Preparation Of Biochar Composites And Their Heavy Metals In Soil Study On The Mechanism Of Pollution Remediation

When heavy metals such as mercury,cadmium,lead,chromium,copper,cobalt,nickel and tin,as well as metal-like arsenic enter into the environmental,excessive amounts of them can cause pollution.Heavy metals cannot be biodegraded,but can be enriched and even converted into more toxic compounds that caused the chronic poisoning after into human and animal bodies through the food chain.At present,there are two ways including remove and passivation to remedate heavy metals pollution in agricultural land.The existing technical means are difficult to reduce the total amount of heavy metals in soil,especially in farmland,economically or effectively.Therefore,the activity regulation of heavy metals in soil has become an important direction for farmland soil heavy metal remediation.Biochar is a kind of carbon material with many tiny holes and it iseasily obtained by low temperature and oxygen-free pyrolysis of carbon-rich biomass.Due to its sources and low price,even improving fertility and pH value of soil,biochar is often used as the soil reclamation.The adsorption property of biochar for heavy metal ions is passable,but it can be greatly improved after modification.Thus it was used as passivator for heavy metal ions in soil.In this paper,the adsorption properties of the following three modified biochar were studied of theirpassivation effect for heavy metals in soil.（1）Firstly,biochar of pomelo peel was activated by hydrothermal method,and then ZnSO₄ was uniformly compounded on biochar by secondary hydrothermal process.Finally,the composite was put into a tubular quartz furnace,and nitrogen was injected.The process was heated to 673 K by 3 K?min^-1,and biochar/Zn O composite was obtained by pyrolysis for 4 h.The adsorption behavior of heavy metal Cu（Ⅱ）on the composite in water was detected.The results showed that the pseudo-second order kinetic model and Langmuir isothermal adsorption model could better simulate the adsorption process,and the maximum adsorption capacity(q_max)was 222.2 mg?g^-1.The four-step simple extraction method was used to analyze the fixed form distribution of Cu（Ⅱ）on the surface of biochar/ZnO.The experiment showed that 75.5%of the Cu（Ⅱ）was non-bioavailable state,while 23.6%of Cu（Ⅱ）was the acidlic soluble fraction which could be assimilate indirectly by plants.It indicated that the biochar/ZnO could effectively reduce the activity of Cu（Ⅱ）.The passivation effect of heavy metals on biochar/ZnO was studied by a simulation experiment.The biochar/ZnO was added in soil with 5%of the mass of soil which was polluted by series contents of Cu（Ⅱ）,and plant absorption of Cu（Ⅱ）was simulated with DTPA solution.The results showed that when the soil pollution was 2400 mg?kg^-1,the effective content of Cu（Ⅱ）could be reduced under 200 mg?kg^-1.It showed that the passivator had excellent passivating capacity for Cu（Ⅱ）.（2）The biochar of pine needles was activated by KOH for 1 h,then ethanol was added and KMnO₄ was reduced by ethanol under the alkaline conditions.biochar/MnCO₃ was prepared after the above mixture with alcohol-thermal reaction at473 K.Finally,the biochar/MnO composite was obtained by calcinated in a tubular quartz furnace at 773 K for 2 h in an argon atmosphere.The results showed that the adsorption process of Cu（Ⅱ）by the composite was more consistent with the pseudo-second order kinetic model and the Langmuir isothermal adsorption model,and the maximum adsorption(q_max)was calculated to be 394.3 mg?g^-1.The study found that the main role of its adsorption was oxidation functional groups complexation and mineral precipitation,accounting for 46.7%and 28.3%respectively.It indicated that the main role of its adsorption was chemical adsorption,and there was a strong interaction between the adsorbent and heavy metals.A simple four-step extraction method was used to extract the residue after Cu（Ⅱ）adsorption.It was found that non-bioavailable fraction and acidlic soluble fraction were 65.8%and 29.6%,respectively.Biochar/Mn O was applied to passivate Cu（Ⅱ）in soil,it was found that after biochar/MnO passivator was added in the soil polluted with the Cu（Ⅱ）pollution of 1200 mg?kg^-1,the effective Cu（Ⅱ）extracted from DTPA solution was dropped from504.94 mg?kg^-1（without passivator）to 133.8 mg?kg^-1（3）Coconut shell powder and chitosan was synthesized by alcohol thermal reaction at 473K.The obtained biochar/chitosan composite was mixed with tannic acid（TA）in deionized water to obtain ternary composite biochar/chitosan/TA.It can be seen from the image of SEM that the bare biochar has a three-dimensional structure with porous and its surface is relatively smooth.However,the surface of the composite was rough and full of particles,it indicated that biochar had been modified by chitosan successfully and etched with TA.The composite was characterized by FTIR,and the results showed that biochar/chitosan/TA acid contained a large number of oxidation functional groups.The composite was used to remove Pb（II）from the water,and the adsorption isotherm and adsorption kinetics were used to study the adsorption process.It was found that the Langmuir adsorption isothermal model and the pseudo-second order kinetic model could better fit the experimental data,and the maximum adsorption capacity of Pb（II）was 125.0 mg·g^-1.In this chapter,the adsorption types of Pb（II）by biochar/chitosan/TA are specifically analyzed.The experimental and calculated results showed that the adsorption mechanism is mainly the mineral precipitation and complexation of oxidized functional groups,accounting for 46.9%and 27.5%of the total adsorption amount,respectively.Finally,biochar/chitosan/TA composites were added to soils with different additive amounts at 2%and 5%to achieve the passivation of Pb（II）.The results showed that when the contents of Pb（II）was reached 600 mg?kg^-1 in the soil,biochar/chitosan/TA could significantly reduce the bioavailability of Pb（II）to 120.18 mg?kg^-1 and 69.86 mg?kg^-11 with the two additive amounts,respectively.