Characteristics Of Humic Acid From Sewage Sludge And Adsorption Of Heavy Metals After Modification

The excess sludge from municipal wastewater treatment plant is rich in organic matter mainly composing of humic acid which can play an important role in treating heavy metal by adsorption,complexing and stabilizing because of having different types of functional groups.Although Some progress has been made in the study of sludge organic matter in composting,land use and activated carbon,research on humic acid directly extracted from excess sludge modified as adsorbent is poorly studied?In this paper,physical and chemical characteristics of humic acids extracted from different steps of digesting and different treatment stages in municipal wastewater treatment plant were studied.Properties of complexation between the sludge humic acids with Cu²⁺,Pb²⁺and Cd²⁺and efficiency of adsorbing heavy metal Cu²⁺or Pb²⁺were investigated.This paper is try to obtain new effectual way for comprehensive development of excess sludge.The humic acids extracted from excess sludge were characterized by SEM,elemental analysis,FTIR,and Solid ¹³C NMR.Complexation constants of Cu²⁺,Pb²⁺and Cd²⁺with humic acid were achieved at various conditions by application of ion exchange method and the effect on complexing were also studied.The results showed that the surface of the digested sludge was looser than concentrated sludge with layered stacking shape;with the increase of digestion time,the composting of C%and H%in humic acid decreased gradually,while the content of O,N and S increased gradually;Fourier infrared spectroscopy showed that the humic acid from sludge contained a large number of complex functional groups which influenced by digestion time;solid state nuclear magnetic resonance showed that the distribution of carbon in the humic acid structure of each sludge was aliphatic>carbohydrate>carboxyl compound>aromatic with an increaseing of aromatic carbon and carboxyl carbon after 20 days of digestion.The range of complexation constants logK_f of sludge humic acid with Cu²⁺,Pb²⁺and Cd²⁺were 4.84⁷.94,3.32⁵.38 and 4.48⁴.85 respectively which influenced by temperature,pH and ionic strength,but under the same conditions,logK_f(Cu²⁺)>logK_f(Pb²⁺)>logK_f(Cd²⁺).Humic acids as adsorbents form excess sludge modified respectively by heating,chitosan composite and montmorillonite composite had special structures and carrying a variety of complexing functional groups.The adsorption mechanism and adsorption factors for the humic acid-adsorbents to heavy metals Cu²⁺or Pb²⁺were investigated.The research revealed that condensation and decarboxylation reaction took place for humic acid by heating,the proportion of oxygen-containing functional groups decreased,the absorption efficiency reaches 59.87mg/g of Cu²⁺with more porous,the adsorption process can be well fitted with pseudo-second-order kinetic model,the adsorption data obtained can be described by the Langmuir adsorption isotherm model.The sludge humic acid chitosan composite adsorbent still maintained the typical chemical structure and main functional groups of both sludge-humic acid and chitosan,although the specific surface area decreased because of agglomeration.Adsorption kinetics and isotherm equations and effects of pH of Pb²⁺onto sludge humic acid chitosan composite adsorbent were investgated in detail and the sorption mechanisms were analyzed with spectroscopic technique and the maximum adsorption capacities for Pb²⁺is190.10mg/g.Sludge-humic acid and montmorillonite composite adsorbent prepared by simple and efficient way of direct mixing maintained original structure,which sludge-humic acid was attached on the suiface of montmorillonite.Characteristics of sludge humic acid montmorillonite composite as a adsorbent for Pb²⁺was sdudied and the sorption mechanisms of it were mainly complexation and ion exchange.The adsorption effect of coexisting ions on the adsorption of Pb2+on sludge humic acid chitosan composite adsorbent or sludge humic acid montmorillonite composite were discussed.The effect of commonly coexisting non-heavy metal cations K⁺,Na+,Ca²⁺and Mg²⁺to adsorption of Pb²⁺on humic acid montmorillonite composite or humic acid chitosan complex is not significant,and the effects of K⁺and Na⁺are relatively small.Coexisting heavy metal ions Cd²⁺and Zn²⁺generally will interfere the adsorption of Pb²⁺on sludge humic acid chitosan composite adsorbent or sludge humic acid montmorillonite composite and correlated with the concentration of respective heavy metal ions in solution,with higher level of Cd²⁺.