| Coal biogas is an important methane gas produced by coal degradation and gasification under the action of microorganism.In recent years,The production process and influencing factors of coal biogas are widely concerned.However,the related environmental issue during gas formation has seldom been studied.This paper aims at the potential environmental problems in the process of coal biogas formation by microorganism.The Qinshui Basin was selected as a typical area for coalbed methane development.Based on the investigation and evaluation of produced water pollution from coalbed methane wells in the Qinshui Basin,coal samples from Lu’an,Yima and Pingdingshan were selected to simulate biogenic gasformation experiments.Variable characteristics of constant ions,organic matter,and heavy metals in coal biogas formation process were studied,and the potential environmental risks of organic matter and heavy metals were preliminarily evaluated.The main research contents as follows:（1）In Qinshui block,phenol in produced water exceeded the standard by 4.4times on average,and naphthalene exceeded the standard in other blocks.In Xiangyuan,Taiyuan and Yangquan blocks,the Cr in produced water exceeded the standards 1.55,2.71,and 3.11 times respectively,and the Ni exceeded 1.90 times in Taiyuan block.The comprehensive pollution assessment showed that the produced water quality of Xiangyuan block was Grade IV,was poor.The other blocks was Grade V,was extremely poor.The produced water in Qinshui Basin is polluted by ions and organic matter.（2）Coal biogas is divided into three stages.In the fast gas production stage,there are higher concentrations of fatty acids in the solution.After entering the slow gas production stage,the concentration of fatty acids drops to almost zero and aromatic compounds dominate the system.The typical organic compounds in the gasification process mainly include p-cresol,fatty acids,esters,benzenes,and n-alkanes.The concentrations of fatty acids and esters are high,and the octanoic acid concentration ranged from 519.55 to 1891.10μmol/L.The concentration of methyl ester varied from298.34 to 412.72μmol/L.The concentrations of benzene and n-alkanes are extremely low and n-alkanes are lower than 1μmol/L.（3）The content of constant ions in the liquid phase did not change significantly during gas formation,but the content of As,Ba,Co,Ni,Pb,Se,Cu,and Ti increased significantly.The content of Ba had a range of 23.88 to144.79μg/L,and the content of As and Cu ranged from 5.88 to 25.07μg/L and 5.47 to 14.17μg/L,respectively.The content of other metal elements is low.Comparing with the coal soaking test,non-water soluble metal elements including combine with hydroxyl,carbonyl,and carboxyl groups would enter the liquid phase during gas formation.（4）Coal can adsorb yeast extract.The removal rate of yeast extract within 72 hours is over 40%.With the increase of pulverized coal quality,the removal rate can reach over 60%.Coal did not adsorb Na+,NH4+,K+and Cl-,SO42-and PO43-.（5）The contents of heavy metals such as Ba,Co,Ni,Pb,Se,and Cu did not exceed the III standard values of groundwater,and metal-like As exceeded 2 times.In organic matter,p-cresol,dimethyl phthalate and 1,2,3,4-tetrahydro-9-propyl-hydrazine exceeded the III standard values of groundwater,in which p-cresol and dimethyl phthalate were seriously excessive.The results of the health risk assessment show that:non-carcinogenic risk values for drinking ranged from 4.37×10-12 to 2.10×10-6,which was less than the non-carcinogenic risk threshold value of 1,indicating that the non-carcinogenic risk of organic matter and heavy metals during the biogenic gasification process was at an acceptable level.The calculation results of carcinogenic risk showed that the carcinogenic risk values of As in the three coal samples ranged from 4.61×10-5 to 1.96×10-4,all exceeding the threshold of carcinogenic risk of 1×10-6,indicating that As is the most harmful potential pollutants in the human body in the process of coal gasformation by microorganism. |
