Effects Of Reaction Temperature And EDTA Concentration On Nutrients And Metal Elements In Hydrochars And Hydrolysate Of Hydrothermal Carbonization Of Poultry Manure

In recent years,the livestock and poultry industry has developed rapidly,which meets the needs of people while produces much waste.Poultry manure is not only rich in nutrients,but also heavy metals and other pollutants,that if we can not turn waste into treasure,and green use,it will cause a waste of resources and environmental pollution.Compared with traditional technologies such as composting and other thermochemical conversion technologies,hydrothermal carbonization（HTC）technology has obvious advantages and is widely used for treating organic solid waste.According to previous studies,hydrothermal carbonization can effectively deal with heavy metals of organic solid waste and reduce the risk of pollution,and most studies focus on the change of process parameters or the addition of acid,alkali,salt,and other additives.As a metal complexing agent,EDTA can form a stable complex with metal ions,but its application in hydrothermal carbonization has not been reported.This study controls the peak temperature and the addition of complexing agent to participate in the hydrothermal carbonization of poultry manure.The purpose of this study is to explore the effects of different process combinations on nutrients and heavy metal elements in poultry manure by setting peak temperature gradient and EDTA concentration gradient and fixing other process parameters,so as to obtain cleaner and valuable products and provide a possible new way for the recycling of livestock and poultry manure.In this study,the peak temperature of hydrothermal carbonation and the concentration of EDTA were taken as the two main influencing factors,and the gradients of 150,180,210,240℃and 0,5,10,25,50 mmol L^-1were set,and the duration is 2 h and the solid-liquid ratio is 1:10 were fixed to explore the effects on the morphology and distribution of nutrients and metal elements in solid and liquid products,as well as the application of hydrochars（HC）to plant growth.Through the above analysis,better HTC processing can be obtained.The results show that in terms of physical and chemical properties,the yield of hydrochars decreases with the increase of reaction temperature（63.9-55.1%）,and the solid-liquid two-phase p H is neutral and partial alkali.The release of soluble salt ions makes the trend of EC in the solid and liquid phases the opposite.The C（62.4-72.5%）and N（64.7-90.9%）recoveries of hydrochars are relatively high,indicating that hydrothermal carbonization is a better carbon sequestration and nutrient recovery technology.The surface structure of hydrothermal carbon is rough,has a certain porosity,and is rich in oxygen-containing functional groups.The fragmentation degree of bulk structure will increase with the increase of reaction temperature and EDTA concentration.For the physical and chemical properties of solid-liquid two-phase products,the reaction temperature is the main controlling factor,and the effect of EDTA concentration is small.In terms of nutrient elements,when the reaction temperature is high,HTC it is not conducive to the preservation of ammonia and nitrate nitrogen in hydrochars.P and K show the opposite trend.On the other hand,P will accumulate in the solid phase while K will decrease significantly.At low temperatures,the increase of EDTA concentration will slightly reduce the recovery rate of P,but the overall recovery rate is more than88%,indicating that hydrothermal carbonization technology can well preserve P resources.The SMT results of P show that P mainly exists in the inorganic state of non-apatite,accounting for about 50%of the total P,which can be absorbed by plants,indicating that hydrothermal carbon has the potential to be used as a slow-release fertilizer.The concentration of ammonium nitrogen in hydrothermal solution will increase with the increase of reaction temperature.the deamination and the ring-opening reaction of organic nitrogen will make ammonium nitrogen mostly distributed in the liquid phase,while the concentration of nitrate nitrogen is at a low level.The content of dissolved organic matter in hydrothermal solution is high,and EDTA can significantly increase the concentration of DOC and DON.In terms of metal elements,the concentration of metal elements in hydrolysate is in the following order:K>Na>Ca>Mg>Fe>Zn>Mn>Cu and the order of metal element content in hydrochars is Ca>K,Mg>Na>Fe>Mn>Zn>Cu.Except for K,the increase of EDTA concentration can significantly promote the transfer of metal elements from solid phase to liquid phase.From the results of solid-liquid distribution,it can be concluded that the distribution of different metals species is very different,which can be divided into three categories:（1）mainly distributed in the liquid phase,such as Na and K;（2）mainly distributed in HC,such as Ca,Mg,Fe;（3）,such as Zn,Cu,and Mn.Due to the high affinity of metal ions and EDTA,the increase in EDTA concentration will reduce the proportion of Zn,Mn,Mg,Fe,and Cu in the solid phase by 76%,72%,50%,42%,and 30%,respectively.The increase of EDTA concentration can significantly reduce the content of available zinc in hydrochars,while the content of available copper will increase.According to the results of the BCR continuous extraction of metals,the heavy metals Cu,Zn,and Fe in hydrochars were activated when 50 mmol L^-1EDTA participated in HTC,and the proportion of exchangeable and carbonate-bound states increased.In the treatment without EDTA,the residual state increased significantly and the toxicity decreased.The follow-up Risk Assessment Code analysis showed that the HTC reaction without complexing agents could promote the passivation of heavy metals,while the high concentration of EDTA would increase the ecological risk.In this study,a ryegrass sand culture experiment was conducted to explore the phytotoxicity of HC.The results showed that unwashed hydrochars had obvious phytotoxicity.Washing hydrochars and reducing the amount of application could reduce the growth difference between treatments.The P content of the treatment with hydrothermal carbon was higher than that of poultry manure,and the P content of the HC_210-0treatment was the highest,which was 11.9 g kg^-1.The results of the Bioconcentration Factor showed that the toxicity of hydrochars produced at the reaction temperature of 210℃was low,while the hydrochars with high concentration of EDTA could increase the Bioconcentration Factor and increase the ecological risk,so the best treatment of the seven treatments was the HC_210-0.Previous studies have shown that although the increase of EDTA concentration can activate heavy metals and increase ecological risk,it can still promote the transfer and distribution of heavy metals to the liquid phase in the process of HTC.Therefore,the treatment of the pot experiment lacks the other three EDTA concentration gradients at 210℃,and the best EDTA concentration is not clear,which is needed for further study.