Adsorption Characteristics Of Pigbiochcar And Its Modification On Pb²⁺ And Cd²⁺ In Aqueous Solution

Biochar is a solid product rich in carbon with a complex pore structure and large surface area,which can promote the complexation of heavy metals in water and the formation of carbonate.Biochar can effectively reduce the effective concentration of pollutants and the toxicity of heavy metals in remediating polluted water bodies.Modifying the surface properties of biochar can improve the heavy metal adsorption capacity of biochar.In this study,the tested pig biochar was modified with the nitric acid and ultrasound treatments,after then,element contents,specific surface area,and the pore and surface functional groups were compared and analyzed.Pig biochar and modified pig biochar were used to adsorb Pb²⁺and Cd²⁺in aqueous solution.The effects of pH,time,adsorbent dosage,temperature,and initial concentration of ions on the adsorption of Pb²⁺and Cd²⁺were investigated by mass adsorption experiments.The pseudo-first-order,pseudo-second-order kinetic equation,particle diffusion equation,Freundlich and Langmuir isothermal equation,and thermodynamic equation were used to calculate absorption capacity,besides,the adsorption mechanism was also investigated.The main results are as follows:?1?Pig biochar was strongly alkaline?pH>10?with a highcation-exchange capacity?CEC?ash?>50%?and phosphorus contents were high?>7%?,also rich in small pores?2⁵0 nm?.The carbon content,pH and CEC of adult pig biochar was higher than that of young pig biochar.Modifying with nitric acid decreased the alkalinity on the surface of biochar.CEC was decreased by ultrasonic modification.Nitric acid modification reduced C and S contents,and increased N,H,O,P and ash contents.C,N and P contents were increased by ultrasonic modification.After nitric acid modification,the H/C and?O+N?/C increased,the aromatics content increased and the polarityand stability decreased.The surface area of young pig biochar was larger.Nitric acid and ultrasonic modifications decreased the specific surface area and pore volume.Nitric acid made the structure of pig biochar collapse,but increased the surface carboxyl,phenol hydroxyl and other oxygen-containing functional groups.Some pores were blocked,ash and inorganic mineral components were precipitated in the ultrasound treatment and decreased the amount of surface functional groups.?2?The pseudo-second-order kinetic model fitted well the process of adsorbing Pb²⁺and Cd²⁺by biochar.The Pb²⁺adsorption rate of adult,young,nitric acid modified and ultrasonic modified pig biochar is 0.0038,0.0037,0.0043 and 0.0038 g×mg^-1×min^-1,the Cd²⁺adsorption rate is 0.0091,0.0331,0.0074 and 0.0110 g×mg^-1×min^-1,which was influenced by diffusion and ion concentration.The adsorption process can be divided into three stages:surface point binding,penetration of liquid film,channel diffusion process,and intrapreic diffusion was not the only control rate stage,membrane diffusion also included.120 min was the adsorption equilibrium time.The adsorption of Pb²⁺and Cd²⁺on the four kinds of biochar fitted Langmuir and Freundlich isothermal adsorption models.The former has a higher fitting degree,and the maximum adsorption capacity for Pb²⁺is 81.30,77.52,90.09,81.30 mg×g^-1,for Cd²⁺is 2.72,2.68,3.15 and2.76 mg×g^-1.The surface of adult pig biochar contains rich hydroxyl,carboxyl and other oxygen containing functional groups,phosphorus contents and cation-exchange capacity.Nitric acid and ultrasonic modification improve the carbon and phosphorus contentof pig biochars,so as to adsorb Pb²⁺and Cd²⁺.The adsorption capacity of Pb increased by 1.1 times after nitric acid modified and the adsorption capacity of Cd²⁺increased by 1.15 times.The adsorption capacity of Cd²⁺was increased by 1.01 times by ultrasonic modified pig charcoal,while the adsorption capacity of Pb²⁺was the same before and after ultrasonic modification.The content of pH,CEC,ash and alkalinity of adult pig was higher,so the activity was stronger,and the adsorption capacity of Pb²⁺and Cd²⁺was higher than young pig biochar.?3?The pH range for adsorption ofPb²⁺by adult,young and ultrasonic modified pig biochar is 3⁵.5while that of Cd²⁺is 3.5⁵.5.When pH=5.5,both the adsorption amount is the largest,for the nitric acid modified pig carbon is at pH=7 and 6.5,the nitric acid significantly reduced the pH of the pig biochar andincreased the adsorption scope of pH.For Cd²⁺solution with 20 mL concentration of 8 mg×L^-1,the dosage of the nitric modified biochar,ultrasonic modified pig biochar and unmodified pig biochar is 25 mg,120 mg and 60 mghad thebest remove capacity.The four pig biochars had the maximum adsorption capacity when using the dosage of 200 mg for 20 mL concentration of 200mg×L^-1Pb²⁺solution,and the nitric acid modified pig biochar was greater than the other three kinds of pig biochars in the teatment,so the biochar is fully utilized.?4?The adsorption thermodynamic results showed when T ranged from 283.15 to 303.15 K,the reaction of biochar to Pb²⁺and Cd²⁺is a spontaneous endothermic and entropy increase process,warming up was propitious to the adsorption of adsorbents.