Experimental Study On Remediation Mechanisms Of Heavy Metal Pollution Based On The Effects Of Sediment Adsorption And Plant Uptake

In the present research, mechanisms of heavy metal pollution remediation basedon the properties of sediment adsorption and plant uptake were studied through thelaboratory analysis method. Sediments and wetland plant species were collected fromNanfeihe River in Hefei and a wetland located in Chaohu, respectively. The main contentsand conclusions are summed up as follows:(1) The analysis of cadmium (Cd) adsorption characteristics by sediments. TheBox-Behnken Design（BBD）was used to design experiments with time, temperature andpH as three design factors, the adsorption as the corresponding target. The experiment datawas analyzed by Design-Expert software, and the regression model was concluded.Response surface methodology (RSM) was applied to study individual and interactiveeffects of the solution pH, temperature and adsorption time on Cd adsorption. Meanwhile,the adsorption isotherm and adsorption kinetics were also investigated. The results showthat the solution pH and time have significant effects on the Cd adsorption, while the effectof temperature is little; the interaction of time and pH value is also more significant. Theoptimal Cd adsorption conditions acquired by RSM is as follows: pH value is10,temperature is30℃, time is60min, and the maximum adsorption capacity is2.19mg/g.The adsorption isotherm results show that the Freundlich isotherm is able to provide abetter fit to the experimental data, and the process is also related to the Langmuir model.Compared with the pseudo-first-order, pseudo-second-order kinetic model can betterdescribe the Cd adsorption process.(2) The analysis of chromium (Cr) adsorption characteristics by sediments. TheCr-contaminated sediment (Cr-CS) is used to study the effects of several factors on Crremoval by changing solution pH, sediment dosage, the background Cr concentration,organic matter, and co-existent heavy metals in the sediment. The Cr(VI) removal kineticswere also investigated. Experimental results show that the background Cr concentration inthe sediment has little influence on the removal efficiency of total Cr. The existence oforganic matter favors the reduction and adsorption of Cr(VI), but has a limited effect on theremoval of Cr(III). The Cr removal process involves reduction, adsorption and precipitation,and is highly pH-dependent. The optimum pH values for Cr(VI) and Cr(III) removals aredetermined as1.0and4.0, respectively. The results also suggest that co-existent heavymetals (Cd, Cu, Pb, and Ni) are hard to release to the water under experimental conditions. Compared with the pseudo-first-order and pseudo-second-order kinetic model based onreduction, empirical rate equation derived by Wittbrodt and Palmer can better describe theremoval kinetics of Cr(VI) with different dosages of Cr-CS. The kinetics of Cr removalinvolving the effects of organic matter (OM) and pH was acquired.(3) The study of Cr stress on phytoremediation. In this work, ten kinds of commonwetland plant species with high biomass were tested for Cr removal in the solution of12mg/L Cr(VI). Growth rate, concentration factor (CF), and chlorophyll content of testedplants were determined, and Typha angustata was proven to be an excellenthyperaccumulator. Tolerance experiments and kinetics results show that T. angustata cansurvive in high concentrations of Cr solution up to30mg/L, and it exhibits goodaccumulation ability for9to12mg/L wastewater. Physiological studies on T. angustatareveal that protein, soluble sugar, and malondialdehyde (MDA) contents increase with theincrease of Cr(VI) concentration from9to30mg/L. In addition, Cr in T. angustata wasconcentrated predominantly (94.81%) in roots, with only5.19%in frond.