Toxic Effect Of Activated Sludge Biochar On Earthworms

Recent studies showed that compared to direct application of activated sludge in soil,preparation of biochar by pyrolysis of activated sludge can better promote plant growth in agriculture.But whether activated sludge biochar has a negative impact on the biological population in the soil must be taken into consideration in practical application.In this study,the earthworm,one of the biological population with the largest biomass in the soil,was selected as the research object.Whether activated sludge biochar is harmful to the earthworm was studied and harmful factors were analy Ped.In this study,biochar was made from activated sludge at different final pyrolysis temperatures,350?,550? and 750?.Biochar soil system was developed by adding each biochar to the soil at different proportion,0%,2.5%,5%,7.5%,10%,and12.5%,and mixing evenly to conduct acute toxicity test.LC50 was used as new upper concentration limit and proportion series were reset,0%,2.5%,5%,7.5% and 10%.The avoidance test,neutral red test,and antioxidant defense test were carried out to further investigate harmful effects of biochar on the earthworm at cellular and physiological levels.Based on the above test results,possible harmful factors in activated sludge biochar were selected for further study and verification.PAHs and water-soluble substances in activated sludge biochar were chose to test whether they have harmful effects on earthworms in this study.The main results of this study are as follows:1.Individual level.There was a significant positive correlation between the mortality of the earthworm and the proportion of three kinds of activated sludge biochar(P<0.05).The LC50 of three kinds of activated sludge biochar were respectively1.025×105mg/kg,1.107×105mg/kg,2.69×105mg/kg.The harmful effect of 350? activated sludge biochar>550? activated sludge biochar >750? activated sludge biochar.2.At the cellular level,the harmful effect of activated sludge biochar onthe earthworm is reflected in the destruction of lysosomal membrane of earthworms.When exposed to three kinds of activated sludge biochar-soil system for 7d and 14 d,the Neutral Red Retention Time(NRRT)of earthworms was negatively related with the proportion of biochar(P<0.05).3.Physiological level.CAT,T-SOD,POD and MDA of earthworms in the treatment group were higher than that in the control group.Changes in en Pyme activity and MDA content of three kinds of activated sludge biochar varied,which indicated different harmful effects of three kinds of activated sludge biochar.4.After removing PAHs from 350? activated sludge biochar,the result of the acute toxicity test was significantly lower than before,which showed harmful effects of PAHs have on the earthworm.In addition,the mortality of the earthworm was significantly positively correlated to the proportion of modified activated sludge biochar in artificial soil system(P<0.05).5.In the water extract of 350? activated sludge biochar,the mortality of the earthworm was significantly positively correlated to the concentration of the water extract(P<0.05).Indicating that the water-soluble substances in350? activated sludge biochar contain factors that have a harmful effect on earthworms.