Surface-active physicochemical characteristics of spent bleaching earth on soil-plant interaction and water-nutrient uptake: A review

Applied Clay Science 140 (2017) 59–65

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Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Review article

Surface-active physicochemical characteristics of spent bleaching earth on soil-plant interaction and water-nutrient uptake: A review Soh Kheang Loh a,⁎, Kah Yein Cheong a,b, Jumat Salimon b a b

Energy and Environment Unit, Engineering and Processing Division, Malaysian Palm Oil Board, No.6, Persiaran Institusi, Bandar Baru Bangi, 43000 Kajang, Selangor, Malaysia. School of Chemical Science and Food Technology, Faculty of Science & Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia.

a r t i c l e

i n f o

Article history: Received 21 April 2016 Received in revised form 20 January 2017 Accepted 21 January 2017 Available online 3 February 2017 Keywords: Edible oil Wineries Spent clay Waste acid bentonite Co-composting Soil amendment Plant nutrient

a b s t r a c t The activated or neutral form of bentonite-based spent bleaching earth/clay (SBE) is a by-product generated during the bleaching process in edible oil refinery. Its untreated form is disposed of directly at landfills involving high cost and land area, and possibly causing environmental problems. Recently, this undesirable dumping exercise has been prohibited. To overcome this, SBE is regenerated and reused for value addition, e.g. as bio active materials for water/wastewater treatment. A more recent approach being converting SBE into bio fertilizers; of which the fertilizer characteristics in relation to physical, chemical and biological interaction with soil and its surrounding ecosystem (nutrients, water, pollutants, microorganisms, climate, etc.) is vital in agricultural applications associated with soil fertility management and crops productivity. Previously, SBE's structural characteristics, surface chemistry and activation have been disclosed. This paper provides an insight on soil-crop interactions and agronomy with SBE functions as a soil amendment. © 2017 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . Characteristics of SBE and its effects on soil properties 2.1. Physicochemical properties . . . . . . . . 2.2. Macroelements . . . . . . . . . . . . . . 2.3. Micronutrients . . . . . . . . . . . . . . 2.4. Cation exchange capacity (CEC) . . . . . . . 2.5. Heavy metals . . . . . . . . . . . . . . . 2.6. Soil bulk density . . . . . . . . . . . . . 2.7. Soil water repellency . . . . . . . . . . . 2.8. Soil pH . . . . . . . . . . . . . . . . . . 3. Crop productivity . . . . . . . . . . . . . . . . 3.1. Nutrient availability . . . . . . . . . . . . 3.2. Plant growth . . . . . . . . . . . . . . . 4. Research gaps and future prospects . . . . . . . . 5. Conclusions . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction

⁎ Corresponding author. E-mail address: [email protected] (S.K. Loh).

http://dx.doi.org/10.1016/j.clay.2017.01.024 0169-1317/© 2017 Elsevier B.V. All rights reserved.

For centuries, man has been applying agro-industrial wastes on land as fertilizer or just to dispose of them, e.g., plant residues, animal manures, fly ash from thermal power plants, grape wastes from wineries,

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S.K. Loh et al. / Applied Clay Science 140 (2017) 59–65

etc. (Khalid et al., 2000; Arvanitoyannis et al., 2006; Jala and Goyal, 2006 and Petric et al., 2009). Among the world's major 17 edible oils and fats, palm oil is the largest, constituting 62.8 million t or one-third of the world production (MPOB, 2016), mainly from three major producers - Indonesia, Malaysia and Thailand. As the world's second largest grower, Malaysia produced ~ 20 million t from ~ 56,000 km2 (5.64 million ha) of oil palm plantations, or about 32% of the world supply (MPOB, 2016). In palm oil extraction, vast amount of by-products are generated (Boey et al., 2011; Loh et al., 2013; Kong et al., 2014 and Liew et al., 2015). In particular, the crude palm oil (CPO) produced is refined before use, which, process is, inter alia, bleaching with a surface-active substance, usually a claybased bleaching earth - to adsorb the impurities although other adsorbents such as activated carbon and silica hydrogel can also be used (Gunstone et al., 2007 and Hussin et al., 2011). In this review, bleaching earth refers to bentonite clays of mainly montmorillonite (Mt) - natural/raw (e.g. fuller's earth) or activated having the capacity to adsorb coloring matter i.e. pigments and the undesirable residues from the edible oil processing, such as soap, trace metals, phospholipids, oxidation products and polyaromatics (Loh et al., 2006; Soda et al., 2006 and Hussin et al., 2011). The fully adsorbed earth is termed spent bleaching earth (SBE). Other industries that use similar functionalized materials for refining/bleaching include wineries, but their spent material is known as acid waste bentonite (AWB). Both of them belong to the 2:1 dioctahedral phyllosilicates group due to their similar arrangement of silicate and alumina components in a specific orientation, hence do not differ much in their adsorption capacity. Globally, based on a 1 mass% bleaching earth usage for some 200 million t of edible oils and fats refined, an estimated 2 million t of SBE equivalent is available yearly (Loh et al., 2013; Beshara and Cheeseman, 2014 and MPOB, 2016). In Malaysia, ~10–15 kg bleaching earth per t CPO (1–1.5 mass%) is used in palm oil refining. This alone generates up to 240,000 t/yr SBE (Boey et al., 2011; Loh et al., 2011 and Loh et al., 2015). In the past, research mainly looked into improving the surface activity and adsorption capacity of the clay minerals via structural modification and surface reactivation (Pollard et al., 1993 and Hussin et al., 2011), and recovering the residual oil in the SBE (Al-Zahrani and Daous, 2000; Lee et al., 2000 and Loh et al., 2006) via solvent extraction and other environmentally-friendly means. SBE normally contains ~20–40 mass% in residual oil and in some cases, up to 72 mass%. It is usually disposed of directly in a landfill, either dry or as a slurry (Loh et al., 2006; Ho et al., 2010 and Meziti and Boukerroui, 2011). Attempts to extract the oil have also not proven particularly successful as the oil is of low quality containing free fatty acids, oxidation products, trace metals, etc. and cannot be used as food, and so cannot command a good price. The excessive use of clays in oil refining poses issue in its disposal and the cost involved. The high oil content of SBE is an environmental hazard as the oil can rapidly oxidize to the point of spontaneous ignition via clay-catalyzed auto-oxidation reactions, posing fire hazard particularly if the oil is highly unsaturated (Pollard et al., 1993 and Boey et al., 2011). Besides, such disposal also poses a potential hazard to aquatic life as the containing fatty materials can leach into water (Lee et al., 2000). In view of the increasing disposal costs of SBE coupled with stringent regulatory requirement, efforts have been made to transform it into a useful product(s). The potential uses of SBE are as (1) animal feed (Ng et al., 2006 and Damodaran, 2008), (2) adsorbents via thermal, physical and chemical reactivation (Pollard et al., 1993; Ma and Lin, 2004; Gunstone et al., 2007; Wambu et al., 2009 and Mana, et al., 2011) and pyrolysis (Tsai et al., 2002), (3) raw materials for making cement and bricks (Beshara and Cheeseman, 2014 and Eliche-Quesada and Corpas-Iglesias, 2014), (4) expanded clay granules for the construction industry and gardening (Gunstone et al., 2007), (5) fermentation facilitator in biogas plants (Gunstone et al., 2007), (6) fuel briquettes (Suhartini et al., 2011) and (7) soil improver in agriculture, etc. Of these, the last is the most

researched today, even for AWB, owing to their naturally inherited nutrient-binding properties as proven in several field trials conducted on intended crop species (Croker et al., 2004; Soda et al., 2006; Arias-Estévez et al., 2007; Ho et al., 2010; Wang et al., 2010 and Loh et al., 2013). Although activation of SBE as adsorbents is the most widely practiced, the presence of contaminants such as soaps or phosphatides in residual oil of SBE would cause vitrification, thus loss of surface and bleaching activity of the thermally-treated adsorbents (Gunstone et al., 2007). The liquid residual oil, on the other hand, can be used as a substrate for (1) edible fungi fermentation to produce riboflavin as medicine, food and fodder use (Park and Ming, 2004), (2) biofuel (Lara and Park, 2004; Loh et al., 2006; Dwiarti et al., 2010 and Boey et al., 2011), (3) biolubricants (Loh et al., 2007), (4) industrial grade oleochemicals (Chanrai and Burde, 2004) and (5) animal feed (Damodaran, 2008). In the case of AWB, it has been readily used in its disposed form for direct application to soil without further treatment (Arias-Estévez et al., 2007 and Pateiro-Moure et al., 2009). In gist, the difficulty in recovering/reusing SBE makes dumping the solid (de-oiled) residue the only practical way of disposal. Interest has thus been generated in using it, either uncomposted or composted, as a fertilizer/soil amendment to nurture plant, improve soil quality and promote microbial rejuvenation (Gunstone et al., 2007). However, even the simple application of SBE to the soil, more so if wellcomposted, has its risk of potential adverse effects on the soil, environment and food safety due to its possibly high heavy metals content. One the other hand, a poorly composted SBE might affect crop productivity and farmers' well-being. Therefore, a proper assessment of its suitability as fertilizer/soil amendment is of utmost importance if it is to be used this way. This paper reviews the key physicochemical characteristics of SBE for agricultural use, and its potential agronomic benefits. 2. Characteristics of SBE and its effects on soil properties 2.1. Physicochemical properties According to Gunstone et al. (2007), bentonite from open pits is mainly Mt., a complex alumino-silicate based clay mineral from the smectite (Sm) group produced by in situ devitrification of volcanic ash. Other minerals such as beidellite (Bd), saponite (Sp), hectorite (Ht), illite (I), kaolinite (Kaol), gypsum (Gp) and quartz (Qz) are also present in smaller quantities. Two types of bleaching earth – usually bentonite-based clay containing mainly SiO2 (65–75%) and Al2O3 (15– 20%) - are sold commercially i.e. the virgin and acid-activated clay (Ho et al., 2010). The latter has higher adsorption than the virgin type as it is activated - the octahedral metal cations (Al3+, Fe2+ and Mg2+) exchangeable sites in the interlayer bentonite back-bone are dissolved and replaced by protons (H3O+) in the acid treatment (Hussin et al., 2011). This commonly used bleaching earth in crude edible oil refineries is essentially SBE. Physically, the colour of activated bleaching earth is white, turning brownish after bleaching. The particles are very fine, mostly irregular in shape and porous. The original Mt has a total pore volume of 26.5 μL g−1 (Weng and Pan, 2007) with a low specific surface area of 40–160 m2 g−1 (Hussin et al., 2011) while the activated one is much higher 150–350 m2 g−1 (Gunstone et al., 2007; Weng and Pan, 2007 and Hussin et al., 2011). Generally, the increase is due to the acid attack and heat at the exchangeable sites which dissolves the impurities

Table 1 Chemical compositions (mass%) of spent bleaching earth (SBE). SiO2

Al2O3

Fe2O3

MgO

CaO

Reference (author, year)

79.8 56.9 37.45 65–75

8.7 9.24 8.01 15–20

1.9 8.27 0.83 2

3.2 4.32 1.28 2.5

0.7 3.9 0.78 0.5

Lara and Park (2004) Loh et al. (2013) Mana et al. (2011) Weng and Pan (2007)

S.K. Loh et al. / Applied Clay Science 140 (2017) 59–65 Table 2 Main nutrient concentration in spent bleaching earth (SBE). Nutrient/element

Concentration

Proteina Total nitrogena Organic mattera Organic carbona

1.32 0.25; 0.06 31.25; 30.0 12.40; 15.30; 17.40–22.41 128; 290–293

Carbon/nitrogen, C/N Available phosphorus (mg kg−1) Ex.b Ca (cmolc kg−1) Ex.b Mg (cmolc kg−1) Ex.b K (cmolc kg−1) Ex.b Na (cmolc kg−1) SO2 a b

Reference (author, year)

Wang et al. (2010) Ho et al. (2010); Loh et al. (2015) Wang et al. (2010); Loh et al. (2015) Ho et al. (2010); Croker et al. (2004); Loh et al. (2015) Ho et al. (2010); Loh et al. (2013); Loh et al. (2015) 59, 486; 2.36a (as Wang et al. (2010); Croker et al. P2O5) (2004); Loh et al. (2015) 14.6; 3.58a Croker et al. (2004); Loh et al. (2015) 8.6; 1.55a Croker et al. (2004); Loh et al. (2015) a 0.7; 0.27 Croker et al. (2004); Loh et al. (2015) 1.9 Croker et al. (2004) a 1.32 Loh et al. (2013)

Unit in mass%. Ex. = exchangeable.

(metal cations), departs them to make way for substitution by protons. After oil refining, SBE appears in clusters with a smooth surface due to the embedded oil (Lara and Park, 2004). The specific surface area is greatly decreased to 2.04 (Hassan, 2006) - 62.32 m2 g−1 (Loh et al., 2013) with an increased total pore volume of 165 μL g− 1 (0.165 cm3 g−1) (Loh et al., 2013). The specific surface areas for adsorption or chemical reactions is reduced, and the original empty spaces/ pores occupied by metals - the micropores (Meziti and Boukerroui, 2011) - are now filled by the adsorbed oil, reducing its adsorption capability. The main chemical component of SBE is SiO2 followed by Al2O3, MgO, Fe2O3 and CaO (Table 1), whereas its elemental contents are C, Ca, O, Fe, Mg, Al, Si, P and K (Boey et al., 2011 and Loh et al., 2013). Si and Al are major constituents while Fe and Mg minor elements in the octahedral and tetrahedral interlayers (Weng & Pan., 2007). This configuration orientates the active charge sites and surface functional groups in different locations of dissolutions/substitutions, and on interacting with the soil and surrounding media in favorable condition, chemical reaction takes place, leading to active surface binding activities, thus nutrient adsorption. The octahedral and tetrahedral arrangement eases the coordination via complexation of the transition and alkali metallic ions with the charge sites within the geometry and is regiospecific to the entry of nutrients from all directions in the soil matrix. This assists in exchanging cations during soil-plant interaction. 2.2. Macroelements SBE exhibits all the six essential macronutrients - N, P, K, Ca, Mg and S (Table 2) - in sufficient amounts for plant growth. Hence, its potential as fertilizer. The organic matters and nutritive elements in loose forms make application of SBE to soil one of the most promising alternatives compared to landfill disposal and other applications. However, direct applications should be avoided as SBE alone is not readily decomposable thus limiting the nutrients bioavailability in soil-plant system. According to Pusparajah (1977), 93% of tropical agricultural soils have N deficiency. In particular, land application of livestock and poultry farm wastes leads to N 50% loss of total N in two months (Arkhipchenko et al., 2005). Soil amended with SBE/AWB may be a solution to this. The N in AWB is mostly originated from the amide moiety of protein character (Pateiro-Moure et al., 2009). With high organic matter, its cation exchange capacity (CEC) increases. As high CEC improves water holding capacity and provides slow release property, all nutrients from the fertilizer/water will be supplied to the plant in a controlled release efficient manner, and in exchange heavy metals adsorbed; thus promoting long term soil fertility and pollutants management (reduce contaminants in

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Table 3 Effect of palm oil-based spent bleaching earth (SBE) on soil chemical properties (adopted from Croker et al., 2004). Treatment

Equivalent application rate (t ha−1)

Available P (mg kg−1)

Ex.a Ex.a Ex.a Ex.a Ca Mg K Na (cmolc kg−1)

Soil (Arenic Acrisol) Palm oil SBE Soil + palm oil SBE (3.5 kg + 8.75 g) (3.5 kg + 17.5 g) (3.5 kg + 35 g) (3.5 kg + 70 g)

– –

7 486

0.6 0.3 14.6 8.6

0.1 0.7

0.2 1.9

5 10 20 40

11 13 20 32

0.8 0.7 0.8 0.9

0.1 0.1 0.2 0.1

0.2 0.2 0.3 0.3

a

0.3 0.3 0.4 0.4

Ex. = exchangeable.

soil) (Arias-Estévez et al., 2007 and Pateiro-Moure et al., 2009). Similar observation was found in the case of SBE (Loh et al., 2013). Under aerobic condition, soil bacteria will readily transform protein-borne N to − NH+ 4 (ammonification) and NO3 (nitrification) (Roy et al., 2006). If the N uptake rate is slower than the rate of ammonification and nitrification, then NO− 3 will be subject to leaching and/or denitrification (Kim and Owens, 2010). According to Bröckel and Hahn (2004), stabilized fertilizers are able to reduce NO− 3 leaching by increasing the lifetime of NH+ 4 -N to be retained in the soil from b 1 week under normal conditions to 6–10 weeks. Being high in CEC with lot of active charge sites, SBE is expected to be able to hold tight to NH4+. This is indeed demonstrated by Sitthaphanit et al. (2010) for an effective N loss reduction performed by bentonite (CEC of 34.3 cmol kg−1 at 50 t ha−1 treatment) in sandy soils with a 15-day delayed NH+ 4 leaching. P deficiency is a major growth-limiting factor in many acidic soils. Inadequate P supply results in decreased synthesis of RNA - the protein maker - leading to depressed growth (Schachtman et al., 1998 and Hue and Silva, 2000). The amount of P is either inherently low or present in the forms that are unavailable to plants due to high P-fixation by Fe and Al oxides and hydroxides in the clay fraction of soil (Borggaard, 1986 and Zulkifli et al., 2003). Thus, to increase bioavailability of P in acidic soils, SBE and AWB are of particular relevance as they exhibit high extractable P (Fernandez-Calvino et al., 2015 and Loh et al., 2015). Accordingly, adding SBE from palm oil refining (i.e. palm oil-based SBE) to acidic soils at a rate N 20 t ha−1 could increase available P to N20 mg kg−1 (Croker et al., 2004). Similarly, high dosage of perlite waste from wineries also shows significant increase (N20%) in available P (Nóvoa-Muñoz et al., 2008). From the calculated palm oil-based SBE fertilizer equivalent of 280 kg ha− 1 of triple super phosphate at 40 t ha− 1 treatment ratio (Croker et al., 2004), and assuming 240,000 t of SBE are generated yearly in Malaysia (Boey et al., 2011; Loh et al., 2011 and Loh et al., 2015), up to 6000 ha of oil palm planted land can benefit from SBE treatment, thus an annual saving of 1680 t of triple super phosphate. The exchangeable K in soil amended with AWB is dependent on the types of solid wastes used. Although the exchangeable K is unchanged (Table 3) in Nam Phong soil series (Arenic Acrisol) treated with palm oil and soybean-based SBE at an equivalent rate of 5 to 40 t ha− 1 (Croker et al., 2004), the winery clay wastes show otherwise as are affected by the original available K content proportionately with the quantity of AWB or perlite waste (K-rich) applied (Nóvoa-Muñoz et al., 2008; Pateiro-Moure et al., 2009 and Fernandez-Calvino et al., 2015). In these studies, the exchangeable Ca, Mg and Na contents remain largely unaffected (Table 3). Lastly, although S is present in SBE (0.05 mass%) co-composted with chicken litter (0.10 mass%) (Loh et al., 2015), no research has been conducted so far on its likely effects on soil-plant system interaction and uptake. In summary, SBE is highly capable of increasing the soil N, P and K, thus endowing it with considerable potential as a substitute for conventional fertilizers.

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2.3. Micronutrients The plant micronutrients or oligoelements such as B, Cl, Cu, Fe, Mn, Mo and Zn are essential for healthy growth in supporting the various biological developments. SBE from edible oil refinery has got 7.30 mg kg−1 Cu (Ho et al., 2010), 5.94–9.76 mg kg−1 Fe (Pandey et al., 2003) and 18.8 mg kg−1 Zn (Ho et al., 2010), respectively while that from winery (AWB) has abundant Cu (1481 mg kg−1), Zn and Mn each 37 mg kg− 1 (Pateiro-Moure et al., 2009). Thus, SBE/AWB from these sources can be a potential slow release soil amendment for soil low in extractable micronutrients. Nevertheless, if the used winery AWB has high Cu content originated from the Cu-based fungicides used, and is released uncontrolled exceeding the allowable limits for land application (Section 2.5), it will potentially cause phytotoxic. In reality, the negatively-charged large organic molecules associated with AWB in acidic condition have ability to chelate Cu forming very stable complexes. This immobilizes Cu in soil matrix, hence has tendency to reduce the resulting phytotoxicity (Pateiro-Moure et al., 2009). The fractionation into soluble/exchangeable vs less soluble Cu (Nóvoa-Muñoz et al., 2008) and similar mechanism for Zn and Mn have convinced that there is hardly any oversupply of micronutrients in acidic soils treated with higher dosages of winery clay waste. It further reveals that although the bioavailability of Cu, Zn, Mn is low in SBE-treated soil, their supplies to the soil and eventually to the plants is still sufficient and manageable via slow release ability. As SBE is high in CEC and ion adsorption capacity, it regulates the solubility of the micronutrients in the soil solution and binds them in a loose manner that can be easily released when in need.

the organic matters (Table 2) and high CEC (Table 4) in SBE and or its composted forms would suffice to address the above problem by enhancing the soil chemical characteristics (Soda et al., 2006), though its counterparts i.e. AWB, beneficiated bentonite and pure bentonite having higher CEC may be more outstanding (Croker et al., 2004; Pateiro-Moure et al., 2009 and Gillman, 2011). SBE alone in its isomorphic environment is able to exchange cations between the surface of internal structure and external source, creating high affinity towards external (exchangeable) cations from a nutrient source. This unique characteristics provides not only resistance to leaching force but also slow nutrient release ability for root uptake, thus improving crop productivity (Noble et al., 2001). SBE combined with other agricultural substrates e.g. rice husk, chicken litter and oil palm biomass shows synergistic effect in fertilizer characteristics (Soda et al., 2006; Ho et al., 2010 and Loh et al., 2013). Apart from their fertilizer values, the products may be beneficial as soil amendments when added to light-textured soils because each application will result in an increase in clay content, giving the soil increasing capacity to retain nutrients and water. Evidently, the bentonite (saturated with Ca, Mg and K with CEC 59 cmol kg−1) (Table 4) amended tropical Australian sandy soils show N200% increases in the soil exchange properties (Croker et al., 2004). However, the commercially available cation beneficiated bentonite is very costly, while SBE or AWB is readily available and could be more cost-effective to farmers as it could effectively increase soil's CEC from 23% up to 40% with a CEC less than half of that in cation beneficiated bentonite. Increasingly, it is more preferably used in leaching preventive measure in sandy soils. 2.5. Heavy metals

2.4. Cation exchange capacity (CEC) Knowledge about the soil characteristics on which fertilizers are to be applied is important for better fertilizer management and application efficiency. In general, there are 3 groups of problematic soils, namely sandy soils, peat soils and acid-sulphate soils (FAO, 2004). Sandy soils are among the most abundant soils in the world, covering N900 million ha (Croker et al., 2004). Malaysia has a total land area of 32.97 million ha of which 6.86 million ha are used for agricultural purposes such as oil palm plantation (66.1%), rubber (17.1%) and cocoa (0.65%) (Sabri, 2009). The sandy soil in Malaysia covers about 200,000 ha (Wong and Chen, 1998). Sandy soil is high in sand particles and characterized by low organic matter content and CEC. It features high porosity posing high risk of leaching (Sitthaphanit et al., 2010) e.g. soluble fertilizer is prone to leach out due to lack of binding ability (adsorption affinity and bonding strength) and thus results in soil's declined nutrient holding capacity (Gillman, 2011). It usually requires specific amendments for successful agricultural use (FAO, 2004). Thus, suitably, Table 4 Cation exchange capacity (CEC) of different spent bleaching earth (SBE) sources. Type of SBE

CECa (cmolc kg−1) Reference (author, year)

Edible oil SBE

37; 40

Palm oil SBE

15; 31.5–36.0; 42.6

Soybean oil SBE

28; 47.5

Rice oil SBE

11; 19.2

Cation beneficiated bentonite (bent) Winery AWBb Bent manure fertilizer Bent Pure bent

59 124 40.9–66.3 34.3 80–100

Mana et al. (2011); Ho et al. (2010) Croker et al. (2004); Loh et al. (2013, 2015); Soda et al. (2006) Croker et al. (2004); Soda et al. (2006) Croker et al. (2004); Soda et al. (2006) Croker et al. (2004) Pateiro-Moure et al. (2009) Redding (2011) Sitthaphanit et al. (2010) Gillman (2011)

a CEC for clay, loam and sand are N40, 12–25 and b6 cmolc kg−1, respectively (Roy et al., 2006). b AWB is acid waste bentonite (bentonite-based winery waste).

High concentrations of heavy metals in soil cause toxicity, inhibit microbial activity and damage soil quality resulting in degraded and unsuitable soil for plant growth (Kim and Owens, 2010). As a soil amendment, SBE in use must be free of heavy metals or at least their presence is at the lowest possible concentrations. Several heavy metals in SBE (Cd, Cr, Ni and Pb) (Ho et al., 2010 and Eliche-Quesadaa and Corpas-Iglesias, 2014) are found much lower than the stipulated maximum pollutant concentration limits for land application set by US EPA (1993), China and several other countries (Table 5) (Wang et al., 2010). Hence, their presence is negligible (not in the case for winery wastes be it bentonite-based AWB or perlite waste) and application of SBE as soil amendment/fertilizer is safe without heavy metal contamination in plants. This in fact is supported by a high germination index (70–80%) in the growth of Chinese cabbage (Ho et al., 2010) and moderate ones (30–70%) that of winery wastes (AWB and perlite)-treated L. multiflorum seeds (Arias-Estévez et al., 2007 and Nóvoa-Muñoz et al., 2008) without the presence of the herein possible deriving metabolite destroying phytotoxic compounds (acetic, propionic, butyric and isobutyric acids) that would inhibit germination (Ho et al., 2010). Furthermore, co-composting of SBE with biosolids and rice husk could enhance the immobility of heavy metals; hence remarkably reduce phytotoxicity (Ho et al., 2010). Probably, it has transformed the highly labile metals into immobile components. In the case of winery waste, the potential Cu phytotoxicity has been mostly reduced as its large content of Cu becomes immobilize in the presence of stable organic matter in both the clay waste itself and soil (Arias-Estévez et al., 2007 and Nóvoa-Muñoz et al., 2008) coupled with the ability of the soil-clay waste interaction to induce soil-Cu sorption capacity and binding energy making significant diminution in its mobility and bioavailability in the amended soil (Rodríguez-Salgado et al., 2016). 2.6. Soil bulk density Soda et al. (2006) demonstrates a decrease in soil bulk density with increasing amendment rates of co-composted SBE (15–120 t ha−1) for light-textured sandy soil (Satuk series). The attributed increased

S.K. Loh et al. / Applied Clay Science 140 (2017) 59–65

63

Table 5 Comparison of heavy metal concentrations (mg kg−1) in spent bleaching earth (SBE) with the standard limits set by various countries for land application. Heavy metal

SBEb

SBEa

Chinab

European Unionb

Franceb

Italyb

Hollandb

Belgiumb

Cadmium Lead Mercury Arsenic Chromium

0.26 12.9 0.82 1.65 8.03

0.11 6.37 – – 6.28

3 100 5 30 300

20–40 750–1200 16–25 – 1000–1500

2 100 1 – 150

20 750 10 – –

1.25 100 0.75 15 75

1 50 1 – 100

a b

Ho et al. (2010). Wang et al. (2010).

organic carbon content in the amended soil has somehow reduced soil compaction and increased soil aggregation resulted in reduced bulk density (Khaleel et al., 1981 and Soda et al., 2006). Accordingly, a bulk density of 1.55 Mg m−3 is often regarded as the minimum value at which root restriction may be observed in slit and slit loam soils (Logsdon and Karlen, 2004). Thus, the 37% decrease in the original soil bulk density of 1.45 Mg m−3 is a beneficial amendment as it increases the total porosity of the soil. Contrarily, in other case, similar soil type amended with SBE or beneficiated bentonite does not show significant changes in soil bulk density (Croker et al., 2004). 2.7. Soil water repellency SBE has commonly been reported to have high hydrophobic nature resulting in extreme water repellency (Soda et al., 2006 and Loh et al., 2013). The extreme resistance towards water in the soil matrix especially at higher treatment rates is due to the presence of high concentrations of oil residues (Croker et al., 2004 and Ho et al., 2010). On the other hand, palm oil-based SBE co-composted with rice husk, rice husk ash and chicken litter as described by Soda et al. (2006) is able to reduce soil's acidity and hydrophobicity because the presence of microorganisms has been able to decompose and mineralize the oil. Therefore, it is anticipated that poor water retention characteristics of a soil treated with uncomposted SBE is a short-term phenomenon, and once it is domesticated by soil microorganisms a few months later, the soil's hydrophobicity will eventually be improved. This, in part is also the reason why a co-composted SBE is preferred for soil amendment. However, whether a naturally hydrophobic soil can be remediated using SBE is uncertain and poorly understood. 2.8. Soil pH Having been a negatively-charged lamellar aluminosilicate structure, SBE is neutralized by mobile cations (Gunstone et al., 2007). During acid activation, the H+ replaces other mobile metallic ions such as Fe3 +, Al3 + and Ca2 +, causing acidity in SBE (Weng and Pan, 2007). Maintaining an appropriate soil pH is essential for sustaining productivity of cropping systems (Fageria, and Nascente, 2014). Soil pH can be amended via SBE application; e.g. a controlled addition of acid-activated SBE (preferably composted ones) to an alkaline soil could reduce the pH to a desirable level for the intended crop (Soda et al., 2006 and Loh et al., 2013). Contrarily, adding acidic bentonite-based AWB (pH b5, 30 g kg− 1) from winery increases the pH of soil (Arenic Regosol) to N7 (Arias-Estévez et al., 2007). This pH increase is more likely caused by the formation of potassium bitartrate from the byproduct of winemaking than NH4OH deriving from the microbial degradation of N protein in winery AWB waste. Interestingly, H+ is removed from the soil solution to form bitartrate, K+ is released as nutrient for plants and OH– into the amended soil; hence pH increases (Arias-Estévez et al., 2007 and Fernandez-Calvino et al., 2015). However, this phenomena is untrue for perlite winery waste which originally is already alkaline (pH 7.4–9.1) (Nóvoa-Muñoz et al., 2008 and Rodriguez-Salgado et al., 2016). It has become evident that soil pH effect is influenced by the nature of soil type, clay waste dosage and type of clay waste used (cocomposted or uncomposted, SBE or AWB or perlite waste) (Soda et al.,

2006). The co-composted ones tend to influence the soil reactivity and pH more. In particular at highly acidic areas, it is suggested that SBE application allows for short-term savings in lime-based soil pH amendments. 3. Crop productivity 3.1. Nutrient availability The bioavailability of macro and micronutrients e.g. N, P, K, Ca and Mg in plants is affected by many factors including the type of soil amendment such as applications of SBE (Croker et al., 2004). Besides stimulating growth, SBE enriches nutrients by making them (N, available P and K) easily accessible for uptake in plants even though in low dosages level (Croker et al., 2004 and Wang et al., 2010). As SBE contains residual oil, and in the case of palm oil, the inorganic P present is eight times that of the phospholipids P (Gibon et al., 2007); hence it promotes cation exchange and bioavailability in soil making it much easier for plants uptake. In other study, addition of bentonite is effective in retaining N-NH4 thus decreasing N leaching and increasing the probability of plant uptake (Sitthaphanit et al., 2010). SBE with high CEC could make the base cations e.g. Ca, Mg and K more available for plants uptake as these cations are made available to adsorb onto the negatively-charged surfaces in Mt. and therefore, preventing nutrients leaching (Roy et al., 2006). According to Arias-Estévez et al. (2007), although AWB contains micronutrients such as Cu, Mn and Zn, their extraction by plants is slowly released and controlled so that the soil treated will be able to regulate and provide sufficient quantities of these essential micronutrients for plants growth rather than causing unintended plant phytotoxicity. 3.2. Plant growth Field and greenhouse trials conducted so far have confirmed that several plants e.g. Chinese cabbage (Brassica chinensis), Shiitake mushroom (Lentinula edodes), Italian ryegrass (Lolium multiflorum), maize (Zea mays L.), sorghum (Sorghum bicolour), kangkung (I. aquatic), okra (A. esculentus), eggplant (Solanummelongena var. esculenta L) and groundnut var. magenta have benefitted in their growth from the macro and micronutrients contained in SBE (Croker et al., 2004; Soda et al., 2006; Ho et al., 2010; Wang et al., 2010; Loh et al., 2013 and Cheong et al., 2014), AWB (Arias-Estévez et al., 2007) and perlite waste (Nóvoa-Muñoz et al., 2008). In general, the application rate of spent clay (SBE or AWB), harvest cycle of the crops and type of spent clay used affect crop productivity (Croker et al., 2004). E.g. the biomass yield in decreasing order after one harvest cycles of sorghum treated with 40 t ha−1 SBE is beneficiated bentonite N soybean oil SBE N rice oil SBE N palm oil SBE whereas the combined yields after the 2nd and 3rd harvest cycles, palm oil SBE N soybean oil SBE N beneficiated bentonite N rice oil SBE (Croker et al., 2004). Interestingly, small dosage of palm oil SBE (5 t ha−1) seems more beneficial to plant growth compared to those treated with ≥10 t ha−1 with detrimental long-term effect (Croker et al., 2004). As most of the SBE is hydrophobic, an increased treatment in soil tends to repel water (Croker et al., 2004

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and Arias-Estévez et al., 2007), making nutrients less bioavailable to plants. Besides, crops such as maize, shiitake mushroom, okra and groundnut show positive growth when treated with palm oil SBE cocomposted with other biomass e.g. biosolids, rice husk, rice husk ash, chicken litter, oil palm biomass (Soda et al., 2006; Ho et al., 2010; Wang et al., 2010 and Loh et al., 2013). The dry matter production of these crops has increased N30% compared with those grown on commercial fertilizer. Probably, the co-composted SBE has potentially improved the physical soil attributes including specific surface area, bulk density and bioavailability of water/nutrients for beneficial plants uptake. Therefore, the application of co-composted SBE in soil rather than direct application can promote beneficial soil amendments leading to significantly increased plants growth and yields. By optimizing resources recovery and revenues generation, maximizing crop yields while maintaining a good soil ecosystem, the SBE as soil amendment in long run has potential contributing to advancement in agricultural economics. 4. Research gaps and future prospects The use of SBE or AWB is appropriate as potential amendment for the management of acidic and sandy soils. Application of SBE in agriculture undoubtedly provides benefits to the soil and the environment and is cost-effective in soil nutrient management. Depending on the nutrient requirement of crops, SBE can be recycled and used alone or in combination with other biomass (normally co-composted) e.g. biochar to further enhance the fertilizer nutrient quality and amendment ability. In particular, the ability of biomass amendment associating with SBE for degraded soil needs to be performed. Most of the efficacy assessments (pot assays or field trials) using short rotation cash crops are able to accommodate the moderately NPK-containing SBE and able to show beneficial advantages in promoting biomass productivity for commercial sale. Nonetheless, additional field evaluation on the long-term effects of SBE application to soil's physicochemical characteristics and plant productivity are needed to further verify the findings under a broader range of climatic conditions as these will greatly impart crop production systems as well as soil- and plant-water relationships. These findings will be able to ascertain nutrient/fertilizer use efficiency leading to efficient fertilizer management. Soil conditions may change when exposed to severe or sudden weather change i.e. drought vs deluge or monsoon that alters the physicochemical properties e.g. hydrophobicity etc., and the exact role of SBE to amend those conditions, plus the interactions between the different variables controlling the soil's and plant's water-nutrients uptake need to be more clearly defined. The changes in the soil from hydrophobic to hydrophilic and vice versa in associating with soil amendment function by SBE under different climate are so far only poorly understood. Crops demanding high NPK requirement e.g. oil palm might only accommodate SBE as a slow-release fertilizer supplement for sustainable long-term soil fertility nurturement, and the usual supply and quick release of nutrients such as the one accomplished by chemical fertilizers is still very much required for economic growth. On top of that, as soil biological community plays vital role in determining many characteristics and functions of soil so as to provide a healthy and beneficial ecosystem for its surrounding, it is of paramount importance to further investigate the changes in soil's microbial and physical properties (e.g. bulk density, porosity, water-holding capacity, aggregation) in order to provide an insight on more sustainable agriculture in the future. Notably, excessive use of SBE for soil remedy could potentially turn beneficial nutrients and the containing residual oil into contaminants affecting aquatic environment if they are leached unexpectedly and uncontrolledly in high concentrations into a watercourse. This adverse effect needs to be monitored closely during SBE application and amendment to the intended soil.

5. Conclusions Application of SBE in either acidic or sandy agriculture soils is an efficient soil amendment to not just reducing the waste but recycling beneficial nutrients back to soil-plant system. SBE acts as a nutrient source for N, P and K as well as a modifier for soil structure enhancement. Its naturally inherited fertilizer characteristics coupled with high CEC and organic carbon content allow for a slow release of ionic nutrients (macro and micro levels), providing and maintaining sufficient soil nutrients to nurture the plants throughout the growth. Besides, the resulting increase in the CEC of the soil treated with SBE provides opportunity for long-term fertility management. More importantly, SBE from edible oil refinery does not perform any major heavy metal contamination to soil, except for some clay waste counterparts from wineries and other industries. As many studies have shown promising positive soil physicochemical attributes and plants productivity, SBE especially in the composted form has huge potential as amendment to degraded soil. More in-depth agronomical research to understand the intimate interactions/limitations within the SBE-amended soil, crops intended and its surrounding climatically vulnerable ecosystem is much needed. In the future, its co-existing amendment with other potential nutrient rich biomass may contribute to sustainable agriculture.

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