This study is the pH recovery of the P’company’s slag landfill leachate using the injection of carbon dioxide from the LNG exhaust gas and the restoration of ecosystem by ecological engineering approach. LNG exhaust is injected through pipes with flow rate Q = 30 m³/min and pH value is below 8.0. Results of running the MINEQL+ program, the value of carbonic acid species are given as follows. The concentration of TDC(Total Dissolved Carbon) is 0.0018 M at 0.00035 atm of PCO₂ and 9.25 of pH in leachate condition and 0.102 M at 0.03 atm of PCO₂, and 8.18 of pH in LNG exhaust gas injection(30 m³/min) condition, respectively. The amount of LNG exhaust gas is eliminated about 132.60 tonC/year. Elimination of excess TDC results in creation of artificial seaweed beds which is consumed about 159.46 tonC/year by seaweeds. The primary production increase and some new algae, benthos and fishes species in artificial seaweed beds. This result indicate that the small-scale ecosystem occur around artificial seaweed beds with succession.