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Influence of pH, Sodium Adsorption Ratio, and Salt Concentration on Settling Kinetics of Suspended Solids in Coal-Mine Ponds

Agronomy Dep., N-122 Agric. Sci. Bldg.-North, Univ. of Kentucky, Lexington, KY 40546-001.

^* Corresponding author.

ABSTRACT

Colloidal particle-settling experiments were conducted in the laboratory to test the influence of pH and dissolved solids on the kinetics of settling of suspended solids in a coal-mine spoil sample obtained from western Kentucky. Potentiometric titrations suggested that the pH range of colloid coflocculation for the sample tested was around 4.2. Repulsive diffuse double layers at pH above 4.2 appeared to control-suspended solid behavior. The rate of settling of suspended solids decreased as the pH of the suspension increased. This rate was highly influenced by the ionic strength, sodium adsorption ratio (SAR) and type of divalent cation (Ca or Mg). With Ca as the saturating cation and SAR values below 1, the suspended solids dropped below the required 35 mg/L limit within a 2–h period. At SAR values above 4 and pH near and/or above 7 suspended solids exceeded those required by law. When the divalent cation was Mg, the coal-mine spoil solids appeared to be less dispersive. For sedimentation ponds neutralized with NaOH, this type of kinetic information is necessary to improve sediment trapping efficiency and design ponds with appropriate water detention times.

Contribution from the Dep. of Agronomy, Univ. of Kentucky Agric. Exp. Stn. Journal no. 89-3-98.

Received for publication March 4, 1991.