Un­der­ly­ing Chem­istry of AMD For­ma­tion

Chemical Industry Digest - - Waste Treatment -

Ox­i­da­tion of pyrite is the first re­ac­tion that re­sults in gen­er­a­tion of fer­rous, sul­phate and pro­tons as shown in Eq. 1. The pro­tons gen­er­ated as a re­sult of this re­ac­tion con­trib­ute to low­ered pH caus­ing acid­ity.

FeS +7/2 O+ HO Fe ++2 SO+2H+(Eq .1)

2 -2

2 2 2 4

Fer­rous iron ox­i­da­tion de­pends on the ox­i­diz­ing en­vi­ron­ment, pH and bac­te­rial ac­tiv­ity. Sev­eral mi­crobes flour­ish in AMD sites (Chen et al., 2016) and con­trib­ute to fer­rous iron ox­i­da­tion. It can be ox­i­dized chem­i­cally when suf­fi­cient oxy­gen is avail­able in wa­ter. How­ever, the mi­cro­bial fer­rous ox­i­da­tion is a faster process. The ox­i­da­tion of fer­rous is shown as be­low:

Fe+ +1/4 O + H+ Fe+ + ½ H O

2 3

2 2

(Fe2+ ox­i­dized by IOB) ( Eq. 2) With the in­crease in pH (greater than 2.3), the Fe+ 3 (as in Eq. 2), pre­cip­i­tate as Fe(OH) as shown in Eq. 3

3

Fe+ + 3H O Fe (OH) + 3H+ ( Eq. 3) 3

2 3 (solid pre­cip­i­tate)

There­fore, the over­all re­ac­tion (com­bin­ing Eq. 1 to 3) will be as in Eq.4.

FeS + 15/4 O + 7/2 H O Fe (OH) + 2SO -2 + 4H+

2 2 2 3 4

( Eq. 4) Sol­u­ble Fe3+ i.e. when it does not tend to pre­cip­i­tate will ox­i­dize the metal sul­phide (FeS ) as in Eq.5. 2 FeS + 14Fe+ 3 + 8 H O 15Fe+ 2 + 2 2 2SO -2 + 16H+ 4 ( Eq. 5) Thus, the over­all re­ac­tion (com­bin­ing the in­di­vid­ual steps) un­der the above con­di­tion will be as in Eq. 6. FeS + 15/8 O + 13/2 Fe+ 3 + 17/2 H O 2 2 2 15/2 Fe+ 2 + 2SO -2 + 17/2 H+ 4 ( Eq. 6) A num­ber of other sul­phide min­er­als (apart from pyrite), shown in Ta­ble. 1, are ox­i­dized and they re­lease met­als. How­ever, they do not suf­fi­ciently/nec­es­sar­ily con­trib­ute to­wards acid­ity in

AMD.

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