Releasing behavior of copper in recirculated bioreactor landfill

Monitoring of copper adsorption on biochar using spectral induced polarization method

Copper contaminant generated from mining and industrial smelting poses potential risks to human health. Biochar, as a low-energy and cost-effective biomaterial, holds value in Cu remediation. Spectral Induced Polarization (SIP) technique is employed in this study to monitor the Cu remediation processes of by biochar in column experiments. Cation exchange at low Cu2+ concentrations and surface complexation at high Cu2+ concentrations are identified as the major mechanisms for copper retention on biochar. The normalized chargeability (mn) from SIP signals linearly decreased (R2 = 0.776) with copper retention under 60 mg/L Cu influent; while mn linearly increases (R2 = 0.907, 0.852) under high 300 and 700 mg/L Cu influents. The characteristic polarizing unit sizes (primarily the pores adsorbing Cu2+) calculated from Schwartz equation match well with experimental results by mercury intrusion porosimetry (MIP). It is revealed that Cu2+ was driven to small pores (∼3 μm) given high concentration gradient (influent Cu2+ concentration of 700 mg/L). Comparing to activated carbon, biochar is identified as an ideal adsorbent for Cu remediation, given its high adsorption capacity, cost-effectiveness, carbon-sink ability, and high sensitivity to SIP responses - the latter facilitates its performance assessment.

Investigation of Cu Adsorption and Migration with Spectral Induced Polarization in Activated Carbon

In this paper, the adsorption process of copper ions on activated carbon (AC) was simulated in a column test. It was deduced that it is consistent with the pseudo-second-order model. Cation exchange was observed to be the major mechanism of Cu-AC interactions through scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) measurements. Adsorption isotherms were fitted well using the Freundlich model. Adsorption thermodynamics at 298, 308, 318 K demonstrated that the adsorption process is spontaneous and endothermic. Spectral induced polarization (SIP) technique was used to monitor the adsorption process, and the double Cole-Cole model was used to analyze the SIP results. The normalized chargeability was proportional to the adsorbed copper content. Two measured relaxation times from the SIP testing were converted into the average pore sizes of 2, 0.8, 0.6, 100-110, 80-90, and 53-60 µm by the Schwartz equation, which are consistent with the measured pore sizes from both mercury intrusion porosimetry and scanning electron microscopy (SEM). The reduction in the pore sizes by SIP during the flow-through tests suggested that the adsorbed Cu²⁺ gradually migrated into small pores as with continued permeation of the influent. These results showcased the feasibility of using SIP technique in engineering practice involving the monitoring of copper contamination in land near a mine waste dump or in adjacent permeable reactive barriers.

Effect of air and water on the release of chlorine from semi-aerobic landfill

Landfill leachate has a high chloride (Cl^-) content. Because it is highly mobile, and cannot be sorbed or transformed bio-chemically, it is important to have detailed information about how it migrates in landfill sites. In this study, we set up four lab-scale simulated landfills, including an anaerobic landfill (AL), an anaerobic landfill with leachate recirculation (RAL), an anaerobic/semi-anaerobic landfill with leachate recirculation (RASL), and an anaerobic/semi-aerobic landfill (ASL), to explore how, when regulated, moisture and air affected the migration of chlorine. We found that water and air had a strong influence on the release of Cl^-. Leachate obviously promoted Cl^- dissolution in refuse when recirculated. When air was introduced into landfill, thereby changing it from anaerobic to semi-aerobic, the leachate Cl^- concentration increased sharply from around 4-9 g L^-1 (RASL) and 18 g L^-1 (ASL), respectively. In principle, Cl^- is released continuously when leachate is recirculated in landfills (RAL and RASL), but it can also be found a terminal when the leachate recirculation stops (AL and ASL). Cumulative amounts of 64, 66, 27, and 53 g of Cl^- were released from the AL, RAL, RASL, and ASL, respectively. Lower COD/Cl and NH4+-N/Cl ratios in ASL and RASL after day 175 indicated that lower Cl^- pollution risk than that in AL and RAL.

Release behavior of chloride from MSW landfill simulation reactors with different operation modes

The chloride ion (Cl^-), a very common monatomic anion, has high ecological toxicity at high concentrations because of its non-biodegradability, and can easily migrate from landfill site into the surrounding environment. Four lab-scale landfill simulation reactors were established to investigate Cl^- release behavior: the anaerobic landfill mode (R1), the semi-aerobic landfill mode (R2), the anaerobic landfill with leachate re-circulation mode (R3), and the semi-aerobic landfill with leachate re-circulation mode (R4). The landfill operation modes had a great influence on the release of Cl^-. In 256 days, the cumulative release amounts of Cl^- in the four reactors were 64.52, 132.07, 56.10, and 33.1 g for R1-R4, respectively. Once air enters anaerobic landfill, the leachate Cl^- concentration may sharply increase. The highest leachate Cl^- concentrations were 6.6 g L^-1 in anaerobic reactor and 18 g L^-1 in semi-aerobic reactor. However, the leachate re-circulation can maintain the release of Cl^- at dynamic equilibrium state. Theoretically, the Cl^- release behavior from anaerobic landfill with leachate re-circulation (R3) will be continuous. In contrast, under the other conditions, it can be anticipated to occur once the leachate recirculation stops (R1) or when the landfill encounters air incursion (R2 and R4). The semi-aerobic operation modes had significantly lower COD/Cl and NH₄-N/Cl ratios than the anaerobic modes. This indicates that the Cl^- pollution risk from semi-aerobic modes is lower than that from anaerobic modes.

Effect of Mass Proportion of Municipal Solid Waste Incinerator Bottom Ash Layer to Municipal Solid Waste Layer on the Cu and Zn Discharge from Landfill

Municipal solid waste incinerator (MSWI) bottom ash is often used as the protection layer for the geomembrane and intermediate layer in the landfill. In this study, three sets of simulated landfills with different mass proportion of MSWI bottom ash layer to municipal solid waste (MSW) layer were operated. Cu and Zn concentrations in the leachates and MSW were monitored to investigate the effect of MSWI bottom ash layer on the Cu and Zn discharge from the landfill. The results showed that the Zn discharge was dependent on the mass proportion of MSWI bottom ash layer. The pH of landfill was not notably increased when the mass proportion of MSWI bottom ash layer to MSW layer was 1 : 9, resulting in the enhancement of the Zn discharge. However, Zn discharge was mitigated when the mass proportion was 2 : 8, as the pH of landfill was notably promoted. The discharge of Cu was not dependent on the mass proportion, due to the great affinity of Cu to organic matter. Moreover, Cu and Zn contents of the sub-MSW layer increased due to the MSWI bottom ash layer. Therefore, the MSWI bottom ash layer can increase the potential environmental threat of the landfill.

Content and fractionation of Cu, Zn and Cd in size fractionated municipal solid waste incineration bottom ash

Municipal solid waste incinerator (MSWI) bottom ash was size fractionated into six fractions, with the respective particle size of <0.45 mm, 0.45-1 mm, 1-2 mm, 2-4 mm, 4-8 mm and >8 mm. The contents and fractionation of Cu, Zn, Cd in the size fractionated MSWI bottom ash were investigated. The results showed the contents and fractionation of Cu, Zn and Cd varied among the different particle sizes, which were related to their thermodynamic characteristics. High content of Cu was found in the bottom ash with the particle size of <0.45 mm and >4 mm, due to its lithophilic property and the function of entrainment. The content of Zn showed a relatively even distribution among the various particles. The content of Cd showed a decreasing trend with the increase of the particle size, due to its high volatility. Besides, the carbonate bound fraction of Cd showed a decreasing trend with the increase of the particle size, while the carbonate bound fraction of Cu showed an increasing trend. The organic matter bound fraction of Cu increased when the particle size increased. The results also showed the fine ash contained a higher level of unstable Cd than the large ash, while the large ash had a higher level of unstable Cu comparatively.

Heavy metal source analysis in municipal solid waste (MSW): case study on Cu and Zn

This research contributes to the knowledge of the heavy metal sources in municipal solid waste (MSW). Samples were collected from 8 cities of Zhejiang province, Eastern China. Cu and Zn, the most two conventional heavy metals with extensive distribution in many kinds of MSW components, were investigated. It shows components of kitchen waste (KW), ash (AS), plastic (PL), and paper (PA) have high universality in MSW and accounted for 55.1-95.5% in each MSW sample. Moreover, these four components are also the main heavy metal sources of MSW, which accounted for 76.3% and 82.3% contribution of the Cu and Zn contents, respectively. The Cu and Zn contents in the gross MSW sample were 41.2-1643.7 mg kg(-1) and 109.3-1077.9 mg kg(-1), respectively, which on different degree exceed the set standard for "environmental quality standard for soil" (Cu, 100 mg kg(-1); Zn, 400 mg kg(-1)) of China and have high potential of environmental risk. The heavy metal contents in the gross MSW do not have spatial variation but present high seasonal variation, significantly higher in summer than winter (P<0.01). Much more attention should be paid on the MSW management in summer to avoid heavy metal pollution.

Bio-immobilization of Cu and Zn in recirculated bioreactor landfill

PURPOSE

To protect the environmental quality of soil, groundwater, and surface water near the landfill site, it is necessary to make an accurate assessment of the heavy metal mobility. This study aims to present the bio-immobilization behavior of heavy metals in landfill and provide some reference suggestion for the manipulation of heavy metal pollution control after closure.

MATERIALS AND METHODS

Two simulated bioreactor landfill system loaded with real municipal solid waste (MSW), namely, conventional bioreactor landfill (CL) and leachate recirculated bioreactor landfill (RL), were operated. Cu and Zn, the two conventional heavy metals with the highest contents in MSW, were chosen to track the heavy metal bio-immobilization behavior in landfill.

RESULTS

The MSW in landfill is a great threat to environment because much of the heavy metal is "hidden" in different components. The weight ratio of Cu and Zn in landfill amounts to 0.00427% and 0.00437%, respectively. The accumulated effluent masses of Cu and Zn in CL increased all along, while they still kept at a stable level after day 105 in RL.

CONCLUSIONS

The microbes like sulfate-reducing bacteria mediate the behavior of Cu and Zn in bioreactor landfill system. Cu and Zn can be bio-immobilized in bioreactor landfill system with leachate recirculation like RL.