**2.1 Acidity/alkalinity property**

The pH is the measure of H+ concentration; in the case of pure water, H<sup>+</sup> and OH are in the same concentration. If the H+ concentration is higher, it is acidic and if OH is higher it becomes alkaline. The pH value of water decreases when it comes in contact with oxidized sulfides. For heavy metals, the lower pH value improves the solubility of the solution and is converted to a toxic solution.

The alkalinity is a measure of the base concentration of a solution and measured by the ability of the solution to absorb protons or the capacity to neutralize a strong acid. It also depends on the CO2 content and mineralization process. Thus total alkalinity is also known as the sum of OH, CO3 <sup>2</sup>, HCO3, NH3, HS, PO4 3 , H2BO3 , and organic anions. The measurement of acidity indicates the total acid in the solution; also known as the capacity to neutralize the base.

The solution of a lower pH may contain different abundant acids. For pH higher than 7.0, total acidity rarely exists due to the lower value of sulfuric acid (H2SO4). The carbonate/bicarbonates convert into carbonic acid for pH less than 4.2, which then leads to the rapid dissociation into water and CO2 [4].

### **2.2 Concentration of different heavy metals**

Different heavy metals of high concentrations are the common feature of AMD. Some metals in the metal deposits often incorporate into the AMD at specific geochemical conditions. The toxicity level is a greater problem to aquatic life as well as human health. The control mechanism of heavy metals is quite complex and highly precise to metal and site. In the initial stage where AMD forms, the type of metallic minerals and their solubilities and or dissolution rate control the concentrations of heavy metal. In the second development stage, effluent evolves in contact with regional rocks, atmospheric conditions, and water, changes occur in the complex of metal which favor adsorption and precipitation, so control and mitigate AMD flow [5].
