| In recent years, the treatment of water heavy metal pollution has attracted more and moreattention. Adsorption is one of the commonly used control technologies for water heavy metalpollution, especially using all kinds of agricultural wastes as adsorbent. As a technology fullof potential, it is not only economical, but also shows good adsorption properties aftermodification. In order to improve the adsorption efficiency of agricultural waste, modificationis a hot spot in the present study. And in order to develop effective modification,understanding the mechanism of modification and the mechanism of improving the efficiencyof adsorption are necessary.The peanut shells modified by potassium permanganate which were made in the earlystage of our research group’s work, showed good adsorption performance on Cd2+and Pb2+.On the basis of previous work, this paper investigated the micro changes of peanut shell in theprocess of the modification by detecting the physical and chemical properties of peanut shell’ssurface and the distribution of functional groups before and after the modification. This paperalso explored the adsorption mechanism of Cd2+and Pb2+before and after the modification,analyzing the changes of the adsorption mechanism after the modification, finding out whichchange of the microstructure and properties lead to the improving of adsorption efficiency.In this paper, the influences of adsorbent additive amount, initial solution pH,temperature, time and so on factors on adsorption of Cd2+and Pb2+by the block peanut shellsbefore and after modification has been investigated, and systematically studied the adsorptiveproperty of the block peanut shells before and after the modification by compared with theresults of the corresponding study of modified peanut shells. Besides, isoelectric point ofpeanut shells before and after the modification, influence of ionic strength on adsorption, pHchange of adsorbate solution before and after adsorption and desorption efficiency of fivedesorption solutions (viz. distilled water, NaCl, HNO3, citric acid and EDTA) to Cd2+and Pb2+were investigated to infer the adsorption mechanisms of Cd2+and Pb2+on peanut shells beforeand after the modification. Furthermore, the potential adsorption mechanisms were confirmedby comparing surface features of the peanut shells before and after the modification beforeand after adsorption characterized by X-ray photoelectron spectroscopy and Fourier transforminfrared spectroscopy. The results indicated that:(1) The adsorptive property of peanut shells is greatly improved after the modification.Compared with the peanut shells, the biggest adsorption capacity of modified peanut shellsfor Cd2+and Pb2+is4times and5times respectively; The process of adsorbing Cd2+and Pb2+by modified peanut shells get balanced after18h, whereas36h of the peanut shells.(2) After modification of potassium permanganate, the surface features of peanut shellsare changed, more conducive to the adsorption of positively charged metal ions:after themodification, the point of zero charge of peanut shells became4from6, the negative chargeon the surface of the modified peanut shell increased; The BET specific surface area of thepeanut shells is failure to detect for it is below the instrument detection limit, BET specificsurface area of modified peanut shell increased to4.368m2/g and could provide moreadsorption sites for adsorption; The oxygen-containing functional groups on the surface of themodified peanut shell are increased, which is more conducive to the adsorption of heavymetal ions through complexation, and the manganese dioxide generated during themodification, also played a role in the adsorption of Cd2+and Pb2+.(3) The potential adsorption mechanisms for peanut shells adsorbing Cd2+and Pb2+are asfollows: Cd2+is adsorbed through the joint effect of electrostatic adsorption and otherphysical function and the outer complexing; whereas Pb2+is adsorbed via electrostaticadsorption and other physical function and ion exchange with H+on the peanut shell surfaceand the outer complexing.(4) The potential adsorption mechanisms for modified peanut shells adsorbing Cd2+andPb2+are as follows: Cd2+is adsorbed through the joint effect of the outer and inner complexingand ion exchange with K+on the modified peanut shell surface; whereas Pb2+is dominantlyadsorbed via inner complexing with O, N active groups on the modified peanut shell; Inaddition, the manganese dioxide on the peanut shell surface generated during modificationalso plays a role in the adsorption of Cd2+and Pb2+. |
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