Characterization Of Cadmium Accumulation With Corn Stover That Enhances Biomass Enzymatic Saccharification For Bioethanol And Carbon Quantum Dots Production

Cadmium(Cd)is one of major toxic heavy metals polluting agricultural lands and daily-life locations,and thus plant phytoremediation has been considered as a powerful approach for heavy metal removal.Meanwhile,crop straws provide enormous biomass resource convertible for biofuels and other bioproducts such as fluorescent carbon quantum dots.However,little has been yet reported about one crop straw available for heavy metal removal,cellulosic ethanol production and fluorescent carbon quantum dots preparation using biomass process technology.In this study,we used two corn cultivars(Zm03,Zm 18)with distinctive biomass yields to grow in the soil pots co-supplied with Cd,and then determined Cd accumulation in corn stovers.Meanwhile,we detected cell wall compositions and major wall polymer features of the Cd-accumulated corn stovers and examined their enzymatic saccharification and ethanol yields under three physical and chemical pretreatments,with the Cd released during biomass process for bioethanol production.Furthermore,we used the supernatant of pretreatment and sequential biomass residues after enzymatic hydrolysis to prepare carbon quantum dots and then added it into the T.reesei RUT-C30 incubation culture for secreting high cellulases enzymes production.Here,the main results were described below:1.While grown in the soil pots co-supplied with Cd,corn Zm03 cultivar could accumulate much more Cd than that of the Zm18 in their mature stovers,and both corn cultivars showed significantly increased soluble hexoses levels in the stovers by 22.12%and 25.21%,compared to the controls of two corn cultivars without Cd supply.2.Due to the Cd accumulation,two corn cultivars showed significantly reduced cellulose and lignin levels in their mature stovers,with reduced rates of 14.37%and 4.29%(cellulose)and 9.83%and 8.71%(lignin),respectively.By contrast,the Cd accumulation could increase hemicellulose by 5.87%and 4.92%and pectin by 2.63%and 8.20%.Meanwhile,we detected significantly decreased wall polymers features in the Cd-accumulated mature stovers including the degree of polymerization(DP)and crystallinity index(CrI)of cellulose and the Xyl/Ara ratio(reverse branching degree)of hemicelluloses.3.Under three pretreatments(1%H2SO4,5%CaO,10 min LHW pretreatments)with the Cd-accumulated mature stovers,two cultivars showed much enhanced biomass enzymatic saccharification,compared to their controls(without Cd).Notably,under 5%CaO pretreatment with the Cd-accumulated mature stovers,two com cultivars showed almost completely enzymatic hydrolysis while co-supplied with 1%Tween-80,with hexoses yields of 100%(%cellulose),leading to the ethanol yields of 15.43%and 15.53%(%dry weight)achieved.Even though under 10 min LHW pretreatment,the corn Zm03 cultivar was of the hexoses yield at 100%,with the bioethanol yield of 13.68%in the Cd-accumulated stover.4.Chemical analysis indicated that 51%～63%of total cadmium accumulated in corn stover were tightly associated with wall polymers in two corn cultivars.However,three pretreatments and sequential enzymatic hydrolyses performed above could release 86%～98%of total Cd accumulated in the mature stovers of two corn cultivars for Cd collection and recycling.Notably,only three pretreatments performed could release all soluble Cd and partial Cd associated with wall polymers,but both 1%H2SO4 and 10 min LHW pretreatments led to more Cd release than that of the 5%CaO pretreatments in two corn cultivars.5.Using either the liquid supernatant of 10 min LHW pretreatment or the solid residues remained after sequential enzymatic hydro lysis,this study prepared to synthesize the carbon quantum dots(CQDs)that were of blue fluorescence.While the CQDs were co-supplied into the culture medium of T.reesei RUT-C30,the secreted cellulases were of either significantly enhanced degradation activity with the filter paper or much higher enzymes yield,but their specific activity was not significantly altered,compared to the control(without CQDs)performed in this study.In conclusion,this study has demonstrated that corn stovers could largely accumulate Cd for phytoremediation,and it has also explored why the Cd-accumulated corn stovers could be of much enhanced biomass saccharification and bioethanol production,due to much improvement of cell wall compositions and major wall polymer features,with almost complete Cd release for recycle from biomass process.In addition,this work has also applied biomass to obtain visible CQDs for enhanced fungi secreted cellulase production and enzymatic activity.Hence,it has provided a powerful and green-like strategy for Cd phytoremediation,cellulosic ethanol and CQDs production using crop straws.