Removal of colour and suspended solids from landfill leachate using Tin tetrachloride (SnCl₄): The effects of pH, zeta potential, and particle sizes

Old landfill leachate is normally rich in colloidal particles, has low organic compound, poor biodegradability and hard to be biologically treated. The suspended particles are generally dispersed, negatively charged and do not settle by gravity due to their instability and repulsion. This paper aimed to evaluate the effectiveness of SnCl4 as a positively charged coagulant in treating concentrated colour and suspended solids (SS) from an old leachate sampled at one of the local landfills in Malaysia (Alor Pongsu Landfill Site, Keria Perak), which may also represent other landfills with similar property. The mechanism and the effect of pH, zeta potential (ZP) and particle size of colloids on the reduction of the pollutants were evaluated through standard coagulation and flocculation experiment with ZP measurement. The standard jar test was examined at a pre-determined SnCl4 dosage and varied from pH 3 to pH 12. The raw leachate was found negatively charged throughout pH 3 to pH 12. Adding a positively charged SnCl4 increased the leachate surface charge to close to the point of zero charges (IEP) at pH 8 with -0.4 mV. Generally, as the pH value increase from acidic to basic condition, the raw leachate becomes more negatively charged as at higher pH values It was also found that the pH of the solution correlated with the size of the particles, where 221.3 d.nm was recorded from pH 4 to pH 11 and improved to the largest size 660 d.nm at pH 3. The highest colour and SS performance were observed at optimum pH 8 with reductions of 98.9% and 100%, respectively The reduction of repulsive forces at this pH promoted the collision of particles and thus made the aggregation possible. ZP measurement in is important as it provides an early estimation of the effective pH for the optimum coagulation and flocculation process of landfill leachate contributed by the colloidal particles.