OXIDATION REDUCTION. Soil Chemistry 9-1

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1 Sil Chemistry 9-1 OXIDATION RDUCTION In sil systems, rapid changes in misture cntent strngly affects sil aeratin status. In additin t changes in bulk cnditins, there are micrsites r zne where diffusin f xygen is restricted, such as small pres filled with water and the interir f aggregates where xygen cnsumptin is mre rapid than xygen diffusin. In areas were there is better access t sil air and pckets f easily decmpsed rganics, and intense decmpsitin can deplete available xygen and prduce a number f redx-active rganic cmpunds. Frm a glbal perspective redx is an imprtant aspect f energy and carbn transfer. Carbn reductin and its subsequent xidatin fuel the bilgical wrld. Carbn reductin r gain in electrns is ften called phtsynthesis by the uninitiated and the reverse xidatin reactin is called respiratin when applied t humans and ther large animals. Micrbial respiratin accmpanies r is synnymus with mineralizatin. Regardless f yur perspective, redx reactins are imprtant aspects f sil chemistry. Redx reactins change the speciatin and slubility f many elements, create new cmpunds and alter the bichemistry f sils. In a cmplex mixture such as sils the interpretatin f redx relatinships is difficult. Since the dynamics f sil xygen which drives the changes in redx ptential are rapid, equilibrium may nt be attained. Frm a thermchemistry view pint redx is nt at equilibrium, because all f the energy yielding cmpunds by definitin cntain excess free energy and are unstable with respect t carbn dixide and water. Prcesses which reduce xygen levels and decrease redx ptentials are driven by micrbial cnsumptin f xygen. Cnditins necessary fr lwering redx ptentials include, a surce f decmpsable rganic materials (energy surce), a ppulatin f micrbes capable f utilizing this energy surce fr metablism, and a restrictin n the resupply f xygen. These requirements are nt unifrmly distributed in sils and sediments. Thus, redx reactins and redx ptentials are nt unifrm thrughut the sil matrix. In fact, redx ptentials are highly variable and therefre are best used as an indicatin r relative status f the sil. Sectin 9- Oxidatin Reductin

2 Sil Chemistry 9- h -ph RLATIONSHIPS Thermchemistry backgrund The Gibbs free energy (G) may be defined in differential frm as: dg = - Sd T - V dp - w (1) where w' is defined as the useful wrk in a chemical system ( nn-pressure, vlume wrk) in ur case this will be the electrical wrk f the system. At cnstant T and P we find: dg = - w () Als remembering that at equilibrium dg = 0 and therefre the useful wrk r electrical wrk in the system is als zer at equilibrium. In an electr-chemical system the wrk is derived frm the transfer f electrns frm ne cmpund t anther and is equal t the ptential and the charge transferred w'= dq, where is the ptential and Q is the quantity f charge transferred. In a chemical reactin where n electrns are transferred per mle f reactant, the electrical wrk is nf where is the emf f the system, F is the Faraday cnstant. Therefre: d G = - nf and d = - G nf (3) (4) Frm thermdynamics it can als be shwn (but nt here) that: 0 dg = dg + R T ln Q (5) Q is the reactin qutient. quating (3), (4) and (5) we find (6) which is the familiar Nernst quatin = - RT nf lnq Other useful relatins are derived frm (4) and the relatinship dg = -RT ln K t btain: - nf = -RTln K (6) (7) Sectin 9- Oxidatin Reductin

3 Sil Chemistry 9-3 Rearranging and substituting gives: lg K = 16.9 n (8) When is expressed in vlts. Slving fr evaluating cnstants and specifying temperature as 98 K expressing the result in millivlts gives: 59. = lg n K The Nernst quatin (6) is the basis fr the measurement f redx ptential in sils and sediments. Replacing with h and with h and writing the reactin as a reductin prduces the familiar equatin used fr sil redx ptential ( h ) shwn in McBride (page 40). h (reduced species) = h - lg (10) + n (xidized species) ( H ) m In aqueus systems the bunds f h is dictated by the stability f water. In xidizing systems the xidatin f water t yield xygen and prtns via the fllwing reactin is the upper bundary fr h. Since the reactin yields prtns, the stability is a functin f ph. (9) H O O + 4 H e - = 1.9 vlts (11) Using the Nernst quatin evaluated at 5 C gives the fllwing: ( ) PO H h = + lg 4 (HO) (1) fr cnditins where the activity f water is taken t be unity the equatin reduces t: P 4 + h = + lg O lg(H ) h = lg O ph P (13) h = lg P ph O Sectin 9- Oxidatin Reductin

4 Sil Chemistry 9-4 Water xidatin depends n bth ph and xygen pressure. If an arbitrary xygen pressure is chsen, than the plt f h vs ph will have a negative slpe f V r 59 mv per ph unit and an intercept f (1.9 vlts). Nte that if (H + ) is 1 mle/liter (i.e. ph = 0), and P O is 1 atmsphere standard cnditins are met and h =, which is the intersectin f the line with the vertical axis in h-ph plts. At the ther extreme f reducing cnditins in aqueus systems, hydrgen in reductin t hydrgen gas is the lwer stability f the system. As written belw fr as an xidatin ptential, the reactin is: H g H + aq + e - = vlts (14) Sectin 9- Oxidatin Reductin

5 Sil Chemistry ( ) h = H + lg PH h = lg H ph P but since = 0 fr the Hydrgen half cell, (15) h = lg P ph H At standard cnditins where P H = 1 atmsphere and ph = 0, h = = 0. Fr a given hydrgen pressure the h decreases with a slpe f vlts per ph unit and has an intercept f Thus the stability limits fr h are bunded by hydrgen and xygen gas evlutin frm water. Limits f 1 atmsphere f xygen and/r hydrgen are nt realistic, hwever they give the bunds fr the system. Other values fr h given xygen levels f 0. atmspheres can easily be calculated and will be parallel t the 1 atmsphere lines. At ph = 0, lg P O = -1.9 is the xygen pressure required fr h t equal 0. In sil and sediment systems reasnable bunds fr ph are between abut 4 and 9, therefre the h - ph relatinships f sils are defined in the regins bunded by these values. Nte that in fr standard cnditins f 1 atmsphere f xygen and ph = 0, O = 1.9 vlts. Since pe = h / r The slpe f the h vs ph line is therefre, as ph increases ne unit h decreases vlts and pe + ph = a cnstant r Similarly fr the hydrgen water cuple, pe + ph = 0. POURBAIX DIAGRAMS These diagrams are an extensin f the ther equilibrium diagrams we have discussed this quarter. In this case the plts are values f r p in relatin t ph. ph is chsen as the independent variable because xidatin reductin reactin are strngly affected by ph and this is mre cnvenient than partial pressure diagrams invlving P O, f P H. Hydrgen ins r hydrxide ins are ften invlved in the reactins and are cnveniently cvered by ph. As an example lets lk at the diagram fr irn superimpsed n the water stability graph discussed in McBride. Sectin 9- Oxidatin Reductin

6 Sil Chemistry 9-6 The necessary equatins are: = 0.771V; p = 13.0 e (1) ( OH ) (s) + 3 ( OH ) pk = sp () ( OH ) (s) + ( OH ) pk = sp (3) The value fr (OH) 3 is that fr sil ferric hydrxide given in Lindsay (1979). Since equatin (1) is independent f ph, it represents a line parallel t the ph axis. + 0 ( ) = lg ( 3+ ) (4) quatin (4) cntains the activity rati f + t 3+, cncentratin is nt needed, hwever the straight line with an intercept at vlts implies that the activity rati f the tw irn species is always = 1. rric irn level and ph will determine the line separating 3+ and (OH) 3. Redx reactins are nt invlved and the line is parallel t the h axis. Chsing a 3+ value will determine the psitin f line A separating 3+ and (OH) 3. K = ( ) K 3+ sp H w Taking lgs f bth sides and substituting - 3 ph fr 3 lg (H + ), fr K sp lg ( ) =.706-3pH; r ph = lg( ) 1 3 (5) and fr K w gives: (6) If 3+ is taken t be 1 mm, separatin between 3+ and (OH) 3 is drawn at ph Sectin 9- Oxidatin Reductin

7 Sil Chemistry 9-7 At the intersectin f the line derived frm equatin (6) and the redx relatin fr + and 3+ (equatin 1), slid phase (OH) 3 can be reduced t +. The redx equilibrium is derived by summing equatins (1) and () and adding 3 H + ins t each side f equatin () t give: 3 HO + (OH ) + 3 H + e (s) (7) This cuple has an f vlts and has the frm: (8) + h = ph lg( ) If the cncentratin f + is taken as 1 mm, then the equatin is: h = ph (9) At the ph where ur assumed irn cncentratin will cause precipitatin f ferrus hydrxide, the relatinship changes t: (OH) 3 + H + + e - (OH) + H O with an f vlts. h = ph (10) Other phases such as carbnates, sulfides and xides culd be added t this diagram as necessary fr the system being cnsidered. Bhn et al discuss these diagrams as des McBride, The mre cmplete discussin is cntained in Slutin Minerals and quilibria by Garrels and Christ (1965). (Freeman Cper. San Francisc, CA ) This bk is ut f print, hwever if yu ever get a chance t pickup a used cpy d s!! It is an excellent intrductin t gechemistry. All f the thermdynamic data is based n the infrmatin given by Sadiq and Lindsay in Lindsay Sectin 9- Oxidatin Reductin

8 Sil Chemistry 9-8 Figure 9.1 Purbaix diagram fr irn in relatin t h and ph. Sectin 9- Oxidatin Reductin

9 Sil Chemistry 9-9 Review questins 1. Flded sils may r may nt exhibit lw redx ptentials. What are several reasns fr this bservatin?. If sulfides were present in a sil, what might yu infer abut the nitrate and manganese levels? 3. Why are sils usually nt at redx equilibrium? 4. Why d irn and chrmium xidize mre readily at higher ph levels? D they? Hw wuld yu knw? 5. Irn phsphates are cnsidered t be mre prblematic as surces f P in aquatic systems that are reduced. Why? What ther infrmatin wuld be useful t answer this questin fully? 6. Why d reduced sils becme mre alkaline? 7. What cnditins cause irn and manganese staining f tilet bwls? 8. Can yu determine the chemical reactins taking place frm a measurement f sil ph and h? 9. What are the bunds fr h and ph in aqueus systems? 10. What type f wrk is dne in redx cells? 11. Is the h f a sterilized sil likely t be high r lw? 1. What cmpunds and/r elements are the electrn acceptrs and dnrs in the reductin f nitrate? 13. Why is it said that redx is a cupled reactin? Sectin 9- Oxidatin Reductin