Removal Efficiency of Some Toxic Heavy Metals from Water During Coagulation Using Polyaluminum Chloride, Adsorption Using Natural Clay (Alrawag) or Durah Activated Carbon and Reverse Osmosis

Abstract

The removal efficiency of some toxic heavy metals from raw water was examined using four techniques: coagulation of turbid water using an inorganic polymer, adsorption by natural clay called Alrawag, adsorption by prepared durah activated carbon (DAC) and reverse osmosis (RO). The efficiency of poly-aluminum chloride (PAC) as a coagulant in removing the turbidity and dissolved organic matter (DOM) from raw water was attempted and gave good results. The PAC used was prepared in the laboratory by reacting aluminum chloride solution with sodium carbonate at room temperature or with sodium hydroxide solution at temperatures 85oC, 70oC, 60oC, 50oC and 40oC. The pH of all prepared PAC solutions was between 3.30 and 4.15. The optimal pH, effective PAC dose, retention time for coagulation to commence were found to be above 5.0, 0.8 cm3/dm3, and 20 minutes, respectively. The removal percentage of the toxic heavy metals, chromium, copper, arsenic and lead gave 99% during coagulation, while that for cadmium and nickel was negligible. The removal percentage of heavy metals copper, arsenic and lead at concentrations, 1, 2, 3, 4 and 5 mg/dm3 were found to be in the range 93.56% to 19.11%, it decreased with increased concentration. The competitive removal of arsenic, lead, chromium, copper, nickel and cadmium from their mixture during coagulation gave removal percentage in the range 25.12% to 53.54% at low PAC dose of 0.25 cm3/dm3 and 2.43% to 99.39% at high PAC dose of 2.0 cm3/dm3.The removal ofcadmium and nickel during coagulation process improved in presence of other ions, gave a removal percentage range of 2.43 to 76.67% and improved further to 95.05% and 86.35% respectively, when 3.0 g of natural clay was added for the individual solutions of the metals. The remaining aluminum was found to be 4.84 mg/dm3 in the final treated water when 0.25 cm3 of PAC was added to one liter of raw water, decreased to 0.381 mg/dm3 and then increased to 41.89 mg/dm3, at PAC doses of 1.0 cm3 and 3.0 cm3 per liter, respectively. It was found that the presence of sulphate ions was not important in metal removal, while carbonate and bicarbonate ions helped in removing the heavy metals. The analytical results of the natural clay (Alrawag) by Fourier Transform Infrared (FT-IR) proved the presence of OH and Si-O groups, and the X-ray fluorescence (XRF) analysis gave 43.1% silicon, 24.0% iron, 13.1% aluminum, 10.9% calcium, 3.26% potassium and other elements. The optimum pH for adsorption of six toxic heavy metals was obtained: it is > 1.8 for lead and nickel, > 3.8 for cadmium and copper, > 2.9 for chromium and > 8.5 for arsenic. The adsorption capacity of the clay was found to increase with increased initial concentration of the metal. The adsorbed amount of a metal in mg/g clay for cadmium, lead, arsenic, chromium and copper were 1.242, 3.11, 0.04, 3.23 and 3.96 mg/g at initial concentration of 25 mg/dm3, and 3.006, 26.35, 0.525, 13.58 and 13.34 mg/g at 200 mg/dm3, respectively. The clay could be washed and regenerated with diluted nitric acid and reused for the second, third or even for the forth time to remove the heavy metals from water. At 50 mg/dm3 the removal percentage of Cr3+ gave 100%, while that of Cr6+ did not exceed 24%, and that of phosphate ions gave only 5.61%.5.61%. V In the present work, the activated carbon was prepared from Sudanese durah. The FT-IR spectrum of durah activated carbon (DAC) shows the presence of OH, Si-O groups. The X-ray Fluorescence (XRF) analysis gave 24.4% potassium, 23.2% silicon, 18.6% calcium, 7.42% iron, 6.8% phosphorous, 5.57% zinc, 5.1% magnesium, 3.6 chloride, 2.4% aluminum and other elements. DAC was used to remove copper metal from the solution and to remove Cu2+, Cr3+, As5+, Ni2+ and Cd2+ from a mixture containing them using the fixed-bed column technique. The effect of inlet concentration of cadmium on removal efficiency and breakthrough time was studied. As the inlet concentration of the metal increased, the removal efficiency and breakthrough time decreased. The removal efficiency for all metals gave more than 95%. The rejection efficiency of the total dissolved solids (TDS) and heavy metals from water using reverse osmosis (RO) technique was found to increase with decrease in feed temperature, decrease of feed TDS, decrease of pH and increase of feed pressure of the membranes. The removal percentage of the different metals at low pH became more stable and can be arranged as follows: Cd2+ > Cr3+ > Cu2+ > As3+ > Pb2+ > Ni2+. The removal of heavy metals by RO at low concentration was found to be efficient and gave 99% to produce fresh water with metal concentrations which agreed with the permissible standards stated by most international organizations. The results obtained showed that the investigated methods of heavy metal removal during coagulation of turbid water using poly-aluminum chloride (PAC), adsorption on natural clay (Alrawag), or durah activated carbon (DAC) or by reverse osmosis (RO) gave up to 100% efficiency.