Comparison of chemical reactivity between inorganic and synthetic polymers in the treatment of AMD

Abstract

This study involves the treatment of the acid mine drainage (AMD) sample using inorganic coagulants such as FeCl3, Fe2(SO4)3, FeSO4, AlCl3, Al2(SO4)3 and acid-free polyferricchloride (af-PFCl) of Ca(OH)2 or Mg(OH)2 and acid-free polyaluminiumchloride (af-PACl) of Ca(OH)2 or Mg(OH)2 in a jar test during rapid and slow mixing, for 250 and 100 rpm for 2 and 10 minutes respectively, settled for 1 hour, after which the pH, conductivity, turbidity measurements were conducted. A similar set of experiments was conducted using the AMD sample with the same dosages in a shaker using the same timing, settled for 1 hour, after which similar measurements were conducted. A third similar set of experiment was conducted with dosage without mixing, settled for 1 hour, after which similar measurements were conducted. In a few selected experiments extra measurements such as dissolved oxygen (DO) and oxidation reduction potential (ORP) were conducted. In the first experiment, 200 mL of the AMD sample was poured into five 500 mL glass beakers. A dosage of 20, 30, 40, 50 and 60 mL of 0.043 M FeCl3 and Al2(SO4)3 was added in the middle of the samples respectively using plastic syringes. The samples settled for 1 hour after which the pH, conductivity and turbidity were measured. A second similar set of experiments was conducted by pouring the AMD sample into five 500 mL Erlen Meyer flasks, equal quantities of coagulants were added and the samples were placed in a shaker using similar stirring method. A third similar set of experiments were conducted in a jar test with rapid mixing for 2 minutes, settled for 1 hour, and similar measurements were done. The results showed turbidity removal in the AMD samples with FeCl3 and Al2(SO4)3 dosage during mixing, shaking and without mixing is similarly identical. Turbidity removal was still effective in the AMD using inorganic coagulants/flocculents. Experimental results show that the ionic concentration and charge density in the system (wastewater) during treatment determines the rate of destabilization-hydrolysis. In a second set of experiments, 200 mL of acid mine drainage sample were poured into 5 glass beakers and dosed with FeCl3, Fe2(SO4)3 and polymers of a mixture of FeCl3 and Ca(OH)2 (af-PFCl) respectively. The samples were placed in a flocculator and stirred at 250 rpm for 2 minutes. The samples were allowed to settle for 1 hour after which the pH, conductivity and turbidity were measured. A similar set of experiments was conducted by pouring 200 mL of the sample into five Erlenmeyer flasks with the same dosage and were placed in a shaker using similar timing and measurements. A third similar set of experiments was conducted but without mixing, settled for 1 hour and the same measurement taken. The fourth and fifth sets of experiments were conducted with Ca(OH)2 and af-PFCl polymer respectively. The results showed that the pH and residual turbidity in the samples with Fe3+ salts, Ca(OH)2 and af-PFCl dosages in corresponding dosages are almost identical. The difference between the pH and residual turbidity in the corresponding sample dosages with mixing, shaking and without mixing exhibit is insignificant. In a third set of experiments, 200 mL of the AMD was poured into 5 glass beakers and thereafter dosed with Fe3+ and Al3+ salts and a synthetic polymer of FeCl3 and Mg(OH)2. The samples were treated in a jar test at 250 rpm for 2 minutes, settled for 1 hour after which the pH, conductivity, turbidity, dissolved oxygen (DO) and oxidation reduction potential (ORP) were measured. A similar batch experiments was conducted by placing the samples on a shaker using the same timing and measurements. A similar third batch of experiment was conducted with samples without mixing and employed similar settling time and measurements, and the fourth similar set of experiments was conducted dosing the AMD sample with 0.1 M af-PFCl of Mg(OH)2 in mixing and shaking. The turbidity removal in the samples in the samples with FeCl3, AlCl3 and af-PFCl dosage was effective and comparable, whereas FeCl3 yielded better performance. The SEM images show that the sludge in the samples with af-PFCl of Mg(OH)2 dosage consists of a large cake-like structure, with the likelihood of optimal adsorption. The turbidity removal in the AMD sample with 0.1 M af-PFCl polymer of Mg(OH)2 with mixing and shaking is similarly identical, and also ORP results showed that redox reaction is predominant during destabilization-hydrolysis. The experimental results in all the AMD samples dosed with the coagulants and flocculants used in this study revealed that the concentrations, dosages, dispersion techniques (mixing, shaking and no-mixing), dosing (prior or during mixing), yielded optimal turbidity/TSS removal potential. This shows that the experiments achieved the desired objective, i.e. investigation of the reactive potential of the coagulant/flocculent in the destabilization-hydrolysis of the AMD sample without pH adjustment.