Containment and Dewatering of Heavy Metal Contaminated Sediments using Reactive Soil Minerals and Cellulose Materials
Date of Award
Master of Science (MS)
Civil and Environmental Engineering
Shobha K. Bhatia
The aim of this study is to investigate effective and sustainable measures to contain heavy metal contaminated sediments inside a geotextile tube during dewatering process. The efficacy of cellulosic materials (Jute fibers, Peanut hull, and Kraft pulp) on adsorption of selected heavy metals (Pb2+, Cu2+, Zn2+, and Cd2+) and dewatering performance inside a geotextile tube was studied by a series of Batch Adsorption and Pressure Filtration tests (PFT). The cellulosic materials were chosen keeping in mind their cost, accessibility, ease of handling and use, and sustainability. The studied adsorbents are inexpensive, easily available, and sustainable. In order to understand the optimum amount of cellulosic materials to be added into the slurry as well as the optimum mixing time for maximum removal, an independent study was conducted with the heavy metal ions and the cellulose materials. In this study, batch adsorption tests were conducted on a 500 ppm metal solution by changing the amount of cellulose materials from 0.5 g to 2g equilibrated for 4 hours. Two filtrate samples were collected at tested for metal concentration using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) at 0.5hr, 1 hr, 2 hr, and 4 hr to understand the time of mixing on removal efficiency of studied heavy metal ions. The adsorption data were fitted using Langmuir isotherm to quantify their maximum adsorption capacity to heavy metal ions. It was found out that among the studied metals, all adsorbents exhibited highest affinity towards Pb ions, the order being jute> peanut hull> kraft pulp. A removal efficiency of 47% and maximum adsorption capacity of 100 mg/g Pb ions was highest for all combinations of adsorbents and metal ions. A maximum removal efficiency of 28% of Cu was achieved by the kraft pulp and the decreasing affinity was kraft pulp>peanut hull>jute. The maximum adsorption capacity of kraft pulp determined from Langmuir isotherm for Cu was 13 mg/g. Similarly, kraft pulp exhibited highest affinity for Cd and Zn followed by peanut hull and jute. The removal efficiency of Cd and Zn by kraft pulp was almost 28%, significantly higher than 13% removal of Cu or Pb. The maximum adsorption capacity of kraft pulp for Cd and Zn were 24 mg/g and 11 mg/g respectively.
It has been seen that the dredged sediments contain soil particles with varying sizes and properties. Hence, understanding the role of different sediments in adsorption and retention of heavy metal ions inside geotextile tube is very important to predict the fate of contaminants leaching out from the tubes. To address this issue, a separate study was conducted where two soil sediments, Tully coarse (55% coarse and 45% fine fractions), and Tully fines (100% fine fractions) were mixed with heavy metal solution (500 ppm-2500 ppm) to form a 15% solid concentration slurry. The slurries was mixed for 1 hour and the filtrate samples collected after mixing were tested for metal concentration using ICP-OES. Moreover, Kaolinite clay was also used in this study. It was seen from these tests that the presence of fine fractions, hydrous oxides of iron and aluminum, reactive clay minerals Illite and Chlorite, surface charge, and pH of a soil play dominant role in adsorption and retention of heavy metal ions specially Pb and Cu. Tully fine sediments exhibited excessively high affinity towards Pb as the more than 99% of Pb was adsorbed and retained. Tully coarse also had high affinity to Pb and Cu with removal efficiency ranging between 85-95%. Kaolinite (1:1 clay) being less reactive and having high molecular stability than most of the 2:1 clays (e.g. Illites, Smectites) exhibited less affinity towards Pb and Cu. However, Kaolinite had better affinity towards Cd ions (approximately 3 times) than both Tully fine and Tully coarse soils. In terms of adsorption of Zn, both Tully soils adsorbed more than Kaolinite. Generally, it was concluded that the presence of reactive clay minerals plays a significant role in adsorption of Pb and Cu.
After the understanding of the role of cellulose materials as well as soil sediments in studied heavy metal adsorption, PFT tests were conducted to see the role of cellulose materials in dewatering performance. A 15% solid concentration contaminated slurry was prepared by mixing soil, cellulose materials (2% weight of solids) and heavy metal solution (1000 ppm Pb+500 ppm Cu+500 ppm Cd+500 ppm Zn). It was observed from the PFT tests that in both sediment slurries, the addition of cellulose materials except kraft pulp significantly increased the dewatering rate irrespective of the contamination. However, profound effect of jute fibers and peanut hull on increasing dewatering rate was observed in case of contaminated slurries. A reduction in turbidity of more than 80% was observed with the addition of jute fibers. Peanut and kraft pulp were successful in reducing the turbidity of the filtrate by 78 and 69% respectively. Addition of peanut hulls and jute on contaminated Tully coarse increased the solid content by approximately 33% and 46% respectively. However, in case of contaminated Tully fines a minor increase of 12% was achieved with the addition of jute. The addition of kraft pulp had no significant effect in the solids content. The filtrates collected from dewatering of contaminated Tully fines showed that approximately 98-99% of Cu and Pb was retained, whereas, in Tully coarse slurries 89% Cu and 96% Pb were retained. Although not very high adsorption, both Tully soils were able to retain more than 75% Cd and Zn.
Rupakheti, Prabesh, "Containment and Dewatering of Heavy Metal Contaminated Sediments using Reactive Soil Minerals and Cellulose Materials" (2016). Dissertations - ALL. 482.