Precipitation Objective: To remove undesirable pollutants by a precipitation reaction M(aq) + N(aq) MN(s) Example removal of hardness by lime soda-ash and removal of iron and manganese by oxidation The focus of this lecture is on removal of hardness by lime and soda-ash, but before proceeding with the lime soda-ash method, we need to know how to express concentration in units of meq/l and mg/l as CaCO 3? meq Concentration (mg / L) Concentration in = L Eq wt (mg / meq) Eq wt = Atomic or molecular wt eq # Concentration of X (mg / L) × 50 ( mg CaCO3 / meq) mg of X as CaCO3 = L Eq wt of X ( mg / meq) Symbol At or M wt (mg/mmol) Eq # Eq wt (mg/meq) Ca2+ 40.1 20 Mg2+ 24.3 12.2 K+ 39.1 39.1 Na+ 23 23 HCO3- 61 61 CO3 2- 60 30 Cl- 35.5 35.5 PO4 3- 95 31.7 SO42- 96 48 CaCO3 100 50 Lime Soda-Ash Softening A precipitation method through which Ca is removed as CaCO3 and Mg is removed as Mg(OH)2 The first step is to remove CO2 from water: CO2+H2O H2CO3 Lime H2CO3+Ca(OH)2 CaCO3 +2H2O Now remove Ca or Mg that has alkalinity in the water: Ca hardness Lime Ca(HCO3)2+Ca(OH)2 2CaCO3 +2H2O Mg hardness Lime Mg(HCO3)2+2Ca(OH)2 2CaCO3 + Mg(OH)2+ 2H2O Soda ash is needed if there is not enough alkalinity in the water: Ca hardness Soda ash CaSO4+ Na2CO3 Mg hardness Lime CaCO3 + Na2SO4 Soda ash MgSO4+ Ca(OH)2 +Na2CO3 CaCO3 + Mg(OH)2+ Na2SO4 Amount of lime and soda ash needed to remove hardness meq/l of meq/l needed of lime Soda ash CO2 - Ca(HCO3)2 - Mg(HCO3)2 - Ca with ions other than alkalinity - Mg with ions other than alkalinity 1 Excess 1.25 Practically, some Ca and Mg will be left in solution after treatment: For Ca: 30 mg/l as CaCO3 =0.6 meq/l in equilibrium with CaCO3 is left For Mg: 10 mg/l as CaCO3=0.2 meq/l in equilibrium with Mg(OH)2 is left Example A groundwater sample has the following characteristics Find: 1.The total hardness 2.TDS 3.The amount of lime and soda ash needed to remove hardness to practical limits 4.The meq/l bar graph after lime soda addition Component Concentration pH (-) 6.3 CO2 (mg/l) 8.8 Ca2+ (mg/l) 40 Mg2+ (mg/l) 14.7 Na+ (mg/l) 13.7 HCO3- (mg/l) 164.7 SO42- (mg/l) 29 Cl- (mg/l) 17.8 Solution Hardness: Convert the concentration of Ca and Mg to mg/l as CaCO3 using Concentration of X (mg / L) × 50 (mg CaCO3 / meq) mg of X as CaCO3 = L Eq wt of X ( mg / meq) Thus Ca= 100 mg/l as CaCO3, Mg=60 mg/l as CaCO3 Total Hardness is 160 mg/l as CaCO3 TDS= sum of all cations and anions in mg/l = 279.9 mg/l To determine the amount of lime and soda ash needed, we need first to convert the concentration to meq/l, draw a meq/l bar graph and then use the table Component Concentration (mg/l) Equivalent weight (mg/meq) Concentration (meq/l) CO2 8.8 22 0.4 Ca2+ 40 20 2.0 Mg2+ 14.7 12.2 1.21 Na+ 13.7 23 0.6 HCO3- 164.7 61 2.7 SO42- 29 48 0.6 17.8 35 0.51 Cl- The meq/l bar graph of water before lime/soda-ash treatment The amount of lime and soda ash needed to remove hardness to practical limits are determined as shown in the table below meq/l of CO2 =0.4 Ca(HCO3)2=2.0 Mg(HCO3)2=0.7 CaSO4=0.0 MgSO4=0.51 Excess Total meq/l needed of lime Soda ash 0.4 2.0 1.4 0.51 0.51 1.25 5.56 0.51 The meq/l bar graph of water after lime/soda-ash treatment but before recarbonation Note, the pH of the treated water is high (about 11), so CO2 is added (recarbonation) to reduce the pH