Adsorption Properties Cd²⁺ And Pb²⁺ From Aqueous Solutions By Sludge-based Humic Acid

Humic acid is the most widely distributed natural organic polymer in nature,accounting for about 50%⁸0%of the total organic matter content in soil and hydrosphere ecosystems,has an important impact on the migration and transformation of heavy metals and organic pollutants.Municipal sludge is a by-product of sewage treatment,mainly composed of microorganisms,bacterial gums,inorganic particles and organic substances and humic acid is the main component of organic matter in sludge.In this study,the sludge humic acid was derived from the waste sludge from Qilihe-Anning sewage treatment plant in Lanzhou city,according to the method which was recommended by the International Humus Association?IHSS?.The surface properties and structural characteristics of humic were characterized by SEM-EDS,FTIR,XRD,XPS,specific surface area measurement and elemental analysis;the adsorption characteristics and mechanism of Cd²⁺and Pb²⁺on sludge humic acid were studied,the effects of adsorbent dosage,solution pH,adsorption time and coexisting cations in solution on adsorption were investigated,and the competitive adsorption mechanism of sludge humic acid on co-existing heavy metals was explored;the results provided a scientific basis for sludge resource utilization and the heavy metal pollution control in water.The main results of the paper as follows:?1?The sludge humic acid?S-HA?derived from sludge was characterized and the result showed that S-HA was mainly composed of C,H,O,N,the specific surface area was small,and the pore structure was not developed,the pore size distribution was mainly composed of large pore size;the surface structure of S-HA was loose and rough,the degree of aromaticity and humification was higher,the structure of aliphatic chain was more,and it contained a large number of oxygenic functional groups,such as carboxyl groups and hydroxyl groups,providing a lot of adsorption sites for the adsorption of heavy metal ions.?2?The single adsorption performance of S-HA for Cd²⁺and Pb²⁺in the solution under the mono-system was studied by the static adsorption experiment.The results showed that the dosage of adsorbent,the pH value,the adsorption time and the co-existing cation strength in the solution was proportional to the adsorption removal rate of the two heavy metals.Under the experimental conditions,the maximum adsorption capacity of Cd²⁺and Pb²⁺by S-HA was19.29 mg/g and 27.59 mg/g,respectively.At the same time,the adsorption mechanism of two heavy metals by S-HA was also discussed in this paper,the results showed that the adsorption process of Cd²⁺and Pb²⁺by S-HA existed both physical adsorption and chemical adsorption but mainly by chemical adsorption.The complexation of Cd²⁺and Pb²⁺by oxygenic functional groups on the surface of S-HA was the main adsorption mechanism.?3?New diffraction peaks were not found in the XRD spectra before and after the adsorption of Cd²⁺and Pb²⁺by S-HA,indicating that that there was no precipitate produced after the adsorption of S-HA to Cd²⁺and Pb²⁺,which meaned the surface precipitation was not the main mechanism of the adsorption of Cd²⁺and Pb²⁺by S-HA.At the same time,the wave number of the characteristic peaks of carbonyl?C=O?,carboxyl?-COOH?and hydroxyl?-OH?on the FTIR spectra before and after the adsorption of Cd²⁺and Pb²⁺by S-HA shifted,and the peak intensity decreased,indicating that-OH,.-COOH and C=O produced surface complexing cooperation for Cd²⁺and Pb²⁺.In addition,the equilibrium pH value of the solution decreased after the adsorption of Cd²⁺and Pb²⁺on the S-HA,further confirming that the surface functional groups of the S-HA and the Cd²⁺and Pb²⁺form complexes,further confirming that functional groups on the S-HA surface formed a complex with Cd²⁺and Pb²⁺.?4?The competitive adsorption behavior of Cd²⁺and Pb²⁺on S-HA under binary system showed that the adsorption of Cd²⁺on S-HA was inhibited obviously by Pb²⁺,resulting the adsorption capacity of S-HA to Cd²⁺decreased obviously.And the selective adsorption coefficient of Pb²⁺in competitive system was higher than that of Cd²⁺,which indicated that the competitive adsorption ability of Pb²⁺in competitive adsorption system was stronger.The adsorption performance of Cd²⁺and Pb²⁺on S-HA was related to the physical and chemical properties of heavy metals.