Adsorption Of Cadmium From Aqueous Solution By Ferrihydrite And Their Coprecipitate With Humic Acid

The problem of heavy metal pollution in water is more and more serious.Among them,cadmium is a potential threat to the environment and human health because of its strong toxicity and easy accumulation characteristics.The remediation methods of cadmium pollution in water mainly includes physical method,biological method and chemical method,and the chemical adsoption method is more mature.Different adsorption materials has different adsorption effect to remove cadmium,the selection of suitable adsorption materials is the key to the treatment of acdmium pollution.Ferrihydrite and humic acid are widespread in the natural environment,and have strong adsorption effect on heavy metal through different mechanisms.And some studies have shown that the coprecipitate of the humic acid and ferrihydrite have good adsorption effect on heavy metal in water compared with single system.At present,there's little research on the interaction and ferrihydrite and humic acid,specific response mechanism between ferrihydrite-humic acid and cadmium needs further study.In this paper,the adsorption and desorption characteristics of cadmium on and from synthetic ferrihydrite,coal humic acid and their coprecipitate were compared by the methods of adsorption-desorption thermodynamics and kinetics,effects of various factors,including the initial concentration of cadmium,ionic strength,pH and adsorption duration were also evaluated.The mechanisms of cadmium adsorption on ferrihydrite-humic acid coprecipitate were discussed by infrared spectroscopy,XRD spectroscopy,scanning electron microscopy(SEM)and specific surface area analyzer(BET).The main results are as follows:(1)The combination of humic acid with ferrihydrite did not change the crystal structure of ferrihydrite.but the addition of humic acid reduced the specific surface area and capacity of the coprecipitate,such changes impact the cadmium adsorption and desorption on the ferrihydrite.(2)The kinetics process of ferrihydrite-humic acid coprecipitate for adsorbing cadmium can be described by pseudo-second-order kinetics equation and is mainly based on rapid response,while the adsorption equilibrium complies with the Langmuir isotherm adsorption model and the adsorption process belongs to the favorable adsorption and chemical adsorption,the wholeadsorption process is probably a result of the combination of various dynamic mechanisms.(3)The increase in the dosage of adsorbents will increase the adsorption rates,while the utilization factor declined.Alkaline conditions ared favorable for the adsorption,adsorption rises with the pH increases.The formation of inner-sphere coprecipitates responsible for the small change of adsorption performance in diffrerent ionic strength.The adsorption capacity decreases and then rises with the increase of ionic strength,that shows the adsorption process includes physical and specific adsorption.(4)The adsorption and desorption kinetics of cadmium on coprecipitate are common,but the desorption of cadmium was significantly lower than its adsorption capacity,demonstrating that the adsorption of cadmium on coprecipitate was relatively strong.