Carbonate and Silicate Minerals: Role in Acid Mine Drainage Neutralization

 Carbonate Minerals

Primary neutralizers in AMD are those minerals containing carbonates such as calcite (CaCO3), magnesite (MgCO3), dolomite (CaMg(CO3)_2), and ankerite (CaFe(CO3)_2) (Lapakko, 2002; Lottermoser, 2010). Dissolution of these carbonates consumes H^+ and release Mg, Ca, (HCO_3)^-, as well as H_2 CO_3. The following equations are dissolution of calcite minerals with acid at different pH. These reactions can reverse if there is loss of (g) or H_2 O_((l)) or change in temperature (Lottermoser 2010).

at pH>6.4: CaCO_(3(s))+(H^+)_((aq))→ ((HCO_3)^-)_((aq))+(Ca)^(2+))_((aq))

at pHCaCO_(3(s))+(H_2 (SO)_4)_((aq))→ CaSO_(4(s))+(CO)_(2(g))+H_2 O_((l))

In these reactions, one mole of acid is neutralized by mole of calcite.

These reactions indicate that the pH of the water will remain neutral until primary carbonate depleted and minerals with lower solubility such as siderite (FeCO_3(s) )form the next pH buffering plateau.

at 4.8≤pH≤6.3: FeCO_(3(s))+(H^+)_((aq))→ ((HCO_3)^-)_((aq))+((Fe)^(2+))_((aq))

When siderite has become depleted, the dissolution of gibbsite Al(OH)_3(s) will maintain the water pH

at 4.0≤pH≤4.3: Al((OH))_(3(s))+3(H^+)_((aq))→ ((Al)^(3+))_((aq))+3(H_2 O)_((l))

Upon the dissolution of the Al hydroxide phase, the dissolution of Fe oxyhydroxides will be the primary buffering mineral maintaining the water pH:

at 2.5≤popH≤3.5: (FeOOH)_((s))+3(H^+)_((aq))→ ((Fe)^(3+))_((aq))+(H_2 O)_((l))

Once all the carbonate minerals and soluble hydroxides have been depleted, the dissolution of silicates minerals takes place as a primary acid neutralization.

Silicate Minerals

Neutralization by silicate minerals is more active in lower pH comparative to carbonate minerals (Mills et al.

2015). Silicate minerals are abundant in nature making them significant in maintaining the pH if the rate acid production is low (Bowell et al. 2000). Like carbonate minerals, the effectiveness of neutralization goes in line with the availability of reactive surface area, high concentration of mineral contents as well as smaller grain sizes (Lapakko 2002; Jambor et al. 2002).

General neutralization by silicate minerals:

(2KAl(Si)_3 O_8)_((s))+((2H)^+)_((aq))+((9H)_2 O)_((l))→((2K)^+)_((aq))+((Al)_2 (Si)_2 O_5 (OH)_4)_((s))+(4H_4 (SiO)_4)_((aq))

Feldspar Kaolinite Dissolved silica

(FeMgSiO_4)_((s))+((4H)^+)_((aq))→((Fe)^(2+))_((aq))+((Mg)^(2+))_((aq))+(H_4 (SiO)_4〗_((aq))

Olivine Dissolved silica

Under dry climate

(2KAl(Si)_3 O_8)_((s))+(2MgSiO_4)_((s))+((4H)^+)_((aq))→((2K)^+)_((aq))+((Mg)^(2+))_((aq))+(Mg)_3 ((Al)_2 (Si)_8 O_20 (OH)_4)_((s))

Smectite (clay mineral)

Under moderate climate

(2KAl(Si)_3 O_8)_((s))+(2MgSiO_4)_((s))+((6H)^+)_((aq)) (+(9H)_2 O)_((l))→((2K)^+)_((aq))+((2Mg)^(2+))_((aq))+((Al)_2 (Si)_2 O_5 (OH)_4)_((s))+ ((5H)_4 (SiO)_4)_((aq))

Kaolinite (clay mineral)

Under very wet climate

(2KAl(Si)_3 O_8)_((s))+((Mg)_2 SiO_4)_((s))+((6H)^+)_((aq)) (+(14H)_2 O)_((l))→((2K)^)〗_((aq))+((2Mg)^(2+))_((aq))+(2Al(OH)_3)_((s))+ ((7H〗_4 (SiO)_4)_((aq))

Gibbsite

Exchangeable Cations

Micas, clays and organic matters may provide active sites for exchangeable cations such as Ca2+, Mg2+, Na+, K+ (Strömberg and Banwart 1999). These cations can be replaced by H+ and Fe2+ ions produced during sulfide oxidation and raise the pH. Lottermoser, (2010), illustrates the cation exchange in the following reaction:

Clay-((Na)^+ )_((s) )+((Fe)^(2+))_((aq))↔Clay-((Fe)^(2+) )_((s) )+((Na)^+)_((aq))

Clay-((Ca)^(2+) )_(1/2 (s) )+(H^+)_((aq))↔Clay-(H^+ )_((s) )+((1/2 Ca)^(2+))_((aq))

Hydroxide Minerals

Hydroxide minerals such as ferric hydroxide and gibbsite (Al(OH))_(3(s)) can act as acid neutralizing. These hydroxides have ability to trap dissolved H+ and produce water. Lottermoser (2010) explains the reaction as follows:

(Al(OH))_(3(s))+(3H^+)_((aq))↔((Al)^(3+))_((aq))+3(H_2 O)_((l))

(Fe(OH))_(3(s))+(3H^+)_((aq))↔((Fe)^(3+))_((aq))+3(H_2 O)_((l))