Unit 11 - Environmental chemistry

list the four major stages of water treatment.

1. primary settling: for large solids to settle and pH is adjusted

2. aeration: remove dissolved gases, increased oxygen content to oxidize the smelly components

3. coagulation: addition of chemicals to bond to small suspended particles called flocs

4. disinfection chemicals like chlorine or uv light are used to kill inactive pathogens and bacteria before the water is distributed

explain and describe the role of settling aeration and coagulation

describe how activated carbon is used in water purification.

• useful in purifuing water when contaminantion is of small quantitiy, to imporve smell taste and colour. 

• removal of contaminants is through physical adosrption as activation carbon has a very high surface area

• created by anaerobically charring high carbon containing compound such as peat at temps below 600 degrees celsius

explain the process of hardness removal.

• Removal of Ca and Mg

• Ca is removed by adding phosphate ions to make ca precipitate

• Ca is removed by precipiation and filtering of caco3, added as Naco3 to convert biocarbonate to carbonate

• mg precipitates as magnesium hydroxide when water is alkaline (at high oh concentrations)

• after removal by filtration Caco3 and MgOh, the pH of the water is reduced back to normal by added dissolved co2

describe four methods of disinfecting drinking water other than chlorination.

1. membrane technology

2. ozonolysis

3. treatment of chlorine dioxide

4. UV radiation

one advantage and disadvantage of membrane technology

• membrane techology is the use of using a fine membrane and forcing viruses and bacteria to pass through the membrane where the pores are microscopic in szie

• con: doe snot remove bacteria or viruses smaller than the membrane, does not remove ions or cations

• pro: can remove very small particles without chemical additives

one advantage and disadvantage of ozonolysis

• Pro: disinfects water without leaving behind a harsh chemical residue, ozone is produced on site and quite expenive kill sbacteria viruses

• disadvatage: expensive on a small scale and cannot be stored as ozone is unstable

• produces oxygen

one advantage and disadvantage treatment of chlorine dioxide

• eliminates taste and odor from chlorinated phenols and does not produce harmful trihalomethanes

• clo2 acts as an oxidizng agent rather than a clorinating agent like cl2

• dis: toxic causing hemolysis, expensive, and must be produced on site as it cannot be stored

one advantage and disadvantage treatment of ultraviolet radiation:

• UV c emmited light is enough to kill toxic micro organisms like cryprosporidium, the uv c disrupts the DNA preventing replication

• cons: small suspendied particles decrease the efficiency does not provide residual disenfection meaning the micro organisms can grow back after treatment

• pro: kills bacteria viruses and pathogens without adding chemicals or altering taste or smell of the water

one advantage and disadvantage treatment of nanofiltration

• a membrane of pores 0.002 to 10 um diameter is used to remove things from water

• things smaller than it can still pass through

• con is it is very expensive

• important in the fight against certain pathogens

• this is bevcause pathogens are becoming more resistant to chlorination for example

write the balanced chemical equations associated with chlorine and chlorite ion dissolved in water.

cl2 + h2O → HOCL + H + Cl -

chlorite in dissolved water

ClO + H2) → HClO2 + OH-

list the practical sources of HOCl used for disinfecting water.

• c0omes from Cl2 gas which is dissolve din the water

• comes from NaOCl (liquid bleach)

• CaOCl2 acid base reaction to form HOCl

explain the advantage of HOCl versus ClO⁻ as a microbial disinfecting agent.

• HOCL is neutral and clo carries a charge

• microbial membranes are negatively charged clo is repelled while HOCl can enter the liphid bilayer and difuse into the cells

• HOCl can attack internal cell components like proteins DNA much more effectively

• HOCl is also a better oxidizing agent than CLO- and denatures proteins damages dna deactivates enzymes better

• HOCl is a chlorinating agent

• low to neutral pH forms HOcl while higher pH forms CLo- which is not good

explain why pH control of water in a swimming pool is important.

• disenfection of the water needs to be effective as HOCl is mainly used (hjypochlorous acid)

• if pH is too high then CLO- is dominant and disenfection is not as effective

• if pH is lower then HOCl will dominate which leads to better disenfection of the pool

• too low pH will lead to equipment corroding

What is the MCLG

The maximum level at which the contaminant is believed to be safe

describe the formation of chloramines and their role in residual disinfection.

• chloraminea are combined chlorine compounds when free chlorine from hypochlorous acid reacts with ammonia or organic nitrogen in water

• at pH of 7-8 monochloramine dominates (best for disenfectant)

• at lower pH dichloramine and trichloramine form (which causes odor and irritation)

explain why chlorine use in outdoor swimming pools is greater than in indoor pools.

• Sulight degradation of UV breaking down free chlorines and breaks down the key disenfectant species HOCl

• high exposure to environmental contamiannts like dust pollen leaves insects

• these chlroine demanding substances to disenfect and penetrate into the molecule consume free chlorine

• due to the sun the temperature is affected of the pool much more which increases the rate of chlorine reactions and microbial growth

explain the problems of chlorinated phenol and trihalomethane formation associated with using chlorine as a disinfectant.

• drawback of chlorination is production of chlorinated organic substances 

• chlorine reacts with phenolic compounds (organics with an OH group) and form a ring (forms chlorinated phenols)

• have an offensive odor and taste 

• trihalomethanes THMs are a big issue as they form chloroform CHCl3

• cloroform causes cancer and poses a health hazard at very low concentrations

discuss the advantages of using chlorination to disinfect water.

• The benefits strongly outweigh the risks

An advantage chlorination has over disinfection by chlorine dioxide or

ozone or UV is that some chlorine remains dissolved in water once it has left

the purification plant, so that the water is protected from subsequent bacterial

contamination before it is consumed.

• powerful oxidizing agent 

• residual disenfectant (combined chlorine)

discuss the disadvantages of using chlorination to disinfect water.

• formation of THMs (chloroform)

• chlorinated phenols

• strong unpleasant odor sometimes

bromine over chlorine as a disenfectant

Hypobromous acid

• reacts rapidly with dissolved ammonia producing NH2BR which is a good disenfectant

How is chlorination of water achieved

• using concentrated lquid hypochlorous acid bleach or sodium or calcium hypochlorite tablets

• cant get rid of cloudy appearance of water

chlorination plus coagulation

• uses chlorination plus a dry coagulant to get rid of suspended particles chlorination alone cant get rid of

What is SODIS

• solar disenfection

• relies of UV a absoprtion of heat refelcted back to atmosphere to absorb chemical contaminants which can then oxidize the pathogens

differentiate between surface water and groundwater.

• Ground water is found beneath the surface of the earth found in the saturated zone

• surface water include ponds rivers lakes 

explain why groundwater contamination was ignored for a long time.

• groundwater moves very slowly and in all general directions so contamination is spread out 

• out of sight out of mind thought process

describe various regions in the soil in relation to groundwater.

• unsaturated zone

• saturated zone

• artesian (confined aquifers)

• unconfined aquifers

explain why groundwater has been traditionally considered a pure form of water.

• groundwater typically has less microorganisms

• soil filters out harmful particles

describe what is meant by the terms “aquifer” and “artesian aquifer.”

• aquifer is not confined and under pressure

• artesian aquifers are under pressure and are confined

identify the three sources for nitrates in groundwater

• nitrogen fertilizers

• atmopsheric deposition

• human sewage

• soil cultivation

describe two health hazards associated with nitrates in drinking water

• methemoglobinemia (blue-baby syndrome)

• non-Hodgkin lymphoma

• Nitrites can react with amines to form nitrosamines. The nitrosamine N-nitrosodimethylamine (NDMA) is a probable carcinogen.

list three uses of ammonium perchlorate.

• oxidizing agent 

• solid fuel for rockets

• fireworks

• airbags

• explosive and propellant

identify the major source of perchlorate ion in the environment.

• introduces by the disposal of the compound as a result of its limited shelf life

• occurs in chilean nitrate deposits

• detecte din garden fertilizersa

explain why perchlorate is difficult to remove from water.

• like nitrate difficult to remove due to it shigh water soluble anion that is very inert and does not adsorb to activated carbon or mineral surfaces

• primary remediatary strategy is water in ion exchange and biological treatment by bacteria

• perchlorate salts are always water soluble so they are very difficult to remove from water

identify two types of organic contaminants found in groundwater, and give two examples of each.

• chlorinated solvents ( TCE or PCE)

• petroleum products like benzene and toluene xylene

predict the vertical location of organic contaminants in an aquifer, based on their densities.

• water density is 1 g/ml so things with a higher density will sink to the bottom of the aquiefr

• things lighter than water will float to the top of the aquifer

• lateral movement of plumes is also possible 

• chlormnated solvents will sink while benzene products like BTX will float

explain plume formation in an aquifer.

as groundwater carries the contaminants they will be carried with direction of flow until they reach a low permeability layer 

• analogous to a plume of smoke in the air that moves downwind 

explain why BTX and MTBE components of gasoline are commonly found in groundwater.

• solubility of these substances makes them prime candidates for groundwater contamination

• highly mobile in GW and persistent ans hard to remove

• BTX is highly soluble in water (LNAPLS less dense than water)

• low sorption to soil particles so will continue travelling

identify the greatest risk associated with drugs present in groundwate

• Have very low chance to predict how they will affect organisms 

• concentrations for pharmaceuticals is low but synthetic hormones have great risk on the environment

• chlorination is effective at degrading

describe how pump-and-treat systems work.

systems that pump contaminated water and remove the organic contaminants and then return the cleaned water

state the difference between bioremediation and natural attenuation.

• bioremediation only refers to the biological removal of contaminants

• natural attenuation is any biological chemical or physical process that removes groundwater contaminants

explain why natural attenuation is a popular treatment technology.

• the process is free and requires no work or money added

write half reactions and overall reactions for the reductive degradation of chloroorganics.

• degraded in the presence of a reducing agent like metallic iron

• redox reaction takes plce and reduces chloroorganic to the right hydrocarbon

describe the application of an underground permeable wall containing iron for treating groundwater contaminated by chloroorganics.

• use of an underground permeable wall with iron treats contaminated groundwater

• could be a pile of scrap metal udnerground 

list and describe the three stages of treatment of wastewater and sewage

• primary: mechanical where large particles are removedby allowing a sludge to form

• secondary: biological treatment where biologically oxidized water and suspended solids are oxidized by micro organisms to co2 and water converted to additional sludge which is then removed from the water

• tertiary: specific substances are removed from partially purified water before its final disenfection to reduce BOD and removal of dissolved organic compounds like chloroform

describe the use of trickling filters and activated sludge reactors in the secondary treatment of sewage.

• water on a bed of sand and gravel or plastic covered with anaerobic bacteria to create a microorganism driven reaction

• system is well aerated to speed up oxidation

• biofilm where they absorb th eorganic matter and oxidized

state the main disadvantage of chlorinating finished sewage.

• formation of chlorinated organic by products such as trihalomethanes and chlorinated phenols which are harmful to human and aquatic life

list at least three types of tertiary treatments.

phosphate removal

heavy metal removal

iron removal

removal of inorganic ions in excess

further reduction of BOD

removal of dissolved organic compounds

list two methods of further removal of nitrogen compounds from wastewaters.

• using nitryifing bacteria to oxidize all the nigrogen to nitrate then denitryify it to produce molecular nitrogen which bubbles out of the water

• convert ammonium into ammonia and then to bubble it out of solution using oxygen

identify the two major sources of phosphates polluting our water systems.

• detergent 

• agriculture fertilizers

describe the effects of phosphate nutrients on algae blooms and BOD values of a body of water.

• algal blooms reduce the amount of oxygen

• too much phosphorous is bad as it is a limiting nutrient

• phosphorous directly impacts algal growth

• kills fish

list at least two builders found in detergents.

STP

NTA

sodium citrare

sodium silicate

zeolites

explain the roles of a builder in a detergent

• to enhance the detergents cleaning potential

• increases the pH of the water which helps remove dirt from fabrics

how do you remove phosphate?

adding CaOh2 to precipitate the insoluble calcium phosphates 

differentiate between point and nonpoint sources of pollution.

• point source: comes from a concentrated singular source aka a land fill or car wreck

• non point source: comes from a large area such as agricultural run off storm water etc

identify 3 methods of desalinating water

• reverse osmosis 

• distillation (where salt water is evaporated and the the evaporated water has no salt)

• electrodialysis : electric field and ion exchange membranes to separate fresh water from brine

describe how sewage can be treated in an artificial marsh.

• artificial marshes aka constructed wetland use plants like bulrushes and reeds with bacteria and microbes to take the pollutant and concentrate it within their cells

• increases the pH which destroys harmful micro organisms

• ammonia is removed from the water by nitrification by bacteria then denitrification to make it into N2 gas in the anerorobic bttom of the marsh

identify two advantages and two disadvantages of using artificial marshes for wastewater treatment.

• large amounts of land

• decaying vegetation increases BOD

• serves to remove ammonia

• raises pH which will kill bacteria

identify a risk associated with using septic tanks for sewage treatment.

• can lead to nitrate contaminantion which increases by having many septic tanks close together like in rural communities

describe two methods of cyanide removal from wastewater.

• cyanide ion binds strongly to many metals , cyanide is used for mining and electroplating

• does not decompose and is a stable pollutant

• if you acidify the cyanide solution it will release the HCN acid (hydrocanic acid)

• to remove it you need to use oxygen under forcing conditions of a strong oxidizing agent like hydrogen peroxide

• electrochemical proccesses 

describe how transition metal pollutants can be removed from wastewater.

• by using precipication or reduction of the metal pollutant

what are the main risks associated with spreading sewage sludge on agricultural land

• many toxic substances that can be encorperated into the crops and the soil

• sludge is very high in nutrients that plants might need 

• sometimes called biosolids

• can uptake leads aresenic and pesticides etc

• after plant absorption will enter the food supply which is bad

describe how VOCs in wastewater are removed and destroyed.

• air stripping and catalyic oxidation or adsorption can remove VOCs

• air stripping air is passed upward through downward stream of water and VOCs are trasnfered into the gas phase only for non water soluble compounds

describe the principle behind advanced oxidation methods AOMs.

• generate radicals to react with organics to convert them entierly to CO2, water and mineral acids

name the OH radical as the key reactive species in AOMs and identify three methods by which it can be generated.

• hydroxyl free radical which works as an effectivce oxidyzing agent (hydrogen peroxide 

• h2o2 + UV → 2OH

• O3 + UV → O2 + O

• O + h2O → h2O2 + 2OH

identify the biggest liability associated with AOMs.

the formation of toxic by products for example ozone peroxide treated water of trichloroethene and perchloroethene created a high concentration of trichloacetic acid and dichloroacetic acid

explain how photocatalytic methods can destroy organic wastes.

• uses a semicondutor catalyst (TI O2) small particles are suspended in solution

• TIO2 is non toxic resistant to corrosion cheap plentiful and absorbs light in the UV A region 

• produces hydroxyl radicals in all cases 

• this oxidzes and mineralizes organic matter into CO2 H2O and mineral ions

• non selective (oxidizes everything)

list three strong oxidants that can be used for the direct chemical oxidation of organics

• peroxydisulphate (S2O8-)

• peroxymonosulfate anion HSO5-

• ferrate ion FEO42- (Instable)