Recovery of nutrients from sewage using zeolite-chitosan-biochar adsorbent: Current practices and perspectives

Highlights

• The properties of zeolites, chitosan, and biochar for nutrient removal were summarized.

• Modification and adsorption process of zeolites, chitosan, and biochar was evaluated.

• The preparation of hybrid adsorbents and the mechanism of attachment were reviewed.

• The application of hybrid adsorbents in nutrient recovery is outlined and discussed.

Abstract

The recovery of nutrients especially from sewage shows a growing interest due to its capability in enhancing environmental and economic benefit. The nutrients such as N and P that are rich in sewage can be recovered to be used as fertilizers. Over the years, many natural adsorbents such as NZs, chitosan, and biochar have been investigated for nutrient recovery. However, those adsorbents have low mechanical strength as well as a low affinity towards various charged contaminants. Hence, grafting them into hybrid absorbents may enhance their adsorption capacity (AC) in nutrients removal. The nutrients that can be recovered are up to 96 %. This review summarizes the physical properties, performance in nutrient removal and recovery of NZs, chitosan, biochar, and respective hybrid adsorbents.

Introduction

About 70 % of phosphorus (P) and 75 % of nitrogen (N) constitute the overall sewage composition [1]. These components would impose hazardous effects if it is not further treated. The adverse effect of N ionic compound can potentially cause blue-baby syndrome or methemoglobinemia in infants and the accumulation of N and P in sewage can lead to eutrophication [2]. The removed nutrients are either lost through biological uptake and/or released to the atmosphere by converting the ammonium ion (NH₄⁺) to nitrogen gas [3,4]. Due to the accumulation of unexploited nutrients, the focus has been a shift to nutrient recovery involving the N and P as a solution to high carbon footprint produced during the manufacturing process of fertilizer as about 500 million tons of ammonia (NH₃) produced annually and every 1 ton of NH₃ produced, will generate 1.87 tons of CO₂. Moreover, phosphate rock cannot be replenished and will be used up in 30–300 years [[5], [6], [7], [8]].

The study approach to recover N and P has been done by physical, chemical, and biological methods. Nutrients can be recovered by chemical precipitation and it requires a tight pH range for optimum efficiency due to the potential volatilization of free ammonia at pH > 9.8 [9]. Physical processes for nitrogen recovery include membrane process [10], filtration [11], adsorption [12] and stripping (vacuum stripping) [13] or thermal stripping [14]. The biological process was also done by bio-electrochemical systems (BES) but the limitation lies in the membrane fouling and substrate variation [15,16]. As compared to other processes, the adsorption process has the easiest controllability, simple operation, and design but requires a downstream process such as desorption [[17], [18], [19]].

Adsorption using NZs, chitosan, and biochar has been used in nutrients removal due to its simple process including for sewage treatment [[20], [21], [22], [23]]. NZs are the naturally occurring minerals that are abundant and low-cost resources [24,25]. NZs are crystalline hydrated aluminosilicates with a framework structure containing pores occupied by water, alkali, and alkaline earth cations such as K⁺, Na⁺, Ca²⁺, and Mg²⁺ that contributes to the high cation exchangeability [[26], [27], [28]]. The ranges of cationic exchange capacity (CEC) ranges from 0.64 to 2.29 meq/g [25]. Pore sizes of NZs vary between 0.3 and 1 nm resulting in porosities between 0.1 and 0.35 cm³/g respectively [29]. Owing to this microporous structure, the surface area tends to be very high ranging from 300 to 700 m²/g and attribute to the excellent AC of NZs [29,30].

Chitosan, a natural and environmentally friendly biopolymer has been used as a biosorbent [31,32]. Low-cost chitosan can be produced from the deacetylation of chitin, extracted from shells of crustaceans, skin fungal cell walls, insects, organisms from industries, fungal chitosan, and wine yeast [[31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42]]. Among the carbon materials, activated carbon, carbon nanotubes, fullerenes, and graphene showed high AC but they require high production cost compare to the production of biochar [[43], [44], [45]]. The reported feedstocks for biochar production are from various fruit peels, plant parts, seeds, shells, crop residue, wood chip, algae, manure, sewage sludge, and municipal waste. The capability of biochar from those feedstocks to treat ammonia nitrogen and other contaminants are well documented [22,[46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57]].

The ability of NZs, chitosan, and biochar has been proven to treat various contaminants but has few drawbacks when being utilized as adsorbents. The low affinity of NZs towards negatively charged nutrients did not demonstrate appreciable AC [58]. Chitosan is a cationic biosorbent that shows a high affinity towards anionic nutrients such as nitrate (NO₃⁻), nitrite (NO₂⁻), and phosphate (PO₄^3-) but, it has low AC towards positively charged nutrients such as NH₄⁺ [31,59,60]. Furthermore, the AC of chitosan is highly dependent on the pH [41,61]. Studies indicated that carbon char recorded low AC because it has a non-polar surface. During the carbonization process, the transformation of macropores (> 50 nm) to micropores (< 2 nm) has attributed to the non-polarity of carbon char that is determined by the concentration of surface oxygen complexes [62]. During the transformation, the formation of oxygen complexes mostly occurs minimally at the outer surface or edge of the basal plane compared to the surface plane of the graphitic structure that has been attributed to low polarity [62]. The non or low polar carbon char can adsorp non-polar nutrients, but inadequate for the adsorption of polar nutrients such as ammonia nitrogen (N-NH₃) and (NO₂⁻) [63,64]. In order to increase the surface oxygen that promotes the polarity of carbon char, surface modification such as physical and chemical treatment has to be carried out [65]. Moreover, the application of these single adsorbents suffers from enormous transfer resistance due to large diffusion lengths during the adsorption process [66]. Hence, the application of organic materials such as chitosan with inorganic NZ, as well as biochar, may increase the AC by compositing these three adsorbents [58,66,67].

There are several reviews focused on the modification of NZs, chitosan, and biochar into hybrid adsorbents. The reviews mainly focused on the hybrid by combining two single adsorbents and focus on the synthesized routes as well as performance in the adsorption process. It is shown that, through the hybridization of adsorbents, there has been an improvement in the AC and affinity towards new contaminants. Some studies revealed that hybrids have higher AC in contrast with single adsorbent [18,68,69]. However, the application of hybrids in nutrient recovery by leaching (desorption process) has not been reviewed. Hence this review will emphasize the modifications of NZs, chitosan biochar into hybrid adsorbents, and the application of various hybrids in nutrient recovery.