water treatment and redox chemisty

steps in water purification

1. aeration

2. settling and precipitation

3. hardness removal

4. disinfection

aeration

removes volatiles and raises the Pe

→ for taste and odor

settling and precipitation

removes colloids by adding coagulates such as aluminum sulfate and ferric chloride to remove clay minerals and hume

→ improves turbidity (look of water)

hardness removal

add phosphate to precipitate calcium phosphate as Ca2+ is bad for pipes

disinfection

remove microbes to prevent disease

optional step

add ammonia and fluoride to adjust the pH and improve dental health respectively

why disinfect?

the spread of pathogenic microbes through water supply poses a serious health concern

3 types of disease causing microbes

1. bacteria → salmonella, ecoli, typhus

2. viruses → norwalk, polio, hep-A

3. protozoans → singe celled animals, giardia lamblia

water disinfection techniques

old methods = boiling and filtering through rocks and soil into deep aquifers (neither are practical on an industrial scale)

modern methods = filtration, reverse osmosis, UV radiation, chemical methods (ozonation, chlorination, bromination)

reverse osmosis

REQUIRES A MEMBRANE → made of an organic polymer

• water is forced through a semipermeable membrane at high pressure, impact side becomes concentrated with contaminants that cant pass through

• can be used to desalinate water, is very effective but is energy intensive, wasteful and creates saline discharge

ozonation

• uses ozone gas which kills everything in its path and reacts with pollutants or generates radicals in water

• just a 10 min contact with O3 destroys bacteria and viruses, but is energy intensive, has no lasting protection (short half life) and produces disinfection by products (trihalomethanes and haloacetic acids)

why are disinfection by products a big problem?

have potential to pose long term human health risks like increased cancer risk

• usually caused by ozone reacting with Cl- and Br-

chlorination

• uses hypochlorus acid HoCl- (can used other chlorine forms as well) as its readily reduced neutral therefore it can diffuse through cell walls and oxidize vital molecules

• has lasting protection BUT can create disinfection by products

why treat wastewater?

eutrophication, disease, drugs, microbes, household chemicals, garbage, smell → wastewater is someone else’s drinking water

steps of waste water treatment

1. primary treatment

2. secondary treatment

3. tertiary treatment

primary treatment

physical treatment (can be chemically enchanted) for the settling of suspended particles (colloids) → sludge settles and grease floats

→ garbage can also be removed in this step

secondary treatment

biological process to convert organic matter to biomass (CO2) followed by settling of products (produces more sludge which poses a disposal problem)

→ reduces BOD by ~90%

tertiary treatment

a variety of advanced processes to remove specific contaminants or for disinfection

advanced chemical processes steps

1. lower phosphate content → to stop eutrophication

2. lower ammonium content → to stop eutrophication

3. remove Ca2+ added for phosphate removal

4. remove organic matter

blue baby syndrome

fertilizers increase nitrate (NO3) concentration in ground water and when humans are exposed it causes respiratory failure

→ NO3- is reduced to NO2- by anaerobic bacteria in the stomach → NO2- oxidizes Fe2+ in hemoglobin to Fe3+ therefore it cannot bind oxygen

drinking water guidelines

canada has guidelines with maximum acceptable concentrations measured in mg/L

→ some compounds like arsenic, cadmium and chromium found in drinking water have been established as carcinogens to humans

most powerful oxidant on earths surface

O2

redox reactions

chemical processes in which e- is transferred from one molecule to another → if something is reduced something else must be oxidized

*not the same as neutralization

most important biological redox reaction

aerobic environments = respiration

energy + CO2 + H2O —photosynthesis—> CH2O + O2

←-Respiration——

photosynthesis = driven by sun

respiration = driven by absence of sun

biological oxygen demand (BOD)

mg of O2 required to carry out oxidation of organic carbon in 1L of water

in aerobic environments respiration provides life supporting redox energy but O2 becomes depleted

1. in rivers it is replenished by contact with air

2. in standing water (lake,pond) dissolution of O2 in water is slow compared to microbically mediated decomposition of dead biomass

crucial reaction in carbon cycling

USES UP O2

CH2O + O2 → H2O +CO2

measurements of BOD

very clean = 1mg O2 / L (has very little organic matter in it)

fairly clean = 1-3mg O2/L

doubtful purity = 3-5mg O2/L

contaminated = >5mg O2/L

how to calculate BOD

incubate a sample at a specific temp in an air tight bottle for 5 days → dissolved O2 (DO) is measured before and after incubation → BOD is calculated from the difference between initial and final DO

O2 graph

longitudinal analysis

used to detect pollution of oxidizable organic matter/pollution using [O2]

what happens when a lake/pond becomes depleted of O2

organisms that rely on aerobic respiration cannot survive → anaerobic bacteria takes over and uses oxidants other than O2 (these alternatives cannot produce as much energy as O2 but bacteria can still survive)

alternative oxidants for decomposing organic matter

used in decreasing order: NO3-, MNO2-, Fe(OH)3, SO42- and CO2

PE

used to characterize the extent to which natural waters are chemically reducing in nature

large negative PE

large value for e- activity in solution → REDUCING CONDITIONS → ANOXIC water bodies (anaerobic populations proliferate)

large positive PE

low e- activity in solution → OXIDIZING CONDITIONS → well AERATED surface water

what does dissolved O2 in water determine

the reduction of O2 to water determines e- activity

how does redox potential fall

In a stepwise pattern as BOD increases

PE/pH diagrams

a 2d plot with PE on the Y and pH on the X → water stability boundaries, H2O has a limited range of PE and pH values in which its stable

Region of stability for water

iron systems

equilibrium between dissolved Fe2+ and Fe3+ is only important for pH <3