Synergistic Removal Of Cadmium From Wastewater By Plant-Derived Biosorbent Combination With Microalgae-Endophyte Symbiotic System (MESS)

Heavy metal pollution of aquatic ecosystems is among the most significant environmental concerns.Among all heavy metals,cadmium[Cd(?)]has received most of the attention as it is a non-biodegradable and a non-essential metal ion which tends to accumulate in living organisms,causing significant threats to both environment and public health.In all removal methods,biosorption is considered to be an economic eco-friendly and efficient alternative process for the removal of heavy metals.However,no researchers have conducted a comparative analysis of the adsorption differences between different biosorbents,which has led to the lack of any reference when selecting biosorbents.Therefore,in this study,some plant-derived biosorbents such as submerged plants,emerged plants,hyperaccumulator plants,and floating plants were used to remove heavy metals in wastewater,meanwhile,the differences and adsorption mechanism of these different plant-derived biosorbents were compared in the removal process,which would provide guidance for future people in selecting biosorbents.However,little attention has been paid on this secondary(i.e.,COD)pollution caused by plant-derived biosorbents.Water hyacinth was used as model of plant-derived biosorbent to remove Cd(?)from wastewater.Adsorption kinetics and equilibrium isotherm data were obtained by batch adsorption experiments.Subsequently,endophytes,microalgae and a microalgae-endophyte symbiotic system(MESS)were used to cope with both the COD released by plant-derived biosorbents and Cd(?)left from previous biosorption process.The main contents of study are as follows:1.Hydrilla,Scenedsmus and Chlorella were used as submerged plant-derived biosorbent for the removal of Cd(?),with maximum biosorption capacity of 173.2,167.8 and 166.9 mg g^-1,respectively.Acorus calamus L.,Phyllostachys heteroclada and Canna indica were used as emerged plant-derived biosorbent for the removal of Cd(?),with maximum biosorption capacity of 82.8,70.2 and 67.4 mg g^-1,respectively.Sedum alfredii,Eremochloa ciliaris and Solanum nigrum were used as hyperaccumulator plant-derived biosorbent for the removal of Cd(?),with maximum biosorption capacity of 64.5,60.1 and 57.9 mg g^-1,respectively.Lemna minor,Nymphodies lungtanensis and Water hyacinth were used as floating plant-derived biosorbent for the removal of Cd(?),with maximum biosorption capacity of 39.2,23.7and 21.6 mg g^-1,respectively.The biosorption capacity for Cd(?)removal by these plant-derived biosorbents can be observed that:submerged plants>emerged plants>hyperaccumulator plants>floating plants.This preliminary reflection that the higher hydrophilicity of the plant material,the stronger ability for Cd(?)removal.2.The cell wall compositions of different plant-derived biosorbents have containing large number of complex organic components.The hydroxyl and carboxyl groups,sulfate groups,amino groups,acylamino,and aldehyde groups in the cell wall are functional hydrophilic groups for biosorbents to remove heavy metals.The groups are well established that provide oxygen and nitrogen lone pair electrons to form covalent bonds with Cd(?).Submerged plants have more hydrophilic groups,which can provide more adsorption sites for Cd(?)removal,thereby having higher adsorption efficiency.Relatively few content of functional hydrophilic groups in emerged plants,hyperaccumulator plants and floating plants,so that there is a certain regularity in their ability to remove Cd(?)in wastewater.Morever,the cell of different plant-derived biosorbents have large number of inorganic ions Ca(?),K(?),Mg(?),Na(?),etc.which made Cd(?)adsorbed on these biosorbents by ion exchange.3.In this study,water hyacinth was used as model of plant-derived biosorbent to remove Cd(?)from wastewater.The optimal equilibration pH was 6.5,the dosage of hyacinth powder concentration was 5 g L^-1,and the equilibration time was 60 min.The Cd(?)adsorption process can be considered a second-order process.About 66%of Cd(?)was removed by the biosorbent with a maximum biosorption capacity of 21.6mg g^-1.However,the COD of the filtrate increased from 0 to 292 mg L^-1during this process.4.Endophytes,microalgae and the microalgae-endophyte symbiotic system(MESS)were assessed for the simultaneous Cd(?)and COD removal.In the single endophytes group,six different endophytes(EBL14,LSE02,LSE03,LSE05,LSE06,LSE07)were added to the filtrate.The extent of Cd(?)removal increased from 66%to more than 80%after 48 h of incubation with endophytes,however,COD removal was not observed.In the single microalgal group,about 98%of Cd(?)and 63%COD was removed after 9 d of incubation.In the MESS,after 3 d of inoculation,the extent of total Cd(?)removal increased to 99.2%while COD removal increased to 73.7%.This symbiotic system not only effectively increase the extent of COD removal but also accelerated the process.This study provides a new insight into the application of a plant-derived biosorbent in combination with microalgae and endophytes for the effective treatment of heavy metal-bearing wastewater.