Simultaneous Extraction and Separation of Phosphorous and Heavy Metals from Freshwater Sediments using Electrodialytic Treatment

Lake pollution by phosphorous (P) and heavy metals is a serious issue worldwide, and these contaminants are deposited in the sediments. Dredging of sediments is an effective and commonly practiced technique for lake restoration, as it mitigates the internal loading of P and heavy metals, thereby preventing their release into the overlying water column. The dredged sediments, due to the lack of proper treatment, often end up in the landfills, hindering their repurposing and resulting in the loss of resources, such as P. In light of this, it is crucial to develop a treatment method that can enable the remediation and recycling of freshwater sediments polluted with heavy metals while simultaneously ensuring efficient P recovery.

Therefore, in this thesis, the use of electrodialytic (ED) treatment for simultaneous extraction and separation of P and heavy metals from freshwater sediments was explored. ED treatment is a membrane-based process using an electric DC current over a suspension of particulate materials (freshwater sediment in this study) to extract P and heavy metals. ED cells consist of a sediment suspension chamber and either one or two electrode (anode and cathode) compartments, depending on the type of cell, a 3-compartment (3C) or a 2-compartment (2C) cell. The anode and cathode compartments are separated from the sediment suspension chamber using an anion and a cation exchange membrane, respectively. The sediment suspension is constantly stirred during the ED treatment to ensure a homogeneous suspension. The ED treatment enables selective ion transport, facilitating the separation and recovery of P and heavy metals into separate compartments, thereby supporting the recycling of sediments and the P extracted solution.

The main areas of focus were sediment characteristics, including binding mechanisms of P and heavy metals; types of ED cells (3C and 2C); experimental variables (current, liquid-to-solid ratio, stirring rate, duration, suspension liquid); and ways to repurpose the P-extracted solution and the treated sediments. The study used three different eutrophic freshwater sediments from different catchment areas: 1) sediment with low heavy metal contamination and located in an urban area, 2) sediment significantly contaminated with heavy metals and located near the former metal manufacturing industries, and 3) sediment with low heavy metal contamination (possibly treated with aluminium) and located near agricultural land.

ED treatment for P and heavy metal extraction from the freshwater sediments resulted in a P extraction of up to 54% and the percent-wise order of heavy metal extraction to be: Zn (90%) > Cd (87%) > Cu (60%) > Pb (53%) > Ni (49%) > Cr (10%). The binding of the elements in the sediments sets the ED extraction limits, as it is only the elements desorbed to ionic form that are electromigrated into the electrode compartments. Thus, binding mechanisms were in focus.

Initially, the difference in ED extraction of P and heavy metals was investigated from dried sediment and wet sediment. The aim was dual: both to decide how to store and treat the sediment in the lab and to reflect on the status of the sediment in a large-scale setup. Experimentally, it was seen that there was a difference between the ED extractions from wet and dried sediments, the latter resulting in the highest percentages. It was also found that the different temperatures (freeze drying, air drying, oven drying at 60ºC and 105ºC), chosen for drying, did not have a significant influence on the ED extraction efficiency of either P or heavy metals. The result showed that it is beneficial to dry the sediment prior to ED treatment, and the remaining part of the experimental work was carried out with the air-dried sediments.

To get insights into the binding mechanisms, sequential extraction procedures were used. The procedures were operationally defined and were used to understand the mobility of P and heavy metals, which also gave a potential estimation of the availability of these elements for ED extraction.

The binding mechanisms influence the ED extraction efficiency of P and heavy metals. Cd and Zn were mainly associated with the exchangeable and reducible fractions, resulting in a high extraction during ED treatment. Cu and Pb were dominantly associated with the oxidizable fraction, which was difficult to mobilize, resulting in relatively less extraction during ED treatment. ED treatment was highly efficient in extracting acid-soluble P and partly in extracting P associated with humic compounds. There seemed to be a limit of extraction of about 50% P from the sediments, and this needs to be taken into account when handling the treated sediment.

The extraction and separation of P from heavy metals was more efficient in the 3-compartment (3C) ED cell than in the 2-compartment (2C) ED cell, indicating that the additional oxidation due to oxygen produced at the anode in the 2C-ED cell is not sufficient, and ways to extract P and heavy metals associated with the oxidizable fraction should be considered. The ED extraction of heavy metals, particularly Cd and Zn, was effective even from the sediments with low heavy metal contamination, mainly due to the binding of Cd and Zn with the exchangeable and reducible fractions.

After selecting the 3C-ED cell, multivariate analysis was used to understand the influence of experimental variables (current, liquid-to-solid ratio, duration, stirring rate, suspension liquid: distilled water or tap water) on P and heavy metal extraction. It was revealed that in the investigated experimental domain, current and liquid-to-solid ratio had the most influence on P extraction, while stirring rate and current had the highest influence on heavy metal extraction. The highest ED extraction of P and heavy metals in the investigated experimental domain was obtained at high current, liquid-to-solid ratio, and stirring rate. The influence of stirring rate indicates the importance of oxidizing conditions to release the elements associated with the oxidizable fractions, which should be taken into consideration while optimizing the treatment.

Since P is a scarce resource, the extracted P should be concentrated and processed further to a form directly applicable, e.g., fertilizer production. The P-extracted solution could be potentially recycled as a liquid fertilizer, however, the concentration is low, so a concentration step should be advantageous. Here, precipitation by capacitive de-ionization (CDI) gave promising results due to the formation of a pure nanocrystalline hydroxyapatite. Obtaining a P product suitable for use as a fertilizer could enhance the economic viability of the ED treatment.

The treated sediments met the standards for soil improvers for Cd and Zn, and therefore, the ED treatment can be successfully implemented for the sediments polluted with Cd and Zn. For efficient extraction of Cu and Pb, the use of desorbing agents along with ED treatment should be explored, mainly because these metals are strongly bound to the oxidizable fraction, and the oxidizing conditions during the ED treatment are not sufficient to release them.

In summary, ED extraction has its most beneficial use for the treatment of freshwater sediments polluted with Cd and/or Zn. With Cd being one of the major polluting heavy metals, this is an encouraging result. The method at the current state of development can be used for the remediation of Cd/Zn polluted freshwater sediments for recycling of the sediment, e.g., as soil improver, and the extracted P is in a clean form in a solution, though it needs to be concentrated before being a resource in the fertilizer industry. Nonetheless, the contaminated sediments can be transferred into two distinct resources by using ED treatment.