Oxidation and Reduction Environmental Chemistry

ENVIRONMENTAL CHEMISTRY

Environmental Chemistry

1.

a) An exothermic redox reaction involves the release of energy in form of heat after an oxidation-reduction reaction.

b) The oxidation state of an element represents the charge of an atom after a redox reaction (Khan, n. d)

c) A redox reaction involves a reduction and oxidation reaction where an atom either gains or loses electrons. The process can also involve the gain or loss of oxygen atoms in an atom (Khan, n. d).

d) From the aspects above, it is important to note that energy is only released when electrons are transferred thereby losing their potential energy.

2.

a. During denitrification process which has the following balanced redox reaction: 5CH2O+ 4NO3-+4H+?2N2+ 7H2O+5CO2, CH2O is oxidized to CO2 while NO3- is reduced to N2. In addition, H+ is oxidized to H2O. In both CH2O and H+ there is addition of oxygen atom. In NO3 oxygen atom is reduced. Therefore, it would be prudent to note that NO3- acts as the oxidizing agent while CH2O and H+ are the reducing agents.

b.

i. The part that relates to air-water partitioning happens to be “when water fills the pore spaces, the rate at which oxygen can diffuse through the soil drastically reduces” (Reddy, 2008, pg. 173).

ii. Conditions quickly become anaerobic when soils are filled with water. This is more so the case owing to the fact that the diffusion of oxygen in such conditions tend to be low. In addition, after flooding begins, the rate at which oxygen is depleted increases leading to anaerobic conditions (Reddy, 2008). As a consequence, normal aerobic root respiration is not carried out owing to lack of oxygen that prevents the said process. In addition, availability of toxic materials and plant nutrients in the soil is largely affected. For this reason, plants in such soils must have certain adaptations to survive in the said environment.

iii. The thin oxidized layer of the soil consists of oxidized ions which are inclusive of, but they are not limited to; SO4, NO3-, Mn4+, Fe3+. On the other hand, compounds in the reduced layers of the soil include the sulfides, ammonia, manganous and ferrous salts.

iv. Figure 5.6 is an illustration of what happens during flooding whereby the soil is filled with water. In the inner layer of the soil, ions such as NO3, Mn2+, SO4-2, and Fe2+ are present. The upper layer consists of NH+, PO4-3, and H2S ions. The said figure happens to be corresponding with sequence of redox reactions whereby electrons are lost in one part and gained in the other part. In addition, hydrogen ions are either gained or lost. For instance, reduction occurs when SO4-2 gains an electron into the NO3-. It undergoes further reduction to O2. On the upper later of the soil, oxidation takes place where a hydrogen atom is added to ammonium ion.

3. a. In both benzene and organic matter degradation by bacteria, there happens to be growth of both cultures. Benzene is most likely degraded by bacteria when the reducing conditions are Fe3+/Fe2+. The free energy change at this condition happens to be -3070Kj/mol (Vogt et al., 2011). Given that the relationship between free energy change and redox potential is ?G0=-nFEo, then the redox potential for reduction of benzene would be 3070Kj/mol.

b. The stoichiometric equation for the reaction where Fe3+ is the oxidizing agent is

C6H6?+?18 H2O?+?30 Fe3+?? 6 HCO3-?+ 30 Fe2+?+?36 H+

Concentration of benzene is 64mg/L

1mg=10-3g thus 64mg=0.064g/L

Mass of benzene is given by mass of 6 carbons added to the mass of 6 hydrogen. Therefore,

Mass of C6H6=6×12.01+6×1.008=78.108g

Concentration=mass/volume

Therefore, volume of benzene=mass/concentration

V=78.108/0.064=1220.4375L

Moles=mass/volume

Moles of benzene= 78.108g/1220.4375L

=0.064g/L

The mole ratio for benzene: Fe3+=1:30

Therefore moles of Fe3+ would be 30×0.064=1.92g/L

Mass of Fe3+=55.84g

Volume of iron (III)=55.84/1.92=29.08L

Concentration of Fe3+=55.84/29.08

Therefore, concentration of Fe3+=1.92g/L=1.92×10-3mg/L

4.

a. The concentration of dissolved nitrate reduced in the lake (Jantti, Aalto, and Paerl, 2021). This is more so the case given the dissimilatory reduction of nitrate to ammonium (DNRA). The high concentrations of hydrogen sulfide leads stimulates DNRA rather than denitrification.

b. The concentration of dissolved oxygen in the lake also decreased. This happens to be the case hydrogen sulfide inhibited denitrification process which led to reduction of NO3- to NO2- (Jantti, Aalto, and Paerl, 2021).

c. Iron happens to be in form of iron sulfide (FeS). According to Jantti, Aalto, and Paerl (2021), hydrogen sulfide reacted with iron (II) chemically forming the said compound which happens to be more stable.

5. According to Mitch et al (2001), wetlands can be used to reduce nitrogen contamination. This is more so the case given that denitrification levels in their soils tend to be high thereby creating nitrogen sinks. The said nitrogen sinks are favored by various microbiological processes. Flooded wetlands have mineralized nitrogen in the form of ammonium nitrogen. Mitch et al. (2001) suggest that plant roots can absorb the said nitrogen owing to the fact that it is a component of nitrogen fertilizer. Anaerobic microorganisms can ne able to absorb the said form of nitrogen and convert it to organic matter. Ammonium nitrogen can be deactivated into electronegative soil particles though the process of ion exchange (Mitch et al, 2001). Wetland soils are anaerobic in nature having a thin layer of oxygen. For this reason, further oxidation of ammonium ions is restricted creating a gradient of high and low concentration in the oxidized layer and the reduced layer. As a result, ammonium diffuses upwards into the oxidized layer. In the said layer, nitrification takes place leading to oxidation of ammonium ions to nitrate-nitrogen which happens to be more dischargeable in solutions (Mitch et al., 2001). Further, NO3- becomes the electron acceptor in the process of denitrification where nitrate is lost owing to its conversion to molecular nitrogen and gaseous nitrous oxide.

Second, wetlands are effective in reducing nitrate nitrogen reaching rivers and streams. For instance, creation and restoration of wetlands in rivers helps eliminate watersheds that have a high discharge of nitrogen (Mitch et al., 2001). Essentially, locating wetlands in agricultural sources of nitrogen that have high subsurface drainage would reduce nitrogen load. Third, wetlands are effective at reducing nitrate-nitrogen if the process of denitrification is accelerated. Nitrous oxide (N2O) happens to be one of the greenhouse gases. Restoration of wetlands tends to change the location of nitrous oxides in rivers and lakes. The said restoration of wetlands transfers denitrification from rivers into the wetlands (Mitch et al, 2001).

6.

a) According to United States Environmental Protection Agency (n. d), the percentage concentration of benzene in gasoline is 1.0 vol%.

Molecular weight of benzene=78.11g

% Concentration=weight /volume percentage concentration

78.11×1/100=0.7811g/L=O.00078mg/L

The EPA standard of benzene in drinking water is 5 ppb OR 0.005mg/L while the solubility of benzene in water is 1750mg/L (Agency for toxic substances and disease registry, n. d). Therefore, the concentration of benzene in water does not exceed the EPA standard.

b) At equilibrium, solubility of a gas happens to be directly proportional to its pressure at constant temperature.

Equation for the reaction: 2C6H6+15O2?12CO2+6H2O

The mole ratio of benzene to oxygen is 2:15

Therefore, mole fraction of benzene in water would be 2/15×0.00078=1.04×-4

Partial pressure of gas=Henry’s law constant× mole fraction

Concentration of benzene which is directly proportional to its partial pressure of benzene would be given by: 1.3×10-3×1.04×10-4=1.352×10-7atm

c) Log Koc=0.82×log Kow+0.14

Log Koc=0.82×log2.1+0.14

=0.4042L/Kg

Kd=Kf and Koc=Kfoc therefore, 5% foc would be 5/100×0.4042=0.02021L/Kg

According to Sheppard, Long, and Sanipelli (2009), Kd=concentration of soil/concentration of water. The concentration of benzene in water at equilibrium is 1750mg/L

Therefore, concentration of soil=0.02021L/Kg÷1750mg/L

=1.1549×10-5mg/Kg

d. Concentration of benzene in water=0.00078mg/L

Bioconcentration factor=concentration of benzene in water/concentration of benzene in fish

Therefore, concentration of benzene in fish=137L/Kg÷0.00078mg/L

=175641.02mg/Kg

Mass of benzene=concentration/volume

6 ounces=170097mg

Therefore, mass=175641.02mg/kg/170097mg

Mass=1.0326kg

7.