Study On Micorowave-assisted Preparation Of Functional Rice Husk Adsorbent And Its Adsorption Performance And Mechanism For Lead And Cadmium In Water

Heavy metal pollution in wastewater has become an important environmental problem in many countries.How to reduce the harm of heavy metals has become a research focus and challenges in today’s world.Due to its simple operation,good removal effect,and low cost,the adsorption method is widely used.The core problem of the adsorption method is the preparation of adsorbents with excellent adsorption performance and the exploration of their adsorption performance and mechanism.Therefore,in this study,rice husk andβ-cyclodextrin were effectively combined by microwave-assisted technology to synthesize a high-performance functional adsorbent.The adsorption performance and mechanism of this functional adsorbent for heavy metal were studied by static sequence batch experiment.Dynamic adsorption experiments and simulated wastewater experiments are used to clarify the application effects in actual wastewater treatment.Finallya fuzzy multi-criteria evaluation model was developed,in order to identify the most favorable technology for the disposal of the heavy metal sludge produced during sewage treatment.The main results and conclusions are as follows:（1）This work explored the fast microwave-assisted synthesis ofβ-cyclodextrin-modified adsorbent(β-CD_MW@RH).First the rice husk was carbonized with sulphuric acid and then the carbonized rice husk was modified under microwave irradiation by epichlorohydrin as crosslinking agent andβ-cyclodextrin as the modified agent.A recommended preparation condition is 0.6 g of carbonized rice husk,4.0 g ofβ-cyclodextrin,2.5 m L of epichlorohydrin,100 m L of 7%Na OH.Theβ-cyclodextrin prepolymer preparation time is 600 s and the grafting time is of 420s.Compared to conventional synthesis methods,the processing time of adsorbent could be decreased from 5 h to 17 min by using microwave-assisted technology and we can get a high-performance adsorbent for heavy metals.And the characteristics ofβ-CD_MW@RH was analyzed by Scanning electron microscopy,Zeta potential analysis,thermogravimetric analysis,elemental analysis,and Fourier transform infrared spectroscopy.The result showed thatβ-cyclodextrin deposits on the surface ofβ-CD_MW@RH.Compared with the original material,the characteristic functional groups ofβ-cyclodextrin such as-OH,C-O-C,and R-1,4-bond skeleton group are found in the functional adsorbent.These result all indicate thatβ-cyclodextrin was successfully grafted on the surface of carbonized rice husk.Increased hydrophilicity was analyzed by the elemental analysis showed that also provides evidence for the higher adsorption capacity of functional adsorbent than original carbonized rice husk.（2）The adsorption performance ofβ-CD_MW@RH was investigated from various aspects.β-CD_MW@RH has a good removal effect in a wide initial p H range of 4-6,revealng that Pb（Ⅱ）and Cd（Ⅱ）could be efficiently removed without p H value adjustment.The adsorption equilibrium could fast achieve within 20 minutes,indicating thatβ-CD_MW@RH has a faster adsorption rate;High adsorption performance can be achieved in the temperature range of 5-35℃;Common accompanying ions(e.g.K⁺、Ca²⁺、,Na⁺、Mg²⁺)have a low effect on the removal of Pb（Ⅱ）and Cd（Ⅱ）byβ-CD_MW@RH,showed functional adsorbent has specific selectivity for Pb（Ⅱ）and Cd（Ⅱ）.Desorption can be achieved by 0.5 M HCl solution,and the functional adsorbent has high adsorption capacity after four adsorption and desorption cycles.Under the conditions of p H 4.6,initial concentration 100 mg/L,and adsorption temperature 20℃,the adsorption capacity ofβ-CD_MW@RH are 216.06 mg/g for Pb（Ⅱ）and 148.19 mg/g for Cd（Ⅱ）.The above results all indicate thatβ-CDMW@RH has good adsorption performance for Pb（Ⅱ）and Cd（Ⅱ）,and can be used for the removal heavy metal in the wastewater.（3）The adsorption mechanism is discussed by adsorption kinetics,isotherms,thermodynamics and X-ray photoelectron spectroscopy.The adsorption kinetics showed that the adsorption process ofβ-CD_MW@RH for Pb（Ⅱ）and Cd（Ⅱ）fits the pseudo-second-order kinetic equation well,indicating that the chemical adsorption Process is the main adsorption process;The Langmuir isotherm can be better fitted to the data of Pb（Ⅱ）and Cd（Ⅱ）,indicating that the adsorption of metal ions byβ-CD_MW@RH is a monolayer chemisorption on the surface of the functional adsorbent.The rmodynamic analysis shows that the adsorption for typical heavy metal ions byβ-CD_MW@RH is an endothermic reaction process and can proceed spontaneouslyin the temperature range of 5-35℃;The analysis of XPS shows that-OH onβ-cyclodextrin and-COOH on carbonized rice husk provide adsorption sites for metal ions to form complex compounds.The adsorption mechanism ofβ-CD_MW@RH for heavy metal ions in water is the combination of complexation and electrostatic interaction.（4）The influence of different process parameters on dynamic adsorption is studied,and in order to investigate the removal effects of functional adsorbent on typical heavy metals in actual wastewater,theβ-CD_MW@RH was used to removal of heavy metals in simulated wastewater from the battery factory in Huzhou Zhejiang Province.The results show that the breakthrough time of the fixed bed gradually increases as the filling height increases.And the breakthrough time decreases as the the inlet water flow rate and the initial concentration increases.Both Thomas model and Yoon-Nelson model can be used in the description of the dynamic adsorption process of Pb（Ⅱ）and Cd（Ⅱ）byβ-CD_MW@RH For the simulated battery factory wastewater with a Pb（Ⅱ）concentration of 11.5 mg/L and a Cd（Ⅱ）concentration of 5.3 mg/L,the processing capacity of 2g functional adsorbent is 6620 m L,and the effluent can meet the Wastewater Discharge Standard of China（GB8978-1996）.The functional adsorbentβ-CDMW@RH has good application potential for wastewater containing Pb（Ⅱ）and Cd（Ⅱ）.（5）Fuzzy multi-attribute evaluation system was established to select the best disposal technology for sludge containing heavy metals.Application evaluation system of“intuitive trapezoidal fuzziness-grey relational analysis-Technique for Order Preference by Similarity to an Ideal Solution”,the ranking of the disposal technology is obtained.The results showed that the materialization technology was perceived as the most appropriate disposal measure for heavy meatal-containing sludge.