Comparative Study On Characteristics Of The Adsorption Of Lead And Cadmium To Activated Sludge

The excess activated sludge discharged from the process of biological wastewater treatment is a waste liquid due to the proliferation of sludge microorganisms', the sludge becomes wet earth type after concentrated and dewatered. Municipal wastewater treatment plant sludge is generated mainly from the excess activated sludge and primary sedimentation tank sludge. At present, lots of unstable sludge has become a heavy burden of sewage treatment plant, how to safely deal with this huge, complex sludge and makes it harmless, minimization, recycling has become an important issue which environmental and social workers concerned about. The current researches are more about the adsorption of heavy metal onto sludge, but little about the ones of the industrial wastewater sludge, more about dry sludge adsorption of heavy metal, and little about the direct application of sewage dewatered sludge. Most of studies did not consider the feasibility of desorption. And there are few comparative studies on the adsorption of heavy metals among the different sludge at the same experimental conditions. In this paper, the excess sludge that comes from different wastewater treatment plant is used to adsorb heavy metal ions, the adsorption characteristics and absorption mechanisms were studied for the aim to supply a theory support for the practical application of treating heavy metal wastewater and find an additional method to cost-effectively treat the heavy metal wastewater. So this study have the dual effect of making sludge resources and treating heavy metal wastewater, This study has the resources to achieve the remainder of the use of sludge, and has a wide range of potential applications.The main content of this study includes basic property analysis of the different dewatered excess sludge, the adsorptive thermodynamic and kinetic properties and it's comparison of lead and cadmium to different sludge, the factors that influence lead and cadmium adsorption to sludge, the desorption of lead and cadmium from sludge, the pretreatment method of sludge and it's influences on the adsorptive thermodynamic and kinetic properties of lead and cadmium to pretreated sludge, the further study involves the contribution of ion-exchange mechanism to the lead and cadmium adsorption to sludge, the primary mechanisms about lead and cadmium adsorption to sludge, the correlations between the adsorptive thermodynamic and kinetic parameters and the chemical composition of sludge. Some valuable results were concluded based on this study. The thermodynamic experimental data of lead and cadmium adsorption onto sludge is analyses through non-linear fitting, high correlation coefficient is derived using both the Langmuir and Freundlich isotherm fitting, better fitting accuracy is got with Langmuir isotherm in general which indicates the monolayer adsorption trend of activated sludge adsorption for lead and cadmium. Study shows that the dewatered excess activated sludge had a high adsorptive capacity for lead and cadmium and are appropriate to remove lead and cadmium as bio-absorbent, but also remains adsorptive capacity difference of different sludge. The adsorption process of lead and cadmium to the sludge is generally endothermic process, the enthalpy variation for the adsorption of lead and cadmium is little, the Gibbs free energy change is negative quantity in all the adsorption process which indicates the adsorptions of lead and cadmium to sludge are spontaneous.Most of the adsorption processes of lead and cadmium by excess activated sludge fit the pseudo-second-order kinetic model well, the kinetic constants of the adsorption of lead and cadmium by dewatered sludge is higher than usual, in which the lead kinetic constant are higher than cadmium kinetic constants. The adsorption processes of lead and cadmium by excess activated sludge is so quick that the high adsorptive capacity is attained within 15 to 30 minutes which is very beneficial to the practical operation. The adsorptive rate of lead and cadmium by the excess sludge is controlled by out layer diffusion or by out layer and intraparticle diffusion.Research shows that sludge dosage has much influence on the adsorption of lead and cadmium to sludge, the lead biosorption efficiency increased rapidly with the sludge dosage increasing, while the cadmium biosorption efficiency increased slowly with the sludge dosage increasing, for the cadmium, much more sludge dosage must be added in order to get the same biosorption efficiency as lead. The optimal sludge dosage for the lead and cadmium adsorption varies to different sludge. The pH value is an important factor that affects the adsorption of lead and cadmium to sludge. The lead adsorption to sludge generally has an appropriate pH range which is from 4 to 7, but has a different optimal range rest on the sludge type, the appropriate pH range of cadmium adsorption to sludge is narrow, in which an optimal pH is between 5 and 6. The sludge adsorptive capacity will decrease at low pH due to the competitive adsorption of proton and metal ions, while high pH leads to the hydrolysis and precipitation of heavy metal ions which is removed not by adsorption. Temperature has less effect on sludge's adsorption to lead and cadmium, so the research and practical application can ignore it. In lead and cadmium coexisting adsorption system, cadmium coexisted inhibits the adsorption of lead to sludge, lead coexisted also inhibits the adsorption of cadmium and even more, this is explained for the heavy metal ion character differences including hydrolysis, electro negativity, plasma potential, ionic radius and redox potential etc.The heavy metal desorption effectiveness is associated with both the metal ion and the sludge type, high desorption efficiency is attained to some of the sludge with HCl and EDTA and problems lies in the low cadmium desorption efficiency to a certain sludge and difficult separation of the sludge from the desorption liquid, which makes the desorption invalidity in practical application.In general, dry sludge adsorption data is fitted the pseudo-second-order kinetic model better than the pseudo-first-order kinetic model, all the kinetic constants decrease after the dewatered sludge is dried and the cadmium adsorptive kinetic constants vary less than the lead adsorptive kinetic constants. The adsorptive mechanism shift from the surface adsorption to the surface and inner sphere together for the lead adsorption to the sludge before and after dewatered sludge is dried and intraparticle diffusion becomes important step for dry sludge.The adsorptive capacity of lead to the sludge treated by the alkali is higher than that of dry sludge, but not necessarily higher than that of wet sludge, therefore, sludge alkali treatment in order to increase adsorptive capacity of lead is valid to dry sludge, but not significance to the dewatered sludge, moreover the affinity of lead to the sludge treated with alkali deteriorations. The absorptive capacity of cadmium on different sludge through alkali treatment changes differently, some higher and some lower, the affinity of cadmium to the sludge strengthen after treated with alkali. Research shows that the sludge adsorptive capacity is not necessarily up after sludge dried and alkali treatment, this reflects the complexity that the differences of structure and composition of the sludge imposed on the absorptive characteristics of lead and cadmium. There is no pretreatment need for some dewatered sludge as it has high adsorptive capacity, the direct use as adsorbent is more simple and convenient without drying.Ion-exchange is one of main adsorption mechanisms that the excess sludge adsorbs lead and cadmium, especially significant on the adsorption of cadmium onto sludge. The contribution of ion exchange to the adsorption of lead and cadmium to sludge is related to both the initial concentration of lead and cadmium and also the type of sludge. Infrared analysis showed that the amines, hydroxyl, carboxyl groups of sludge is the functional groups on the adsorption of lead and cadmium. Little chance of inorganic precipitation happens during the adsorption of lead and cadmium onto various sludges. Ion-exchange, complexation and micro-precipitation should be the main adsorption mechanisms of lead and cadmium to sludge.The correlation analysis shows that the influence of the composition of sludge on the sludge adsorptive characteristic parameters is mainly embodied at the thermodynamics constants. Calcium content influences the sludge adsorptive capacity for heavy metal ions. Moreover, the content of Na, Fe and Mn influence the adsorptive capacity or affinity of the lead and cadmium onto the sludge. The sludge's pH value influences the adsorptive intensity of cadmium as well as the dynamics of cadmium on sludge.In this paper, a comparative study on the adsorption characteristics including thermodynamics, dynamics and impact factors of lead and cadmium to dewatered activated sludge is accomplished for the first time. A more comprehensive study on the changes of the sludge adsorptive thermodynamics and dynamics for lead and cadmium before and after sludge pretreatment is done with a meaningful result. Further study on the ion-exchange contribution to the sludge adsorption for lead and cadmium is calculated, showing ion exchange is the main mechanism of the adsorption. Acid and complexation desorption of the dewatered sludge is tried. The main functional groups are ascertained on the adsorption process. This paper has a powerful theory support for the sludge reuse and heavy metal biosorption treatment.