Research On Surfactant Enhanced Pump And Treat Remediation Of A DNAPL Contaminated Aquifer

Some kinds of organic compounds in groundwater came into envionment as aqueous phase liquids, and some kinds of them as non-aqueous phase liquids. According to their relative density to water, NAPLS should be stapled to light non-aqueous phase liquids (LNAPLs) and dense non-aqueous phase liquids(DNAPLs). Because of the density, DNAPLs could cut through the aquifer to the bottom. It could bing large-scale and long-term contamination, the cost of remediation is large, the tim of remediation is long and involve lots of subjects. So DNAPLs is called the difficulty and challenge of remediation study.The research of DNAPLs has become the advanced topic of national envionment and water resources sphere. But in our country ,it has not been fully attached importance to. There is few report about the study of DNAPLs contaminations. It is not seasoned with the serious position of DNAPLs contaminations. Without countermeasure, the contamination of subsurface environment should be severe, and bing more difficulties to the control and remediation works.According to general reviews and remediation technologies of DNAPLs, at the beginning, the efficiency of pump-and-treat technology is high while the cost is low. But when tailing happened, the efficiency turns lower, the time of remediation turns longer, and cost increases. With surfactant-flushing, the dosage of surfactant is large and both of the cost, but the time of remediation is shortemed. So the tree view of this paper is: pump-and-treat is used first at the beginning of DNAPLs contaminated aquifer remediation, and then, surfactant enhanced pump and treat is used. Compare with pump and treat, surfactant enhanced remediation would short the time. And compare with surfactant washing, using pump and treat at first would reduce the dosage of surfactant and both of the cost. Combineing this two methods to remediate the DNAPLs contaminated aquifer has significances both in theory and practicality.DNAPL studied in this research was nitrobenzene. A series of simulition experiments were conducted to study the aspests as followed: Research on the movement and release of nitrobenzene in unsaturated zone and aquifer; disscussed the pumping efficiency in the source zone; quantify arranged the salts'impact sequence to the solubilization; confected the mixture solution contained Tween80 and salts that used in the enhanced remediation. The remediation work was carried on in a sand box. The permeability coefficient was 16.6m/d.Compared with the prevenient studies, the innovations of this work were:(1)The first remediation research on DNAPL contaminated aquifer internal. Put forward the technology: using pumping first and surfactant enhanced pumping second. Disscussed the problem that when using enhance measure. The results indicated it was feasible to use this remediation method;(2)The mixed solution contained nonion surfactants and salts had been used as flushing solution to remediate DNAPLs contaminated aquifer in the past reseach works. The salts were general used NaCl or CaCl2. no relative reports on the salts'impact sequence to the solubilization. According to the solubilization experiment results of nitrobenzene in the Tween80 solution, the salts'impact sequence to the solubilization was quantify arranged. Enlarged the selection range of the surfactants mixing research.(3)According to the simulation pumping test, the impacts of the different pumping methods and different pumping rates to the pumping efficiency were disscussed. It could provide the theoretics foundation to the practical remediation works.The main results can be concluded as followed:(1)The pumping Efficiency was disscussed in the aquifer source zone of different pumping methods and pumping rates by simulation experiment. If using the continuous pumping method, low pumping rate had high concentration while low mass transfer rate; hight pumping rate had low concentration while high mass transfer rate. Invariable pumping rate was not benefit to the solubility from the DNAPL to the aqouse phase. The total pumping mass of DNAPL and the total pumping water volume match the multinormial function(n≥3):M=AQⁿ+BQ^n-1+CQ^n-2…+kQ+a;If using the pulse pumping method, intermission time was 1h according to the experiment results. And compared to the continuous pumping in the same pumping rate, pulse puming had high pumping concentration and high mass tranfer rate.Pump-treat technology had high efficiency in the junior pumping phase. But the pumping concentration and mass transfer rate turned low when the"tail"appeared. The total pumping nitrobenzene mass of collum D which had the highest pumping efficiency was 17.59g and the removal rate was 58.63%. Other mesures should be carried on to the advance remediation.(2) Four surfactants were tested in the surfactant selection experiments, the results were shown as followed:The solubilization sequence of surfactants to nitrobenzene was: Tween20>Tween80>OP>SDS at 10℃. There was liner ship between the lgKm value and nonion surfactants'HLB value. The adsorption quatity on middle sand sequence was: OP>Tween80>Tween20>SDS at 10℃. Flushing experiment result was: Tween80 > Tween20 > SDS. Nonoin surfactant Tween80 is tested in the following experiments that it is suit for non aqouse nitrobenzene contaminated aquifer remediation.Experimental results indicated that the solubility of nitrobenzene in water was greatly enhancement by Tween80 at surfactant concentration above CMC(critical micelle concentration) and a linear relationship was obtained between surfactant concentration and nitrobenzene concentration from the solubility curve. The molar solubilization ratio (MSR) value was 5.093 and lgKm was 3.499. The solubilization was attributed to the ethoxylation group in Tween80 micellar;Effect of inorganic salts such as NaCl, KCl, CaCl2, MgCl2, NaNO3, Na2SO4, NaHCO3 ,Na2CO3 on water solubilities of nitrobenzene in Tween80 micellar solutions was also investigated by a matrix of batch experiments. Mix the Tween80- inorganic salts at the total mass ratios of 2:1,5:1 and 10:1.The results show that the inorganic salts at a high concentration(≥500mg·L^-1) can enhance the solubilization capacities of Tween80 micellar solution and increase the value of MSR and lgKm. Because of the Salting-out effect between the micellar of Tween80 and inorganic salts, the volume of micelle turn bigger that may provide larger solubility volume for nitrobenzene. The cation's effect sequence is 1/2Mg²⁺>K⁺>Na⁺≈1/2Ca²⁺. The inion's effect sequence is HCO₃^->1/2 SO₄^2->1/2 CO₃^2->NO₃^-≈Cl^-.The effect of medium on the mixed solution could impact the flushing results according to the columns flushing experiment. Tween80 mixed NaHCO3 at the mass ratio 2:1 solution has the highest flushing efficiency. This solution can be used in non aqouse nitrobenzene contaminated aquifer remediation.(3) Two dimension box simulation results indicated that high efficiency appeared in the junior pumping phase of the source zone. If the location of the pumping well is right setted, the non aqouse liquid at the bottom of the aquifer may be pumped out. Pumping well 1 pumps out total non aqouse nitrobenzene 53ml(63.6g).The pumping method used in the nitrobenzene source zone is continuous pumping. Reduce the pumping rate has effect to increase the concentration, but it cound not last long term. total dissolved nitrobenzene that pumps from pumping well 1 is 24.24g and 20.8g in pumping well 2. The whole pumping process pumps total 108.64g nitrobenzene and the total pumping time lasts 5854min. It can be seen from the nitrobenzen plume monitoring picture that the pump and treat technology can control the dissolved nitrobenzene plume.while the surfactant injected to the aquifer, no blockage happened and the surfactant sollution migrates downward to the bottom of the aquifer. Higher concentration apperares in the bottom. In the experiment, the sollubilization is obvious that the concentration of nitrobenzene incerased from 45mg/L to the highest 150mg/L. No clean area contaminated by nitrobenzene after the surfactant sollution injected.The pumping well 1 pumps dissolved nitrobenzene 7.79g and the pumping well 2 pumps out 4.62g. Total pumping time is 3420min and total injected surfactant solution volume is 160L. Caculated by the tail concentration at 45mg/L, surfactant solution shorten the remediation time 1920min.Finally, total 121.05g nitrobenzene is pumped out while 112.6g is residual in the box. The residual nitrobenzene is distributing around the nitrobenzene injecting point. Nitrobenzene distributed on the bottom of the aquifer has nearly been clean up.