Effect Of Isomorphous Substitution Into Inorganic Colloid On Adsorption Of Humus And Heavy Metal Ions In Soil

Goethite（α-FeOOH）is ubiquitous in the terrestrial environment and is one of the most common and important inorganic colloids in soil.Goethite can adsorb dissolved organic matter（DOM）and heavy metal ions in soil,playing a key role in the retention of DOM and passivation of heavy metal pollution.In natural environments,non-iron metals can enter goethite through isomorphic substitution,forming non-iron metals isomorphous substitution-goethite.These non-iron metalswill significantly change the physical and chemical properties of goethite,thereby affecting its adsorption of other substances.This research used manganese（Mn）as a alternative metal,exploring the influence of isomorphic substitution on the physicochemical properties of goethite,and then through adsorption experiments,clarifying the adsorption of DOM and Pb（II）on original and isomorphic substitution goethite samples by adsorption experiments.The main conclusions are as follows:（1）Goethite and Mn-containing goethite samples were prepared by coprecipitation.Substitution amounts of Mn in the sample are 0（Goe）,4.0（MnGoe1）,8.1（MnGoe2）,and13.1 mol%（MnGoe3）,respectively.The results of XRD and FESEM indicate that a large amount of impurity crystal phases appear in the system when the Mn content was 13.1%.However,the XRD results of MnGoe1 and MnGoe2 samples indicated that there was no obvious impurity peak when the content of Mn in the goethite were 4.0 and 8.1,indicating that Mn was mainly dispersed in the crystal structure instead of Fe（III）.This research displayed that the microscopic morphology and physicochemical properties of goethite were significantly changed by isomorphous substitution by Mn.In addition,XPS analysis results showed that Mn in the fabricated Mn–goethite samples was mainly present as Mn（III）.（2）The maximum adsorption capacities of Goe,MnGoe1,and MnGoe2 for HA were113.57,130.70,and 145.75 mg·g^-1,respectively.The maximum adsorption capacities for FA were 71.85,97.29,and 114.32 mg·g^-1,respectively.After normalization analysis of SSA,the maximum adsorption capacities of Goe,MnGoe1,and MnGoe2 for HA were4.42,3.63,and 3.38 mg·m^-2,respectively.The results indicated that the adsorption behavior of goethite and Mn-containing goethite for DOM mainly involved H-bonds,surface complexation,hydrophobic interactions,and electrostatic interactions.（3）The maximum adsorption capacities of Goe,MnGoe1,and MnGoe2 for Pb（II）are 37.67,57.63,and 85.43 mg·g^-1,respectively,After normalization analysis of SSA,the maximum adsorption capacities of Goe,MnGoe1,and MnGoe2 for Pb（II）were 1.47,1.60and 1.98 mg·g^-2,respectively.Incorporating Mn into goethite not only enhanced the adsorption capacity of goethite per unit mass for Pb（II）,but also improved the adsorption capacity of mineral samples per unit SSA for Pb（II）.SEM-EDS research showed that Pb（II）is more bound to Mn sites of mineral.Isomorphic substitution increased the content of hydroxyl groups on the surface of goethite,making it possible to adsorb more Pb（II）per unit of SSA At low p H,the formation of complex products is the main mechanism for goethite to adsorb Pb（II）.At higher p H,due to the transformation of Pb,both Vander Waals force and precipitation are involved in the adsorption of mineral samples for Pb（II）.（4）This research studied the effects of environmental conditions on the adsorption of DOM and Pb（II）by different mineral samples.The results showed that both p H and ionic strengthhad impact on the adsorption of DOM by mineral samples.This was mainly because p H changed the electrostatic attraction between the mineral sample and DOM,while ionic strength had salting-out,or salt particle bridges.In addition,p H can significantly affected the adsorption of Pb（II）by mineral samples,because p H can changed the morphology of Pb ions,thereby affecting the adsorption of Pb by mineral samples.Ion strength did not t have a significant impact on the adsorption of Pb（II）ions by mineral samples（especially for Mn substituted goethite samples）,mainly because Pb（II）formed inner-sphere surface complexes with mineral samples,weakening the impact of changes in ion strength on the adsorption of Pb by goethite.