Question for chemists on sulphates

[JonClarke]

Opportunity has detected the presence of gypsum, Mg-sulphate and jarosite (Fe-Al sulphate at Meridiani, as well as haematite. Jarosite forms only under fairly acidic conditions (pH <4) Most modelling I have seen on salts formed on Mars have ignored pH as a factor as the dominant chloride and sulphate salts are not considered pH dependent (http://www.geology.yale.edu/~ajs/1999/07-09.1999.10Morse.pdf). As a result the co-occurrence of these sulphates is generally taken as confirmation of acid brine models for Martian water chemistry. <br /><br />However, my limited knowledge of chemistry (largely rule of thumb) suggests this assumption is not correct. Mg-sulphates on earth are formed from near neutral marine-derived waters in evaporitic ponds. They are also found in neutral to alkaline saline soils. Epsomite is known to dissolve in acid mine leachate when the pH falls to <5, this is consistent with its use to remediate acid sulphate soils (pH <4) where its addition buffers the pH to more acceptable levels ( />4).<br /><br />This is important with respect to mars in that I suspect that Mg-sulphates are not likely to coexist with jarosite in any large water body. As the Ca and Mg sulphates at Meridiani are clearly very early, to me it seems logical to assume that the waters that deposited these sediments was of relatively low acidity. The higher acidity associated with the jarosite must therefore be a later feature, perhaps associated with weathering. This is supported by the formation of jarosite in modern weathering in the Antarctic.<br /><br />Despite this I have not been able to find any diagram or table that shows the stability of Mg-sulphates with respect to pH. Does anyone have such a table or diagram? Am I onto something here, or simply full of it?<br /><br />Thanks<br /><br />Jon<br /> <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[thechemist]

Well, Jon, this is no easy task, at least for me <img src="/images/icons/wink.gif" /><br />Have a look at this until I can dig up more :<br /><br />"Crystallization kinetics of epsomite influenced by pH-value and impurities"<br />Sattar Al-Jibbouri, Christine Strege and Joachim Ulrich<br />Journal of Crystal Growth<br />Volume 236, Issues 1-3 , March 2002, Pages 400-406 DOI<br /><br />-----------------------------------<br />Abstract<br />The influence of different additives and pH-values of the solution on the crystallization kinetics of Epsomite (MgSO4·7H2O) has been investigated in a fluidized bed crystallizer. For detection of changes in the metastable zone width, an ultrasonic technique has been used. In the experiments the supersaturation (0.2–2 K), the impurity concentration (1–5 wt%) as well as the pH-value of the solution (2.5–8.6) have been varied.<br /><br />The growth rate of MgSO4·7H2O crystals is related to the width of the metastable zone and supersaturation. The effects of the pH-value of the solution and the examined additives (KCl, K2SO4, MgCl2, and NaCl) can be divided into thermodynamic effects and kinetic effects. In the first case the impurities exert influence on the saturation and supersaturation limit whereas in the latter case the crystal growth is being suppressed. The thermodynamic properties of the concentration solutions are considered to explain the results. <br />-----------------------------------<br /><br />From this paper's experimental a reference to a book:<br />"Saturated solutions of Epsomite at 25°C are prepared according to the solubility data of Mullin [3]. "<br />3. J.W. Mullin Crystallization, Butterworth-Heinemann, London (1993). <div class="Discussion_UserSignature"> <em>I feel better than James Brown.</em> </div>

 

[thechemist]

Jon,<br /><br />Fig.11 on this paper has some data that might be what you're looking for. <br /><br />"A molal-based model for strong acid chemistry at low temperatures (<200 to 298 K)"<br />by Giles M. Marion<br />Geochimica et Cosmochimica Acta<br />Volume 66, Issue 14 , 15 July 2002, Pages 2499-2516 [doi] <br /><br />This paper includes a simulaiton of Europa's low temperature acid brines also.<br />Since this is a science forum, I attach Fig.11 here, I hope Elsevier does not mind much <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <em>I feel better than James Brown.</em> </div>

 

[thechemist]

Jon,<br />I 've read the Morse article you provided, and it seems to me that pH is included in the calculaitons there.<br /><br />"In the past, FREZCHEM did not consider pH because none of the chloride and<br />sulfate salts are pH dependent (table 1). In order to assess more accurately the roles of<br />evaporation and freezing of brines containing carbonates, incorporation of carbonate<br />chemistry into FREZCHEM necessitates that pH be explicitly considered."<br /><br />It looks to me that the Gitterman pathway in Fig. 11 corresponds to what comes closer to the rovers mineral findings, at least in Meridiani. <div class="Discussion_UserSignature"> <em>I feel better than James Brown.</em> </div>

 

[JonClarke]

Thanks TheChemsit.<br /><br />For a non-chemist, how does H2SO4 molality convert to pH?<br /><br />I agree Morse did try to incorporate pH, but only with respect to carbonate, it was one of the improvements that paper offered over earlier efforts. But it seemed to consider it only in terms of carbonate, not the Mg sulphates.<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[thechemist]

Hi Jon,<br />I have actually not provided much useful data.<br />H2SO4 is a strong acid thus these simulations deal with <b>very</b> acidic conditions.<br />(For example a H2SO4 molality of 0.01 would give a pH of -log(0.01)=2).<br /><br />With respect to the Morse paper, it should be realized that pH is a property of the whole brine solution. pH is either incorporated in the complex scheme of ionic equations of the model or not. <br /><br />I'll keep looking for data ... <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <em>I feel better than James Brown.</em> </div>

 

[silylene old]

TheChemist: Nice work!<br /><br />I can check SciFinder tomorrow if I am not too busy. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>

 

[Maddad]

Jon,<br />I sent an email to my chemistry professor at school, Mary Ann Beaston. She was the stinker who gave us test questions like calculating the mass of reactants in a hypothetical Star Trek fuel. Anyway, tomorrow is the last day of final exams (I took my last this morning), so she might be at school to see this and respond. Good luck.

 

[thechemist]

If I may add another reference :<br /><br />"The electrolyte NRTL model and speciation approach as applied to multicomponent aqueous solutions of H2SO4, Fe2(SO4)3, MgSO4 and Al2(SO4)3 at 230–270 °C"<br />Ali Haghtalab, Vladimiros G. Papangelakis and Xuetang Zhub.<br />Fluid Phase Equilibria<br />Volume 220, Issue 2 , 30 June 2004, Pages 199-209. (DOI)<br /><br />From the abstract : <br /><i>"This work presents chemical modeling of solubilities of metal sulfates in aqueous solutions of sulfuric acid at high temperatures. Calculations were compared with experimental solubility measurements of hematite (Fe2O3) in aqueous ternary and quaternary systems of H2SO4, MgSO4 and Al2(SO4)3 at high temperatures. "</i><br /><br />Have not looked at it yet but it sure looks interesting. <div class="Discussion_UserSignature"> <em>I feel better than James Brown.</em> </div>

 

[thechemist]

Jon,<br />Here is the general picture I got after some relevant info lookup.<br />MgSO4 is a very soluble salt, thus the effect of pH on its solubility in the pH ranges expected in acid brines will be extremely small. The figure presented some posts above shows that even in very concentrated H2SO4 solutions, it remains very soluble.<br /><br />Even the partially soluble sulfates, like CaSO4, will not have their solubility much increased by lowering pH, because of the high acidity of the HSO4(-) ion.<br /><br />Thus, I tend to think that the low pH needed for jarosite to form cannot exlude its concurrence in acid brines with sulfates. <br /><br />Carbonates, on the other hand, become more soluble in acidic solutions because of the weak acidity of carbonic acid H2CO3.<br /><br />Cheers !<br /><br /> <div class="Discussion_UserSignature"> <em>I feel better than James Brown.</em> </div>

 

[yurkin]

You guys really know your sulfates. <img src="/images/icons/smile.gif" />

 

[JonClarke]

So if pH does not effect sulphate solubility, why is epsomite used to remediate acid soils (and stomachs)? Is there another process happing here?<br /><br />cheers<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[thechemist]

Epsomite is medically used as a laxative and to treat pre-eclampsia, eclampsia and preterm labor. Magnesium hydroxide Mg(OH)2 is used to mediate acidity in the stomach, if I remember correctly.<br />Do you have any references for the use of epsomite to treat acid sulfate soils ?<br />I could not find any myself.<br /><br />However, I found this very interesting (and aussie !) article that explains the creation of acid sulfate soils :<br />http://www.dlwc.nsw.gov.au/care/soil/ass/what_are_acid_sulfate_soils.html<br /><br />As it is explained in there, gypsum and epsomite are produced when components of the soil react with H2SO4 and neutrealize it. If not enough neutrealizing material exists or the acidity is very big, the pH falls below 4 and if the environment is also highly oxidative, jarosite is produced (Fe3+ ions need to be soluble to form jarosite, and this requires pH<4). <br /><br />So my conclusion is that if a soil layer contains only epsomite and gypsum, acidity stayed />4. If jarosite is also included in the layer, pH dropped lower than 4. If two layers (with and without jarosite) are found, these were caused by two separate water episodes of different acidity.<br />Have a look and let me know !<br />And thanks for this entertaining thread, I've learnt some geology !<br /><br />Cheers ! <div class="Discussion_UserSignature"> <em>I feel better than James Brown.</em> </div>

 

[JonClarke]

That is the only source I know. I read that to indicate that eposomite was used to control ASS, but I can see now that perhaps it simply can act as a natural buffer. Limestone or dolomite would certainly be cheaper. As an aside the best way to remediate ASS is not to let them develop in the first place, unfortunately human greed being what it is....<br /><br />Anyway, epsomite does buffer the pH in such situations, the question is how?<br /><br />I could not find the site that talked about the use of epsom salts for acid stomachs, but I did, on a school site, find this reference to the pH of a epsom salt solution being 6.5-7 http://www.wpbschoolhouse.btinternet.co.uk/page03/AcidsBasesSalts.htm<br /><br />Cheers<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>