Effects Of Biochar And Vermiculite Mixed Addition On Leaching Behavior And Plantability Of Dredged Sediment Pollutants

Due to the inadequate construction of urban pipe network and the diversion of rain and sewage,a large amount of domestic sewage and industrial wastewater are discharged into the river without treatment,resulting in seasonal or even perennial black and odor phenomenon in the river water,which seriously affects the production and life of coastal residents.Dredging the sediment is considered to be one of the most effective means to eliminate black and odorous water bodies.However,the dredging project will produce dredged sediment containing a large amount of pollutants.The dredged sediment contains a large amount of nutrients such as nitrogen,phosphorus and potassium,which has the potential to be used as a planting bag filler.Two problems need to be solved when using dredged sediment as a planting bag filler:one is to control the leaching of pollutants in the river sediment to prevent it from causing secondary pollution to the surrounding environment;the other is to improve the plantability of the river sediment.It can maintain the good growth of green vegetation.This study took the river sediment of Ba He River in Xi’an as the research object,and carried out batch dynamic leaching experiments,culture experiments and pot experiments to study the leaching behavior and plantability of biochar and vermiculite additions to pollutants in the river sediment.In order to provide theoretical basis and technical guidance for the use of river sediment as a plant bag filler.The main conclusions of the study are as follows:1.The results of dynamic leaching experiments show that the addition of biochar and vermiculite can reduce the leaching amount of pollutants in the sediment,and the reduction range increases with the addition of biochar and vermiculite.The reduction of pollutant leaching amount is mainly attributed to the adsorption of pollutants by biochar and vermiculite;the maximum adsorption capacities of biochar for Cu²⁺,Pb²⁺,Cr³⁺and PO₄^3-are 22.74 mg/g,39.93 mg/g and 72.40 mg/g and 73.89 mg/g,the maximum adsorption capacities of vermiculite for Cu²⁺,Pb²⁺,Cr³⁺and PO₄^3-were 19.23 mg/g,22.27mg/g,15.80 mg/g and 78.97 mg/g.The adsorption of biochar and vermiculite for various heavy metals and PO₄^3-in the sediment conformed to the Langmuir model;the adsorption capacity of biochar for Cu²⁺,Pb²⁺and Cr³⁺was stronger than that of vermiculite,but the adsorption of PO₄^3-was weaker than that of vermiculite.The experimental results of heavy metal species analysis showed that biochar and vermiculite could change the copper and lead in the sediment from unstable state to stable state,but the effect on chromium species was not obvious.The mineral composition analysis of sediments did not significantly change after adding biochar and vermiculite.2.The culture experiment showed that the mixed addition of biochar and vermiculite led to complex changes in the physicochemical properties of the sediment.The pH value,organic matter content and permeability coefficient of the sediment gradually increased with the addition of biochar and vermiculite.Compared with the control,the additions of biochar and vermiculite were 2%,5%and 10%.and 20%（mass ratio）,the pH value of the sediment increased by 0.06,0.18,0.52 and 0.58 units,the organic matter content increased by 1.04 times,1.16 times,1.25 times and 1.53 times respectively,and the sediment permeability coefficient increased by 1.04 times.times,2 times,2.5 times and3.7 times.The density of sediment,field water capacity,and hydrolyzed nitrogen content decreased gradually with the increase of biochar and vermiculite additions.Compared with the control,when biochar and vermiculite additions were 2%,5%,10%and 20%（mass ratio）,the density of sediments decreased by 0.03 g/cm³,0.21 g/cm³,0.26 g/cm³and 0.41 g/cm³,and the field water holding capacity of sediments decreased by 5.04 g/kg,6.89 g/kg,11.28 g/kg and 20.05 g/kg,and hydrolyzed nitrogen decreased by 3.6 mg/kg,32.6 mg/kg,35.7 mg/kg and 53.6 mg/kg.3.The pot experiment showed that the addition of biochar and vermiculite increased the germination rate of ryegrass.When the addition of biochar and vermiculite is 5%-10%,the plantability of river sediment can be improved;compared with the control sample,after cultivating for 30 days,the plant height of ryegrass increased by 80.1%-96.1%,and the chlorophyll content It increased by 18%～38%,root activity increased by41%～48%,biomass increased by 28%～31%;after cultivating for 45 days,the plant height increased by 83.9%～98.4%,and the chlorophyll content increased by 18%～20%,root vigor increased by 13%～32%,biomass increased by 57%～84%.When the addition of biochar and vermiculite reached 20%,the addition of biochar and vermiculite could not effectively improve the plantability of the river sediment.In addition,the effects of biochar and vermiculite on the plantability of sediments were time-sensitive,and the effects were obvious in the first 30 days of the pot experiment,and after 45 days,the effects gradually decreased with time.