| Date | May 2013 | Marks available | 1 | Reference code | 13M.3.HL.TZ1.7 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Predict | Question number | 7 | Adapted from | N/A |
Question
Waste water from industrial processes contains a range of toxic substances that are harmful to the environment. These toxins include sulphide (S2–) and metal ions such as chromium (Cr3+). Microorganisms such as Brachymonas denitrificans that carry out denitrification of waste water, may be inhibited by these toxins. The effects of different concentrations of toxins on the rates of denitrification by B. denitrificans and a group of denitrifying bacteria named BPT3 are shown in the graph below.
Predict the Cr3+ concentration that would cause 50% inhibition in BPT3.
Waste water from some industrial processes contains high levels of Cr3+. State, with a reason, which of the bacteria investigated should be used to treat this water.
Compare the effect of Cr3+ and S2– on the inhibition of BPT3.
Raw sewage contains high level of nitrates. Explain the importance of denitrification of raw sewage by bacteria such as B. denitrificans and BPT3 before it is released into rivers.
Markscheme
53 mg dm–3
Accept answers in the range 51 to 54 (units required).
BPT3 because it is less inhibited by Cr3+
a. Cr3+ more toxic than S2– (between concentrations of about 67/70 and 80 mgdm–3);
b. Cr3+ toxicity increases more rapidly with increasing concentration than S2–;
c. Cr3+ requires lower concentration to cause same percentage inhibition (between 40 and 70%) as S2– / vice versa;
d. below (about) 30 (mgdm–3) Cr3+ not toxic whereas S2– effect not known/may be toxic / Cr3+ cannot be compared with S2– below concentration of 70/67 (mgdm–3);
a. denitrifying bacteria convert nitrate to nitrogen/N2;
b. nitrate causes eutrophication/algal blooms in rivers / removing nitrate reduces the risk of eutrophication/prevents algal blooms;
c. high levels of nitrate/algal blooms lead to anoxic conditions / increase BOD / reduces oxygen levels;
d. nitrate is toxic for humans (in high concentrations);
e. removing nitrate reduces the risk of nitrate entering drinking water;
Examiners report
This was the least popular of the HL options but it was encouraging to note some schools studying it with some good standards seen.
Almost all candidates read the graph correctly for the 1 mark.
This was the least popular of the HL options but it was encouraging to note some schools studying it with some good standards seen.
Many candidates correctly identified BPT3 as the bacteria that should be used to treat the water but some could not give a reason so missed the mark.
This was the least popular of the HL options but it was encouraging to note some schools studying it with some good standards seen.
The better candidates were able to correctly compare the effects of sulphide and chromium ions on the inhibition of BPT3. Weaker candidates gave very confused and unclear answers.
This was the least popular of the HL options but it was encouraging to note some schools studying it with some good standards seen.
Many were able to get two marks for why denitrification of raw sewage before release into rivers was important with many mentioning eutrophication, algal blooms and reduced oxygen levels as problems of high nitrate levels.
