Preparation Of Municipal Sludge Biochar And Its Application In Environmental Remediation

As a byproduct of wastewater treatment,sludge has a large output and is difficult to treat and manage,but it contains a large amount of nutrients including N、P、K,and some organic matter.Effectively utilizing the value of sludge to achieve"waste"to"pollution"has become a hot topic for current research.The sludge is converted to a stable biochar by pyrolysis.During the process of dewatering and carbonization,pathogenic microorganisms can be effectively removed from the sludge,and the heavy metals in the sludge become more stable.The resulting stable biochar can be used to enhance soil quality for contamination remediation,providing new ideas for the safe use and disposal of sludge.In this experiment,sludge biochar obtained from Chaozhou Second sewage treatment plant was used as the raw material for pyrolysis at 350℃.A series of characterization tools were employed to understand the physical and chemical properties,as well as the adsorption characteristics of the sludge biochar on heavy metals.Additionally,the sludge biochar was further investigated by modifying it through the addition of KOH,KMn O₄,and Mg Cl₂to assess its physical and chemical properties,as well as its adsorption capacity toward various heavy metals.On the basis of this,biofertilizer was prepared using the ratio of sludge biochar/potassium dihydrogen phosphate/ammonium chloride/porcelain clay=10:1:1:2,followed by an investigation of the material’s physical and chemical properties.Different rates of biofertilizer were added to different soils to analyze the distribution of heavy metals in the soil and the heavy metal content in chard to determine the fixation effect of biofertilizers on soil heavy metals and their safety.The main conclusions are as follows:（1）Through characterization,the surface of sludge biochar was found to be rough and has functional groups such as-OH on the surface under a microscope.The adsorption capacity of the sludge biochar was positively correlated with the initial p H,the amount of sludge biochar added,and the initial heavy metal concentration.At an initial p H of 6 for the adsorption solution,Pb²⁺reached the maximum adsorption amount of 17.79 mg/g.At a p H value of 5,the maximum adsorption amount of Cu²⁺was 15.73 mg/g.At p H values between4.5 and 6.5,the maximum amounts of Cd²⁺adsorbed were 2.64 mg/g.With the increase of heavy metal concentration,the maximum amounts of Pb²⁺,Cu²⁺,and Cd²⁺adsorbed were42.55 mg/g,35.78 mg/g,and 6.73 mg/g,respectively.The competitive ability of sludge biochar for adsorbing the three heavy metals was in the order of Pb²⁺>Cu²⁺>Cd²⁺.（2）Adsorption experiments were conducted using sludge biochar modified with KOH,KMn O₄,and Mg Cl₂in single and mixed Pb²⁺,Cu²⁺,Cd²⁺,and Zn²⁺heavy metal solutions.With the increase in heavy metal concentration,the average adsorption rate of KMn O₄and Mg Cl₂biochar for the four metals also increased and could reach more than 80%.The adsorption performance of KOH-modified biochar was the best,and the adsorption rate could reach more than 95%in both single and mixed solutions.The results showed that the adsorption capacity of modified sludge biochar for heavy metals was improved,and the performance of KOH-modified sludge biochar was the best.（3）The prepared biofertilizer showed an increase in surface functional groups,such as-OH and C=O,compared to the original sludge biochar.Moreover,it displayed a significant increase in N,P,and K contents,with a 40.2%increase in total N,110.7%in total P,and101.5%in total K.The results of the pot experiment indicated that the addition of biofertilizer had a non-significant effect on soil p H,which was maintained between 4.5 and 6.5.When the amount of soil organic matter reached 8%,the organic matter significantly increased by 33.3units,compared with the control group.The analysis of heavy metal morphology showed that the available Pb and Cu states in soil decreased.However,the transformation of Zn morphology was not evident,as reflected in the metal content of Chinese cabbage.（4）The chard harvested after the pot experiment was analyzed for its heavy metal content.The results showed that in the experimental groups with 6%and 8%of biofertilizer,the chard did not grow due to the high concentration of mineral ions in the fertilizer,which caused the seedlings to"burn".However,the addition of biofertilizer in other experimental groups resulted in a decrease in heavy metal concentration in the chard.The Cu content in contaminated soil decreased by 61.3%when 2%biofertilizer was added,while the best results were obtained at 4%addition for other experimental groups.