1. Charge Density: MgCl2 dissociates in water to form Mg2+ and 2Cl- ions. The Mg2+ ion has a higher charge density than the K+ ion. This means that Mg2+ ions can interact more strongly with the negatively charged surface of As2S3 particles, leading to better coagulation.

2. Ionic Strength: The ionic strength of a solution is a measure of the concentration of ions in the solution. MgCl2 has a higher ionic strength than KCl. This means that MgCl2 can reduce the electrostatic repulsion between As2S3 particles, allowing them to come closer together and coagulate.

3. Solubility: MgCl2 is more soluble in water than KCl. This means that MgCl2 can be used at higher concentrations without the risk of precipitation. Higher concentrations of MgCl2 can lead to more effective coagulation of As2S3.

4. Hydrolysis: MgCl2 undergoes hydrolysis in water, producing H+ and OH- ions. The H+ ions can react with the surface of As2S3 particles, forming insoluble metal hydroxides that bridge the particles together and promote coagulation.

In summary, the higher charge density, ionic strength, solubility, and ability to undergo hydrolysis make MgCl2 a better coagulant for As2S3 compared to KCl.