Soil contamination and remediation Introduction to soil chemistry Chemistry background History chemical reactions Colloids - soil ph soil s buffer capacity
Chemistry - background atom, molecule, chem. substance periodic table (eg. http://en.wikipedia.org/wiki/periodic_table) atomic number, atomic mass ~ molar mass of a substance (g) molecules (ionic and covalent chemical bonds) oxidation no. of element in molecule = sum of positive and negative charges Basic rules: sum of oxidation number is zero for neutral molecule (eg. NaCl), otherwise is equal to the molecule charge (např. CO 3 2- ) oxidation number of an element in free state is equal to O (O in O 2 ) oxidation number of alkali metals (Group 1: H, Li, Na, K, Rb, Cs and Fr) = +1 oxidation number of alkaline earth metals (Group 2: Be, Mg, Ca, Sr, Ba and Ra) = +2 oxidation number of oxygen is almost always equal to -2 (except v H 2 O 2 whereis-1) oxidation number of hydrogen is almost always equal to +1
Chemistry - background http://en.wikipedia.org/wiki/periodic_table
Units of concentration Molar concentration c A (M) Moles per unit volume 1M = 1 (mol / L) 1 mol is amount of substance of a system which has 6.0225 x 10 23 elementary entities Mole fraction x i number of moles of solute (n i ) / total number of moles in a solution (-) Milliequivalents per Liter (meq/l) equivalent weight = atomic mass / oxidation number Example: Ca 2+ atomic mass 40 / ox. number 2 = 20 g
Units of concentration "Parts-per" notation amount of one substance in another ppm, ppb, ppt (parts per million, billion...) nebo ppmv (parts per million volume) Mass per unit volume Mass of solute per unit volume of solution mg / L common unit 1 L distilled water represents 1 000 000 mg so 1 ppm 1 mg / L
Mole = gram formula weight 6 x 10 23 atoms Formula weight add up atomic weights 2 Hydrogen = 2 x 1 gram = 2 grams 1 Oxygen = 1 x 16 grams = 16 grams 1 mole H 2 0 = 18 grams Molarity = moles / L of solution Molality = moles / kg of solution But 1000 g of water = 1 liter of water For dilute solutions (up to 0.01 molal) molality = molarity Molarity = (mg/l x 10-3 ) / formula weight in grams
Concentration units Example 1: What will be the concentration in ppm, when 1g is diluted in 999,999 liters of water? 1 ppm
Concentration units Example 2: What concentration expressed in is equal to 2000 ppmv 2
Concentration units Example 3: How many kilograms of Atrazin would have to escape to Orlik water dam (300 mil. m 3 ), to reach the concentration 3 ppb of Atrazine in water 900 kg
Concentration units Example 4: What concentration in ppm equals to molar concentration 0,001M Ca 2+ in water 40 mg/l = 40 ppm
Brief history of soil chemistry 1819 Italian chemist Gazerri early leaching experiments J. Thomas Way reported that soils retain cations NH 4+, K + a Na + dissolved in leaching water and release cation Ca 2+ Father of Soil Chemistry
Brief history of soil chemistry F. Stohmann a W. Henneberg introduced adsorption isotherm c on solid c in solution 1859 S. Johnson found that organic matter is capable to absorb NH 4+ even more than soil s inorganic part
Elementary soil composition Prvek % O 49,0 Si 33,0 Al 6,7 Fe 3,2 Ca 2,0 Na 1,1 Mg 0,8 Prvek % K 1,8 Ti 0,5 P 0,08 Mn 0,08 S 0,04 C 1,4 N 0,2 (URE a BERROW, 1982)_ Hydroxides, clays Oxides, hydroxides, organic matter soil air Quartz, silicates, clay minerals
Soil water Water is an exceptionally good solvent Charges strong enough to cause dissociation of molecule Groundwater naturally contains dissolved cations and anions
Major, Minor, and Trace Solutes Dissolved in Groundwater Ca 2+ Mg 2+ Na + HCO - 3 SO 4 2- Cl - Si Major (> 5mg/L) Calcium Magnesium Sodium Bicarbonate Sulfate Chloride Silicon Minor (.01 5 mg/l) B Fe NO3-2+ / 3+ Boron 2+ / 3+ Iron NH + 4 K + Sr 2+ Mn 2+ Nitrate Ammonia Potassium Strontium Manganese Trace (<.01 mg/l) Everything Else!
Chemical reaction in soil (Biogeochemical weathering) (6) Types: 1. Hydration 2. Hydrolysis 3. Dissolution 4. Carbonation 5. Complexation 6. Oxidation-Reduction REDOX All involve water!!!
Chemical reaction in soil Hydration: addition of whole water to a mineral (Adsorption) e.g. clay & mica Hydrolysis: reaction between H + a OH -, the products of water molecule dissociation mineral + water = acid + base CaCO 3 + H 2 0 = Ca 2+ + HCO 3- + OH - hydrolysis products form clays
Chemical reaction in soil Carbonation: formation of carbonic acid from dissolved CO 2 gas (from organism respiration) CO 2 + H 2 0 = H 2 CO 3 = HCO 3- + H + Complexation: organic acids from (decomposed OM) interact with metal ions to form organo-metal complexes (chelates)
Chemical reaction in soil Oxidation Loss of electrons, increased (+) valence Example: Fe 2+ Fe 3+ + e - Oxidation releases energy Lost e- must go somewhere, so always paired with a reduction
Chemical reaction in soil Reduction Gain of electrons. Occurs where oxygen supply is low and biological demand is high Example: Fe 3+ + e - Fe 2+ Reduction often consumes H +, decreasing soil acidity (raising ph)
Key things to remember O 2 > NO 3- > Mn 4+ > Fe 3+ > SO 2-4 > CO 2 The longer the soil is saturated, the further to the right the system moves. So VERY little energy to be gained in permanently saturated systems. There has to be an energy source (O.M. or sugars) for any redox to take place, because reduction (think wet) is MICROBIALLY mediated
Chemical reaction in soil Oxidation and reduction in soil, most influenced element is Fe, e.g.: 4Fe 2+ + O 2 + 4H + = 2 H 2 O + 4Fe 3+ Reaction consists of two half reactions: 4Fe 2+ = 4Fe 3+ + 4e - O 2 + 4H + + 4e - = 2 H 2 O oxidation reduction Standard electrical potential of a half reaction is the voltage represented by the flow of electrons when reaction is at equilibrium http://www.fr.ch/mhn/images/mineraux/goethit.jpg
REDOX Together the oxidation and reduction is often called (REDOX) Oxygen is not a single electron acceptor Electron Donors Sulfur (as sulfide, S 2- ) Iron (as ferrous, Fe 2+ ) Nitrogen (as ammonia, NH4 + ) Carbon (as CH 2 O) Electron Receptors Oxygen (as gas) Sulfur (as sulfate, SO 2-4 ) Iron (as ferric, Fe 3+ ) Nitrogen (as nitrate, NO - 3 ) Carbon (as CO 2 )
REDOX potential Result of REDOX reactions is a flux of electrons electrical potential Oxidation Potential Eh (mv) positive if oxidizing, negative if reducing In soils: from -200mV to 750mV
REDOX potential Eh measurements http://www.soil.ncsu.edu/wetlands/wetlandsoils/redoxwriteup.pdf
Dissolution Chemické procesy Higher temperature = higher solubility examples : Gypsum dissolution: Dissociation of Salt NaCl = Na + + Cl - CaSO4 (s) 2H 2 O Ca + 2 (aq) + SO 2-4 (aq) + 2H 2 O (gypsum) (solute) (solute)
dissolving of minerals by water, all previous reactions are dissolution All chemical weathering processes occur simultaneously and are interdependent Hydratation Redox Dissolution Hydrolysis Carbonation Complexation
Layer silicate clays Product of biogeochemical weathering Surface charge; ability to hold and exchange ions; physical properties (stickiness and plasticity)
Clay minerals structures Silica tetrahedron SiO 4+ one silicon surrounded by four O 2- Tetrahedral sheets tetrahedra are joined by shared oxygen
Clay minerals structures Octahedral sheet one Al 3+ or Mg 2+ surrounded by four O 2- or OH - Octahedral sheets Octaherda are joined by shared O 2- or OH - Al 3+ (Mg 2+ ) (OH - )
Clay minerals structures Clay particles are formed by octahedral and tettrahedral sheets stocked one on the other. Isomorhpous substituition: tetrahedral sheets Al 3+ for Mg 2+ octahedral sheets Si 4+ for Al 3+ Unbalanced negative charges
Clay minerals 1:1 Kaolinite group No effective layer charge No internal surface Several sheets form crystal Small specific surface ~15 m 2 /g Kugler, R.L. and Pashin, J.C., 1994, Reservoir heterogeneity in Carter sandstone, North Blowhorn Creek oil unit and vicinity, Black Warrior basin, Alabama: Geological Survey of Alabama Circular 159, 91 p.
Clay minerals 1:1 Kaolinite group
Clay minerals 2:1 vermiculite vermiculite limited shrink-swell vermiculite
Typy jílových minerálů 2:1:1 Chlorite: octaherdal-like sheet of hydroxides forms the interlayer, no swelling Nonexpanding
Clay minerals 2:1 Chlorite
Clay minerals 2:1 Smectite Substitution of Al for Mg Expanding Water and ions adsorption Small crystals Large speficic surface area 800m 2 /g
Clay minerals 2:1 Smectite
Clay minerals summary
Soil Colloids Chemical properties of colloids = chemical properties of soil (adsorb water and ions) Size < 2 µm Large surface area > 10 m 2 /g (outer) až 800 m 2 (inner + outer) Colloids mineral (clay minerals) organic (humus, humic acid) organic-mineral
Colloids Colloids Net negative (adsorbs cations) Net positive (adsorbs anions) Variable charge (depends on ph) ph... positive ph...negative core negative charge Solution Stern layer Diffuse layer Acidoidní koloid
Ca 2+ Ca 2+ Electric double layer Ca 2+ SO 4 2- Ca 2+ Ca 2+ Ca 2+ SO 4 2- Ca 2+ Ca 2+ Ca 2+ SO 4 2- Ca 2+ 2- SO 4 Ca 2+ Ca 2+ Stern layer Diffuse layer solution
... colloid transport Have a look at movies on colloid transport. http://www.bee.cornell.edu/swlab/colloid s/videos/
References http://old.mendelu.cz/~agro/af/agrochem/multitexty/html/agrochemie _pudy/ (in Czech) Kutílek a kol. Hydropedologie, - skriptum (in Czech) Fitzpartick, E.A. Soils Sharma, H.D., Reddy, K.R. Geoenvironmental engineering, Wiley and Sons, 2004 Kugler, R.L. and Pashin, J.C., 1994, Reservoir heterogeneity in Carter sandstone, North Blowhorn Creek oil unit and vicinity, Black Warrior basin, Alabama: Geological Survey of Alabama Circular 159, 91 p.