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Arsenic contamination of Bangladeshi groundwater arises from a combination of three factors, namely, the source, mobilisation and the transport of Arsenic.
The pandemic arsenic pollution in the Bengal Basin was probably caused by superimposed effects of the preferential entrapment of arsenic in organic-rich deltaic sediments when the sea level rose, impeding entry of air to the aquifier, whilst microbial oxidation of the organic carbon depleted the dissolved oxygen in the groundwater. Arsenic mobility in the aquifiers is largely controlled by adsorption and desorption reactions and by precipitation and dissolution reactions.
Attachment of arsenic to an iron oxide surface is an adsorption reaction, whereas the release of arsenic from such a surface is a desorption reaction. Formation of a solid phase from components present in aqueous solution is Solid-phase precipitation whereas dissolution of volcanic glass within an aquifer is solid-phase dissolution. Arsenic adsorption and desorption reactions are influenced by changes in pH, occurrence of redox reactions, presence of competing anions, and solid-phase structural changes at the atomic level. Solid-phase precipitation and dissolution reactions are controlled by solution chemistry, including pH, redox state, and chemical composition.
Pyrite and arsenopyrite are absent or very rare in aquifer sediments from the Bengal Basin. This and the low concentration of sulphate in groundwater go against the postulated pyrite oxidation hypothesis. Arsenic gets preferentially entrapped in the organic-rich, argillaceous flood-plain and delta sediments, and any delta or floodplain that developed into marshland or swamp is prone to contain As-contaminated groundwater.
Three mechanisms are generally used to explain the release of arsenic into ground water. These are reductive dissolution of FeOOH and release of sorbed arsenic to groundwater, oxidation of arsenical pyrite, competitive exchange of sorbed arsenic with phosphate from fertilizer. However, hardly any of the arsenic pollution is due to the fertilizer-phosphate or pyrite oxidation. On the other hand, it is thought that the distribution of arsenic contamination is due to the distribution of buried peat deposits which drives the reduction of FeOOH, which is not limited by geography or time. (335 words)
Question 1 (b)
Table 1: Frequency table for the Cook Inlet Basin groundwater wells as sampled by the USGS. The Kanai Peninsula Borough has been completed, as an example.
Kanai Peninsula Borough
Total number of wells
Number of wells showing As levels above WHO limits
% of wells showing As levels above WHO limits
Average As concentration
* Number of Wells showing As levels above WHO Limits:
†Calculation of Average As concentration/µg l-1 for the Municipality of Anchorage:
‡Calculation of the % of wells showing As levels above WHO Limits::
Question 1 (c)
Arsenic distribution in environment is complex, due to the chemical forms in which it may be present, and because of the continuous cycling of its different forms through air, soil and water. Most arsenic in the atmosphere comes from high-temperature processes such as coal plants, burning vegetation and volcanic activity. The arsenic is released into the atmosphere as arsenic trioxide where it adheres onto the surface of particles which are dispersed by the wind and eventually fall back to the earth.
Natural, low-temperature biological reactions involving microbes also release arsenic into the atmosphere. Microbes acting on arsenic in soils and sediments generate arsine gas which reacts with oxygen in the air and is converted back to non-volatile forms of arsenic.
In well-oxygenated water and sediments, arsenic is present as arsenate. Some chemical forms of arsenic adhere strongly to clay and organic matter and can affect how they behave, giving a potential for arsenic to be released from water and sediments.
Lastly, weathered rock and soil containing arsenic may be transported by wind or water. Since many arsenic compounds tend to adhere strongly to soils, water percolating down does not usually move arsenic through more than a short distance in soil. (200 words)
Question 1 (d)
Local variability in arsenic concentrations depends on well design, dept, and presence of peat in sediments. Drilled wells are sealed and water enters through the bottom of the tube, so contaminated water from higher levels is excluded.
Wells drilled to layers where peat is absent are unlikely contaminated since anaerobic bacteria that encourage dissolution of Fe (OH)3 and release of arsenic do not have sufficient dead tissue to survive. Wells drilled into organic-rich layers are contaminated due to the dissolved Fe (OH)3 present here. Wells drilled into aerated zones have low levels of arsenic since Fe (OH)3 is stable. (99 Words)
Question 1 (e) (i)
Question 1 (e) (ii)
Question 1 (f)
Question 2 (a)
The diagram above was extracted from the link: http://www.colorado.edu/intphys/Class/IPHY3430-200/image however i have added some labels to the diagram.
Muscle contraction is controlled by motor neurons that release the neurotransmitter acetylcholine at the neuromuscular junction. This is the place in the body where axons of motor nerves meet the muscle and transmit messages from the brain which cause the muscle to contract or relax. Acetylcholine is synthesised by the motor neurons and stored in synaptic vesicles at the axon terminal. An action potential at the axon terminal causes voltage-gated calcium ion channels to open and increase the calcium permeability of the presynaptic terminal cell membrane. Calcium ions enter the presynaptic terminal and cause the synaptic vesicles to release the acetylcholine into the synaptic cleft.
The acetylcholine diffuses across the synaptic cleft and binds to the nicotinic receptors on the postsynaptic membrane of the muscle fibre causing and increase in the permeability of the ligand-gated sodium ion channels. The movement of Sodium ions into the muscle cell results in depolarisation if the postsynaptic membrane. Once a threshold has been reached, a postsynaptic action potential is generated and propagated over the muscle cell membrane resulting in contraction. The acetylcholine is rapidly broken down into acetic acid and choline in the synaptic cleft by the enzyme acetyl cholinesterase. The choline is then reabsorbed into the presynaptic terminal where it combines with acetic acid to form more acetylcholine which enters the synaptic vesicles.
The active ingredients in some natural and synthetic drugs are alkaloids. Alkaloids such as tubocurarine and nicotine show structural similarity to neurotransmitters in that they too possess positively charged nitrogen atoms linked to methyl groups. This structural feature allows the alkaloids to bind at the acetylcholine receptor on the surface of muscle cells and thus preventing the acetylcholine from interacting with its receptors. The can result in either an antagonistic or an agonistic effect. Tubocurarine does not induce muscle contraction like acetylcholine, and is thus known as an acetylcholine antagonist. On the other hand, nicotine mimics the action of acetylcholine causing an action potential to be generated in the muscle cell membrane resulting in contraction. Thus, nicotine can be classified as an acetylcholine agonist. (T4 pgs 36-37). (345 words)
Question 2 (b)
Many medicinal drugs like silphion and nicotine come directly from the plant kingdom. Others, like aspirin are simply derivatives of botanical natural products. Aspirin comes from the pain killer salicylic acid which was originally extracted from the bark of willow trees, whilst analgesics such as morphine are derived from the opium poppy. There may be many more cures for diseases provided by plants, waiting to be discovered. But how do researchers go about in identifying plants that may contain medicinal products? In a world where financial resources are limited, it would be impossible to screen every plant species.
Modern research on plants can be possibly done in various ways. It is possible to research medicinal properties of plants by studying the molecules of plants that are closely related to plants already known to produce useful compounds. Ecological surveys can also be done by selecting plants from particular habitats or that display particular characteristics that indicate that they are capable of producing substances that could be exerting and effect on animals. For example plants that seem to be immune from predation from insects could indicate that the plant is producing substances that are toxic to insects. Such substances could show bioactivity in humans meaning that they could be capable of achieving some therapeutic effects.
Furthermore, a possible approach could be that of ethno botany, which assumes that the indigenous use of plants can offer strong clues to the biological activities of the plant in particular. Indigenous people possess a vast body of knowledge that is of great value. Their creativity is reflected in the rich knowledge systems they have developed to conserve and carefully utilize earth's resources. This knowledge has been passed on from generation to generation not by being written down, but through oral transmission. In my opinion, this could possibly be the most productive of plant surveying methods. (309 words)
Question 2 (c)
People have been using herbal remedies for ages. From the earliest recorded history, human beings have used items from the environment to cure illness and treat symptoms. The most effective of these remedies are still in use today, but they are no replacement for traditional medicine, and naturally occurring herbs or plant must not be considered a primary care option.
Herbal medicine has become increasingly popular in recent years with many patients using them regularly, often without first consulting with their doctor. While many herbal remedies are safe, it is important to understand that there are risks involved when using them and a large number of people could be endangering their health by taking herbal medicine without having previously checked with a health professional. Furthermore, because many herbal medicines have significant pharmacological activity, and therefore potential adverse interactions, healthcare professionals and doctors must be familiar with the therapeutic modality of the substance.
The belief that all herbs are safe due to the natural occurrence is widespread. However, these are far from being free of side effects. Some products are known to be safe, such as natural skin care product ingredients like aloe which is helpful for dry skin and is otherwise relatively harmless. Despite this, even when a natural remedy is safe, it may not always be effective. Using an ineffective remedy for treating serious medical conditions can make the underlying condition worse, and failure to treat the condition is a major cause of complications from use of natural remedies.
Some plants may contain compounds which are toxic or poisonous like for example hemlock. On the other hand, extracts from poisonous plants could be useful if used adequately. Although risk of death is very small, some herbal remedies interacting with other medications result in severe consequences. An example of this is the famous St John's wort that for many hundred years was used for healing wounds. It was found to make many prescription drugs used to treat conditions of heart disease, seizures and depression less effective. It is thus important to look out for harmful drug interactions and try to minimize them. People also run additional risks because they tend to diagnose an ailment themselves and take what they think is appropriate resulting in the danger of incorrect diagnosis and thus taking the wrong remedy. (T4 pg 57-59)
An additional risk in herbal remedies is that although the herb in it self may be safe, it may be contaminated with, or replaced by a much more dangerous substance. This can happen by mistakes being made by consumers or suppliers of the herbs as in the case reported by the Yorkshire Gazette of Saturday, 15 October 1825 page 2 where a woman called upon a druggist to purchase preparative medicine for the small pox, but she mistakenly pick up a small parcel or arsenic that had been wrapped up for someone else (T3 pg 7). This however, can also occur deliberately - in the 19th century, arsenic was well known as a deadly poison, tasteless, and murder by arsenic poisoning could be disguised as natural death from cholera. (T3 pgs 7-8).
The risks involved in natural plant formulations are mainly due to their unknown aspects, and it could be due to the main active ingredient as well as to the secondary ingredients. Risks from prescribed drugs are usually know, described and can be discussed with the pharmacist issuing the drug. On the other hand, although there may be information associated with the natural plant formulations, the user is required to assess and interpret the risks. It is thus recommended that any herbal remedy should always be purchased from places where professional health advice can be given and discussed prior to administration. Further more, by becoming an educated consumer, and with the help of the medical doctor, one would be able to make an informed decision about using herbal remedies, and then decide which option is the best. (646 Words)