Study On Adsorption Properties Of Modified Straw And Its Composite Materials For Heavy Metals

With the rapid development of national economy,more and more people focus on the quality of life and health issues.At present,the restoration of water pollution has become the heat topic in society.Many experts have done a lot of research on water treatment,and all of them agreed that the adsorption method has higher adsorption efficiency on removing heavy metal ions from wastewater.However,to find a low-cost and environmental friendly materials in the adsorption method has become another hot issue in current research.Some scholars have mixed cellulose extracted from straw with chitosan to prepare a composite hydrogel as an adsorbent.However,the composite has the disadvantages of small contact area,complicated production process,contaminant in the preparation process,high cost,poor mechanical properties,and easy to soluble in acid.The straw without dealling as an adsorbent has low adsorption capacity.Therefore,to preparation an adsorbent with high acid resistance,high adsorption capacity,no secondary pollution,simple operation process and low cost to remove heavy metal ions in sewage has become another goal of current research.According to the object,corn stover and chitosan were used as basic materials.Then corn stover was modified by phosphoric acid impregnation heating method and obtained phosphoric acid modified straw with porous structure.In order to improve the adsorption capacity of the adsorbent,the glutaraldehyde was used as a crosslinking agent to further modify the phosphoric acid modified straw by magnetic water-bath stirring method to obtain chitosan/modified straw composite adsorbent.The three adsorbent materials were characterized and the adsorption properties were investigated.The chitosan/modified straw composite adsorbent was also investigated for the multi-component ion competitive adsorption research and recycling performance research.The feasibility of using the modified straw composite adsorbent to remove heavy metal ions in practical applications was estimated.The main research contents and results are as follows:1.The three adsorbents were characterized by infrared spectroscopy,scanning electron microscopy and X-ray diffraction.By those series characterization,we found the surface of phosphoric acidmodified straw adsorbent has a porous hole and increases the specific surface area,and the surface of chitosan/modified straw composite adsorbent has more-NH₂ and-OH group,and rough surface,the crystallinity of the modified material has a certain degree of weakened.The above characterization results indicate that the phosphoric acidmodified straw and chitosan/modified straw composite adsorbent have better adsorption capacity for heavy metal ions.2.The three adsorbents prepared?raw corn stover,phosphoric acid modified corn stover and chitosan/modified straw composite adsorbent?were used to adsorb Pb²⁺and Cu²⁺.The experimental results show that under the conditions of adsorption time of240 min,pH=5,temperature 25°C,dosage 0.0250 g and concentration of 100 mg·L^-1,two-step modification of raw corn stover,the saturated adsorption capacity of Pb²⁺increased from 55.4 mg·g^-11 to 92.56 mg·g^-1,and the saturated adsorption capacity of Cu²⁺increased from 17.1 mg·g^-11 to 64.49 mg·g^-1,indicated that the adsorption capacity of the modified adsorbents were greatly improved.3.By thermodynamic analysis,it is found that the two modified adsorbents conform to the Langmuir adsorption isotherm model and belong to the monolayer adsorption.According to the Van der Hof equation,the?G becomes blow zero,the reaction enthalpy change and the entropy change become greater than zero.It is indicated that the adsorption of heavy metal ions Pb²⁺and Cu²⁺by modified adsorbents is a spontaneous endothermic reaction.The kinetic data indicated that the adsorption of Pb²⁺and Cu²⁺by the modified adsorbent accorded with the quasi-secondary kinetic model,the adsorption mainly based on chemical adsorption and accompanied with physical adsorption.4.Chitosan/modified straw composite adsorbent was used to adsorb multi-component ions of the same concentration.The research data shows that the adsorption capacity of the adsorbent for each ion in the mixed solution is smaller than that of the single component heavy metal ion,which may be due to the existence of competitive adsorption between the multi-component ions and the repulsive force between the same charge.5.The adsorption-desorption experiment of chitosan/modified straw composite adsorbent showes:when it was recycled for 4 times and the adsorption capacity of Pb²⁺was 76.241 mg·g^-1,it decreased by 17.63%than the first adsorption capacity,the desorption rate was 72.66%,and the cycle was used 5 times,the adsorption capacity decreased to 66.31 mg·g^-1,and the desorption rate decreased to 71.22%;The first adsorption capacity of Cu²⁺was 64.49 mg·g^-1.When it was recycled for 4 times,its adsorption capacity was 48.35 mg·g^-1,it decreased by 25.03%than the first adsorption capacity,and the desorption rate was also reduced from 96.14%to 89.76%.When it was used for 5 times,the adsorption capacity was reduced to 40.31 mg·g^-11 and the desorption rate was 89.88%.Overall,the chitosan/modified straw composite adsorbent has good recycling potential for absorbing Pb²⁺and Cu²⁺.