KOPIEREG VOORBEHOU / COPYRIGHT RESERVED

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1 KOPIEREG VOORBEHOU / COPYRIGHT RESERVED UNIVERSITY OF PRETORIA / UNIVERSITEIT VAN PRETORIA DEPARTMENT OF PLANT PRODUCTION AND SOIL SCIENCE / DEPARTEMENT PLANTPRODUKSIE EN GRONDKUNDE GKD 320 SOIL CHEMISTRY / GRONDCHEMIE Internal examiner / Interne eksaminator:mr P.C. de Jager External examiner / Eksterne eksaminator:mr Teunis Vahrmeijer Semester test II / Semestertoets II October / OKtober 2011 Time /Tyd: 120 min Marks / Punte : 50 Question 1 / Vraag 1 1.1) An ash coloured B horizon in a wetland smells like rotten eggs. Discuss the redox status of this soil horizon with reference to the expected Eh and pe ranges and the elements that can potentially be the electron donors and acceptors under these conditions. / ʼn Gryskleurige B horison van ʼn grondprofiel in ʼn vleiland het ʼn vroteier reuk. Bespreek die redoksstatus van hierdie horison met verwysing na die verwagte Eh and pe rykwydte en die moontlike elektronskenkers en ontvangers onder hierdie toestande. (10) 1.2) Briefly discuss the concept of electron lability proposed by Bartlett (1999) / Bespreek kortliks die konsep van electron labiliteit wat voorgestel is deur Bartlett (1999). (6) 1.3) Based on the various redox categories proposed by Bartlett (1999), rank the following soils in terms of decreasing electron lability: a) redoxic, b) manoxic and/ or sulfidic. / Gebaseer op die verskillende redoks katagorieë rangskik die volgende gronde in terme van afnemende electron labiliteit; a) redoksies, b) manoksies and/ or sulfidies. (3) 1.4) Which of the abovementioned soil or soils do you think will be the better growth medium for plants? / Watter van die bogenoemde grond of gronde dink jy sal n beter groeimedium wees vir plante? (1) (20) 1.1) The horizon is highly reduced or anoxic (1 mark). It is expected that the pe would be less than 2 (1 mark) and the Eh < -120 mv (1 mark). The rotten egg smell is gaseous H 2 S (1 mark). The presence of H 2 S indicates that SO 4 2- is the main electron acceptor (1 mark) and in the process gets reduced to H 2 S (1 mark). Considering the wetland conditions, the electron donor is most likely organic material (1 mark). The wet environments are conducive for organic material accumulation (1 mark). Waterlogging is conducive for the development of reduced conditions (1 mark). Oxygen is easily depleted under these conditions (1 mark). If nitrate is presence, it will be the next oxidant the system will use (1 mark). Followed by Mn 3+, Fe 3+ (1 mark) and if all these oxidants were reduced the system will use SO 4 2- (1 mark). 1.2) Free electrons do not exist like free protons (1 mark). Therefore, calculating electron activity and expressing it as a negative logarithm is view by some as not reflecting the reality in the soil (1 mark). Eh can only be measured directly when the soils are submerged (1 mark). (Soils are more often unsaturated). These classic redox approaches also assume the system is at equilibrium, which the soil often is not (1 mark) Bartlett proposed the concept that takes in account the kinetics (how fast or slow) reactions will is likely to take place (1 mark). Thermodynamically some oxidants appear to be strong. However, they can be locked in a crystal structure (for example Fe 3+ in a goethite) and kinetically can be quite slow (1 mark). (Thermodynamics deals with processes at equilibrium, you cannot discern from it how long it will take for the reaction to reach equilibrium). The el concept deals with the balance between electron donors and acceptors (1 mark). Soils rich in electron donors that is labile (easily available for reactions) has a high el (1 mark) and soils poor in electron donor or if it is locked in a stable compound that makes it kinetically slow (for example humus) will have lower el (1 mark). 1

2 1.3) Sulfidic, followed by redoxic and manoxic (3 mark) 1.4) Manoxic (1 mark) Question 2 / Vraag 2 Struvite (MgNH 4 PO 4.6H 2 O) is a mineral that can be synthesised from sewage sludge and can potentially be used as a fertilizer. / Struviet (MgNH 4 PO 4.6H 2 O) is ʼn mineraal wat van rioolslyk gemaak word en die potensiaal het om as kunsmis gebruik te word. 2.1) Calculate the P content of struvite (on a mass basis) and express it in mg kg -1 and % basis. / Bereken die P inhoud van struviet (op ʼn massabasis) en druk dit uit in mg kg -1 en % basis. (4) 2.2) What criteria will you consider when evaluating the suitability of a mineral as a fertilizer? / Watter kriteria sal jy gebruik om te bepaal of ʼn mineraal moontlik as ʼn kunsmis gebruik kan word? (4) 2.3) Will struvite be more soluble in acid or alkaline soils? Answer the question by calculating its solubility at ph 4 and 7? State all your assumptions. (Hint: You can assume only phosphate undergoes hydrolysis, however, sound reasons for this assumption must be given). Sal struviet meer oplosbaar wees in suur of alkaliese gronde? Antwoord die vraag deur die oplosbaarheid van struviet te bereken by ʼn ph van 4 en 7. Gee al jou aannames. (Wenk: Jy kan aanneem dat slegs fosfaat hidroliese ondergaan maar goed gefundeerde redes moet hiervoor gegee word.) (14) (22) 2.1) Molar mass = g mol -1 (1 mark) Considering a mole of struvite % P = g / g = 12.6 %(1 mark) P content on a mass concentration basis: 12.6 % = 12.6 g per 100g = mg kg -1 (2 mark) 2.2) I gave marks for any facts that are logical and make sense. A good nutrient source will be water soluble (1 mark) under all ph conditions (1 mark). It should not have an acidification effect or generate extreme alkalinity (1 mark). It should not contain elements that are potentially toxic to plants (1 mark). 2.3) Assumption (2 mark) a) The activity of water and struvite is taken as unity (2 mark) b) The pka values of NH + 4 (9.25) and Mg 2+ (11.45) suggest that hydrolysis will only occur in very alkaline conditions (1 mark). 2

3 3

4 Alternative way to calculate the solubility: 4

5 Question 3 / Vraag 3 Which form of arsenic (+3 or +5 oxidation state) is considered more toxic and what are the reasons for this? / Watter vorm van arseen (+3 of +5 oksidasie toestand) word beskou as meer toksies en wat is die redes daarvoor? (8) Arsentite, As(OH) 3 0, is considered environmentally more toxic (2 mark). Under anoxic conditions (pe + ph < 9, or pe< 2 at ph 7) arsenite predominates (1 mark). Arsenite is a neutral complex in solution and more soluble (1 mark).it is susceptible to methylation making it lipophilic and easily assimilated by living organisms (1 mark). Environmental remediation of arsenate is less of a challenge (1 mark). It can be removed from water and the soil solution by means of sorption and precipitation (1 mark). A strong analogue exsist between the chemistry of arsenate (AsO 4 3- ) and phosphate (PO 4 3- ) (1 mark). Information sheet / Informasie blad Thermodynamic data Struvite solubility / Struviet oplosbaarheid MgNH 4 PO 4.6H 2 O (s) +2H + (aq) Mg 2+ (aq) + NH 4 + (aq) + H 2 PO 4 -(aq) + 6H 2 O (l) log K 1 = 6.40 Hydrolysis of magnesium / Hidroliese van magnesium Mg 2+ (aq) + H 2 O (l) MgOH + (aq) + H + (aq) log K 2 = Hydrolysis of ammonium / Hidroliese van ammonium NH 4 +(aq) + H 2 O (l) NH 3(aq) + OH - (aq) log K 3 = Hydrolysis of orthophosphate / Hidroliese van ortofosfaat H 3 PO 4(aq) H 2 PO 4 -(aq) + H + (aq) log K 4 = H 2 PO 4 (aq) HPO 4 2-(aq) + H + (aq) log K 5 = HPO 4 (aq) PO 4 3-(aq) + H + (aq) log K 6 =