Extraction And Modification Of The Main Components From Agricultural Wastes And Their Adsorption Performances Toward Lead Ion

As a renewable biomass resource with abundant sources and low price,agricultural waste is widely investigated in the treatment of heavy metal wastewater.However,due to the complex structure of agricultural wastes,the adsorption performance is greatly affected.In this paper,four agricultural wastes of corn stalk,peanut shell,cotton stalk and rape stalk were used as raw materials.The main components:cellulose,lignin and hemicellulose were extracted and surface modified by carboxyl groups.The adsorption properties of the modified and unmodified of raw materials and the extracts for Pb²⁺were studied.The effects of modification on the adsorption properties and stability of the raw and modified materials were studied.Results were as follows:（1）Cellulose,lignin and hemicellulose were extracted from raw materials by acid method,sulfuric acid hydrolysis method and alkali method,respectively.The three extracts and raw materials were characterized by scanning electron microscopy and FTIR spectroscopy.The results showed that cellulose,lignin and hemicellulose from the four plants were successfully extracted.After modification,a large number of carboxyl groups were introduced onto the surface of the three extracts and the raw materials without damaging their structure.（2）The adsorption performances of Pb²⁺on the three extracts and raw materials before and after modification was investigated.Adsorption capacity of each adsorbent was improved significantly compared with that before modification.Adsorption of Pb²⁺on the modified straws and the extracted cellulose,lignin and hemicellulose fitted the Langmuir model well,belonging to mono-layer adsorption.FTIR and XPS analysis showed that carboxyl and hydroxyl groups on the modified adsorbent participate in the adsorption of Pb²⁺by means of electrostatic adsorption and surface complexation.On the whole,the adsorption capacity of the modified adsorbent was as follows:modified peanut shell>modified rape straw>modified maize straw>modified cotton straw;the adsorption capacity of the crop raw materials and extracted components was as follows:modified hemicellulose>modified lignin>modified raw materials>modified cellulose.The adsorption capacity of modified peanut shell hemicellulose was up to 327.43 mg g^-1,which was 10.6 times higher than that of unmodified peanut shell.After the modification of hemicellulose in peanut shell and rape straw,the removal rate of Pb²⁺solution with concentration of 10 mg L^-1 reached 98.56%and98.14%respectively,and the Pb²⁺concentration after equilibrium was lower than the current national emission standard.（3）The effects of time and pH on the release of organic matter from the adsorbents were investigated.The results showed that lignin and hemicellulose were the main sources of organic matter release.Carboxyl groups modification can improve the stability of the adsorbents,organic matter release can still maintain a relatively low level even after adsorption for 30 days.At the pH range of 4.0-5.0,the organic matter release amount of each adsorbent was the minimum.After modification,all extracts from peanut shell and cellulose and lignin from maize straw,cotton straw and rape straw reached the drinking water standard.