Study On The Adsorption Of Lead And Cadmium In Water By Hydroxy Iron Oxide And Its Humic Acid Modifiers

Lead,cadmium and other toxic heavy metals have accumulated in the natural environment for a long time,resulting in heavy metal pollution that poses a serious threat to human health and ecological balance,and has become one of the environmental problems facing the world.In this study,iron nitrate and potassium hydroxide were used as raw materials to find suitable conditions for the synthesis ofα-Fe OOH by examining the reaction p H,ageing temperature and ageing time.On this basis,humic acid（HA）was modified by co-precipitation to prepare a complex（Ge-HA）,which was used in the adsorption experiments of wastewater containing Pb²⁺and Cd²⁺to explore its adsorption law and provide theoretical support for the treatment of Pb²⁺and Cd²⁺wastewater.The main findings were obtained as follows:（1）Reaction p H is a key factor in the synthesis of iron hydroxides,and their crystallinity is influenced by a combination of ageing temperature and ageing time.Well crystallisedα-Fe OOH was produced at a reaction p H of 12,an ageing temperature greater than 70°C and an ageing time of more than 48 h.As C/Fe increases in the composite,the morphology changes from needle-like to granular,and the specific surface area and pore volume gradually decrease,while the stability under acidic conditions increases.（2）The adsorption of Pb²⁺and Cd²⁺byα-Fe OOH was significantly influenced by the initial p H,the ionic strength of the solution and the reaction time.When the temperature was25℃,the equilibrium time for the adsorption of Pb²⁺and Cd²⁺byα-Fe OOH was 240 min,and the theoretical maximum adsorption amounts were 29.82 mg/g and 14.11 mg/g,respectively,and increased with increasing temperature.The Langmuir isothermal adsorption model with quasi-secondary kinetic model can better explain the adsorption ofα-Fe OOH on Pb²⁺and Cd²⁺as a monolayer chemisorption and also as a spontaneous heat absorption process.The material characterisation results show that the complexation of Pb²⁺and Cd²⁺by hydroxyl groups on the surface ofα-Fe OOH is the main adsorption mechanism.（3）HA in solution leads to changes in the adsorption behaviour of Pb²⁺and Cd²⁺on theα-Fe OOH surface.At p H=2～6,the removal of metal ions from the composite adsorption system Pb-HA and Cd-HA byα-Fe OOH was superior compared to the single adsorption system.HA can form new adsorption sites on the surface ofα-Fe OOH,which in turn can ligand-bind to metal ions via carboxyl groups.And at p H=7～8,HA forms a large number of water-soluble complexes with strong migration ability with Pb²⁺and Cd²⁺,inhibiting their transfer to theα-Fe OOH surface.（4）Ge-HA has a better adsorption capacity for Pb²⁺and Cd²⁺compared toα-Fe OOH.When the temperature was 25℃,the theoretical maximum adsorption amounts of Ge-HA for Pb²⁺and Cd²⁺were 96.06 mg/g and 45.37 mg/g respectively.The adsorption of Pb²⁺and Cd²⁺by Ge-HA follows well the Langmuir isothermal adsorption model and the quasi-secondary kinetic model,being a monolayer chemisorption-based heat absorption reaction and a spontaneous process.The material characterisation results show that Ge-HA achieves the removal of Pb²⁺and Cd²⁺from solution by complexing its surface hydroxyl and carboxyl groups with Pb²⁺and Cd²⁺to form-O-M⁺,-（COO）₂-M and-COO-M⁺.