Effect Of Microplastics On The Properties Of Biochar Pyrolyzed From Sewage Sludge

Most of the microplastics are always settled and accumulated in the sewage sludge after municipal wastewater treatment.The occurrence of microplastics can reach thousands or tens of thousands particles in per kilogram dry sludge.With annual production of 56.65 million tons(wet weight)in China,the sewage sludge has become one of the critical environmental issues.At present,there are various treatment and disposal methods for the sewage sludge,including landfill,incineration,pyrolysis,etc.Pyrolysis can change the sewage sludge into biochar potentially for environmental remediation,and therefore,the pyrolysis technology together with the sludge-derived biochar has attracted wide attention.Microplastics will affect the pyrolysis behavior of the sewage sludge and the characteristics of the biochar product,due to the abundant occurrence and unique pyrolytic properties of the plastic particles.However,as the effect of microplastics on sewage sludge treatment is an emerging topic,there have been only very few studies reported on this topic especially on the mechanisms of microplastics' impact on sludge pyrolysis.Therefore,this study conducted a preliminary research on the effect of typical microplastics on the pyrolysis of the sludge and the properties of the sludge biochar.Specially,Polyvinyl chloride(PVC)and Polyethylene terephthalate(PET)were selected as representative microplastics.Through a series of experiments,this study revealed the variation in biochar production,and carbon properties with change in pyrolysis temperature as well as microplastics types and contents.The biochar production rates of all sample series were decreased with elevated pyrolysis temperature.Compared with the raw sludge,the presence of microplastics in the sludge can enhance biochar production rate after 200°C,which might be caused by remaining of undecomposed microplastic in biochar.However,at higher temperature range(300-700°C),the existence of microplastics during pyrolysis resulted in the reduction of biochar production.In detail,compared with the production rate of biochar containing PVC microplastics,the biochar production rate with PET microplastic is reduced by about 2.0%.The carbon content kept decreasing with increasing temperature for biochar production from the raw sludge.However,for the biochar derived from the mixture of sludge and microplastics,the carbon content in the biochar products increased first until reaching the maximum value at 300°C,and then decreased gradually with temperature rising.Besides,at the same temperature,the carbon content in the biochar pyrolyzed from the sludge with microplastics is higher than that in the biochar from the raw sludge.Moreover,the C/H ratio of all biochar samples was increased with the elevated pyrolysis temperature.Meanwhile,the C/H ratio of PVC-sludge biochar was the highest,which was followed by PET-biochar,the raw sludge biochar with the lowest value,indicating that the presence of microplastics,especially PVC,can aggravate the aromatization of carbon in the biochar.Furthermore,the carbon retention rate was calculated from the biochar production rate and carbon content.It was found that the carbon retention rate of all sample series was decreased with temperature going up.The presence of PVC microplastics increased the carbon retention rate in biochar at 200°C.For biochar pyrolyzed at 300-700°C,the PVC series showed higher carbon retention than the raw sludge series.Aside from PVCsludge biochar,the carbon retention rates of PET-sludge series were also higher than those of the raw sludge series pyrolyzed at 200-300°C,but lower at 400-700°C.In addition,the effects of microplastics on carbon structure and stability of the biochar products were further analyzed.For all sample series,the carbon structure became more orderly and the stability of carbon increased in the biochar pyrolyzed at increasing temperature.The existence of microplastics aggravated the ordering process of carbon structure in the biochar.After analyzing the D and G Raman peaks for carbon structure,the results showed that the existence of microplastics enhanced the ordering process of carbon structure in biochar.Similar to the carbon retention rate discussed above,the carbon stability of the PVC-sludge biochar is higher than that of raw sludge biochar at 300-600°C.The carbon stability of 300°C pyrolyzed PET-sludge biochar is better than that of the raw sludge biochar,but is weaker at higher temperature range(400-600°C).