluminum-fulvic acid complex which also absorbs SiO4 tetrahedral to form clay mineral structures Topography influences most of the soil-forming factors Catena- intrinsically linked to the landscape of a region or area 2 key processes control the development of catena: (i) erosion and subsequent transport and deposition of eroded material; and (ii) leaching and subsequent transport and deposition of dissolved materials.
- Exchangeable ions are those that are held temporarily on materials by weak, electrostatic forces At low pH, the CEC decreases & at low pH, H+ ions displace other exchangeable cations from the soil The nutrients are leached from the soil At high pH, the CEC increases & at high pH, the OH- ions remove H+ ions from the hydroxide groups in clay The negative charge is increased w/c increases the CEC of the soil Al & Fe ions are only available at low pH
- Compounds persist if they are of low volatility, low solubility or have a molecular structure that resists degradation. Conversely, if compounds are highly volatile, highly soluble or are easily degraded, they will be destroyed or lost to other environments (e.g. the atmosphere or hydrosphere). The effectiveness of degradation is largely determined by the contaminant availability, although the degree of persistence is influenced by the chemical structure of the contaminant. If the chemical structure of the contaminant is similar to that of a natural substance it is more likely to be degradable.
- Phytoremediation is the use of plants and trees to clean up metals, pesticides, solvents, explosive hydrocarbons, PAHs and leachates at contaminated sites. While microbial bioremediation is usually the fastest and most widely applied clean-up technique, phytoremediation can prolong or enhance degradation over longer timescales on sites where microbial techniques have been used first. It is also useful at remote sites missed during the main remediation campaign, and can be aesthetically pleasing. Plants may accumulate contaminants within their roots, stems and leaves. This is called phytoaccumulation and is known to remove a variety of heavy metals (see Section 5.6) from soils, including zinc (Zn), copper (Cu) and nickel (Ni). Once the plants have had sufficient time to accumulate contaminants they are harvested and usually incinerated to leave a metal-rich ash. The ash typically represents about 10% of the original mass of the contaminated soil, and is either landfilled or processed as a metal ore (bio-ore) if economically viable. Some plants exude enzymes that are capable of transforming organic contaminants into simpler molecules, used directly by the plants for growth, a process known as phytodegradation.