The Adsorption Kinetics And The Adsorption Mechanism Of A New Type Of Adsorbent Expanded Rice Husk-based Heavy Metals Absorption Material

With the National"Eleventh Five-Year"Technology Support Program Project, Mining and metal waste biomass adsorption materials research and applications (Grant No.: 2007BAB18B08). A new type of adsorbent expanded rice husk-based (ERH) absorption material whose raw material was the auspices of the natural agricultural waste rice husk were produced because of using the expanded nature of the waste rice husk. Using ERH adsorbents as adsorption column packing, the following three aspects were studied: the ionic adsorption of six heavy metals including Cu²⁺, Pb²⁺, Zn²⁺, Cr³⁺, Cd²⁺ and Ag⁺, the adsorption kinetics and the adsorption mechanism. The results were as follows:1. The results of the research on the ERH adsorption shown that: When Cu²⁺, Pb²⁺, Zn²⁺, Cr³⁺, Cd²⁺, Ag⁺ initial concentrations were 10, 40.26, 12.8, 11.75, 10, 16.41 mg/L, flow rate is 10 mL/min, and column height changes from 3 cm to 9 cm, the adsorptions capacity was 5.951³.54, 25.677¹7.985, 5.181⁴.33, 6.111⁴.687, 5.15³.366, 6.034⁴.869 mg/g. And when Cu²⁺, Pb²⁺, Zn²⁺, Cr³⁺, Cd²⁺, Ag⁺ initial concentrations were 10, 40.26, 12.8, 11.75, 10, 16.41 mg/L, column height was 6cm, flow rate changes from 5 mL/min to 15 mL/min, the adsorption capacity was 3.763³.005, 26.886¹7.982, 4.51³.807, 5.313⁴.722, 4.624³.639, 5.008⁴.558 mg/g. Further, when the adsorption column height was 6 cm, flow rate was 10 mL/min, and Cu²⁺, Pb²⁺, Zn²⁺, Cr³⁺, Cd²⁺, Ag⁺ initial concentration were 10⁵0, 26¹10, 12.8³5, 8.5³0, 10³0, 7.45³3.23 mg/L, the adsorptions capacity was 3.54⁶.333, 13.44³1.196, 4.447⁵.752, 4.762⁹.579, 4.154⁷.512, 3.211⁷.354 mg/g, respectively.2. Test data at different packing heights were fitted with the Bohrat-Adams model, and the results shown that the adsorption capacity (N0) of ERH on Cu²⁺, Pb²⁺, Zn²⁺, Cr³⁺, Cd²⁺, Ag⁺ reached 509.556, 589.839, 670.353, 631.958, 552.019, 789.731 mg/L, respectively.3.The results of the comparison experiment of activated carbon and natural rice husk shown that when Cu²⁺, Pb²⁺, Zn²⁺, Cr³⁺, Cd²⁺, Ag⁺ initial concentrations were 10, 40.26, 12.8, 11.75, 10, 16.41 mg/L, flow rate was 10 mL/min, and the column height was 6cm, the ERH adsorption capacity was 4.389, 18.87, 4.447, 5.005, 4.154, 4.997 mg/g, that of natural rice husk was 2.634, 10.755, 2.714, 3.387, 1.86 adsorption, 3.106 mg/g and activated carbon adsorption capacity was 3.184, 15.458, 3.664, 4.148, 2.99, 4.121 mg/g, respectively.4. Desorption experiments on the ERH column with the 0.01 mol/L of hydrochloric acid and recycling tests were carried out, and the results shown that the desorption rate of Cu²⁺, Pb²⁺, Zn²⁺, Cr³⁺, Cd²⁺, Ag⁺ reached respectively 96.424, 84.176, 87.047, 92.209, 83.45, 89.16 6%. To the 20th cycle, the ERH column's desorption rate of Cu²⁺, Pb²⁺, Zn²⁺, Cr³⁺, Cd²⁺, Ag⁺ takes up 41.412, 25.661, 35.979, 33.232, 30.433, 28.017 % of the total adsorption of the first time, repectively.5. The results of the experiment on expanded rice husk-based new materials for heavy metal ions adsorption kinetics showed that in the whole adsorption process, the activation energy of intraparticle diffusion controlling step of Cu²⁺, Pb²⁺, Zn²⁺, Cr³⁺, Cd²⁺, Ag⁺ was 49.3, 55.6, 57.9, 41.2, 41.3, 41.8 KJ/mol. And that of the membrane diffusion controlling step was 23.7, 27.3, 26.2, 23.1, 30.7, 21.1 KJ/mol.6. The adsorption mechanism of heavy metal ions were studied from two aspects for the ERH chemical adsorption and physical adsorption. Firstly, the carboxyl and hydroxyl groups on the ERH and heavy metal ions in the solution exchanged with each other, leading to the adsorption of heavy metal ions inevitably. Secondly, the ERH itself is alkaline, which could cause the alkaline precipitation of heavy metal ions. Thirdly, there were a large number of active groups and molecular structure that were advantageous to complexion or chelating heavy metal ions. Fourthly, more active sites of the natural rice hull exposed by the swelling increase its opportunity to contact with the metal ions, which was also good for adsorbing the heavy metal ions. Finally, the loose structure of the ERH could increase the surface area, which is conducive to adsorbing the heavy metal ions. The ion exchange adsorption of ERH on Cu²⁺, Pb²⁺, Zn²⁺, Cr³⁺, Cd²⁺, Ag⁺ respectively takes up 21.396, 10.974, 15.435, 15.336, 33.854, 29.777 % of the total adsorption. And the adsorption by acid precipitation respectively takes up 12.586, 7.013, 8.292, 15.964, 11.285, 8.041 % of the total adsorption.With all the experimental data and theoretical analysis, it could be concluded that the as-produced new expanded rice husk-based absorption material had shown great potential in environmental protection, particularly heavy metal wastewater treatment and environmental restoration.