Rice husk and rice straw based materials for toxic metals and dyes removal: a comprehensive and critical review

ABSTRACT This review investigated depth literature survey on the removal of various heavy metals and dyes contamination. The current study is a broad review of the different methods of preparation of biowaste adsorbent from rice husk and rice straw that has been implemented for the adsorption of many hazardous heavy metals and dyes in order to reduce their harmful effect on the environment. The selection of rice waste-based material for the adsorption process was considered due to its cost-effectiveness, easy availability, high adsorption efficiency, and reusability. This study is a comprehensive review of the adsorption of toxic heavy metals and dyes using rice husk and rice straw-based adsorbents either in bare or in modified forms under different physicochemical processes. In addition, some parameters affecting adsorption capability like pH, initial dye concentration, equilibrium time, temperature, adsorbent dosage, and shaking or stirring speed influence the adsorption mechanism have been discussed thoroughly. The applicability of various adsorption isotherm models and adsorption kinetic models for dye adsorption by various rice husk and straw biomass adsorbents is also reported here. Finally, from the literature reviewed conclusions have been drawn and also proposed a few future research suggestions.


Introduction
In current scenario, purification and preservation of water quality is a major environmental issue due to increasing population, urbanisation, industrialisation and reduction of water resources.Organic and inorganic contaminates play a crucial role for polluting the surface and ground water for many years with human health risk.The existence of carcinogenic compounds in agricultural activities and industrial effluents are the major contamination sources of water [1,2].Dyes are considered as the major organic contaminates for the environment [3][4][5].Chakrabarti et al. [6] reported nearly 40,000 dyes and pigments are listed which consist of over 7000 various chemical structures.Most of them are non-biodegradable in nature [7].It was observed that 10,000 of various commercial dyes and pigments exist over 7 × 105 tonnes are generated annually in worldwide [8].
Commercial dyes can be classified in many ways on the basis of colour, structure and method of application [21].On the other hand, so many complexities are found on considering the study of colour and nomenclature.Therefore, classification according to application methods is mostly favourable [22].According to chemical structure, the classification of common class of dyes are presented in Figure 1.Furthermore, dyes are classified according to the charge on their particle when dissolved in aqueous medium like anionic (acid, direct, reactive dyes), cationic (all the basic dyes), and dispersed or nonionic dyes [23,24].
Due to the toxic nature of dyes and heavy metal ions, it is necessary to remove during treatment method before dispensing into water bodies.Some conventional techniques for removing these contaminants based on chemical, biological, physical and thermal properties include oxidation, biodegradation, ion exchange, precipitation, ion exchange, coagulation, flocculation, reverse osmosis, ozonation, filtration, membrane process, irradiation, adsorption, electrodialysis, solvent extraction and distillation [1,2,15,18,19,26,28,38,39].Among all removal techniques most of them are disadvantages like low efficiency, high cost, less flexibility and preparation of secondary pollutants [2,26,38].However, adsorption is a suitable procedure due to the simplicity of design, inexpensiveness, ease of operation, rapidity and availability of various adsorbent materials without formation of toxic substances [38,[40][41][42].The physical and chemical properties of adsorbate and adsorbent can affect the surface phenomena of adsorption process [43].Several characteristics like non-toxicity, cost effective technology, availability, high surface capacity, and stability in various environmental conditions make an adsorbent suitable and effective [1,38].
Activated carbon prepared from carbon-rich agricultural materials during pyrolysis methods is a suitable candidate for contaminates removal, and this material is one of the best adsorbent in adsorption technologies, according to the US Environmental Protection Agency.But this process has some limitations such as more cost and difficult to recycle as well as requiring chelating agents (for metal removal) have limited its application [38,42,61,62].Besides other materials, agricultural-based bio-adsorbent plays an important role in wastewater treatment.Bio-based adsorbents have certain advantages like more availability, no waste production and negative impact to environment.In this contest, RH and rice straw (RS) as by-products in rice production and milling process can fulfill these criteria.According to the report of Food and Agricultural Organisation of the United Nations (FAO), rice represents the second-largest share of any crop around the world [63].
Therefore, the use of waste products such as RH and RS can be more potential and cost-effective method.Hence, keeping in mind the importance of bio-adsorbents towards wastewater treatment, in this work we review the removal efficiency of toxic contaminates like dyes and heavy metal ions by bare RH and RS, chemically modified RH and RS, functionalised RH and RS are also investigated.The environmental impact, economy, and regeneration of RH and RS were examined.This study promotes the advancement of RH and RS utilisation level and the goal will promote the innovative step towards heavy metal and dyes removal.

Taxonomy and botany of rice
Figure 2 signifies the taxonomy of rice.According to the International Code of Botanical Nomenclature (ICBN), rice is the seed (grain) of the grass plant (family -Poaceae) species most commonly named as Oryza Sativa (Asian Rice) and less commonly as Oryza Glaberrima (African Rice).It belongs to the genus 'Oryza' which comes under the 'Poaceae' family of plantae kingdom.Rice is a cereal food crop that can be regarded as a staple food in developing nations.Next to China in rice production, India stands second across all other countries [64].At present, there exist almost 40,000 varieties [65] of rice but only a small number of them are widely cultivated, milled and polished as well.Despite most people, all over the world consuming white rice, some special varieties of rice are also cultivated such as aromatic rice, coloured rice varieties, etc.These special varieties of rice possess high nutritional profile as compared to the variety of white rice.The colour and aroma of these special rice varieties are the result of settling of the pigments of anthrocyanin and acetyl-1-pyrroline in the inner rice bran layer, respectively [66].Depending upon the soil fertility and variety, the rice plant grows up to 1-18 m in height.The leaves are long and slender, up to 50-100 cm and 2-2.5 cm broad.The tiny wind pollinated flowers are formed in a branched arching to pendulous inflorescence 30- 50 cm long.The edible seed is a grain (caryopsis) which measures about 5-12 mm long and 2-3 mm thick [67].The mass of one grain of rice is approximately 0.020 g [66].Rice can be grown practically anywhere, even on a steep hill or mountain area with the use of water-controlling terrace systems.But it is highly suited in countries having low labour cost and sufficient rainfall because it is labour intensive and requires a large amount of water to be cultivated [68].RH and RS are both the by-products of paddy rice and the main agricultural biomass in India.

Rice husk (RH)
Rice husk is a milling by-products or external layer of rice with 20% w/w amount of the whole paddy produced.India is one of the leading countries with large rice consumption and exporter in the world [15,69].Most of the Indian people consume rice as a staple food.Rice is an important food and cultivated in many countries (approximately 75 countries).About 80 million RH produced annually in the world [28].Some properties of RH such as granular structure, high chemical stability and mechanical strength, local availability, insolubility in water cause to select the RH as potential bio-adsorbents [15,25,28,70].Approximately, 20-25% of RH was obtained from every ton of processed rice and after burning in the farms site produces carbon dioxide and other contaminates [71].So by using RH as a potential bio-adsorbent towards wastewater treatment, the huge amount of secondary pollutant produced from rice milling can be well managed and pollution is also prevented.
RH has several applications such as bio-fuel, making bricks, biochar, animal feed, organic fertiliser additives, production of bioethanol, and composites [69,72,73].Some synthesised materials from RH such as silica, activated carbon, zeolite and ash have also been used for removal of pollutants from contaminated water [40].Silica and carbon are the main components of RH for adsorption [40].
The composition of RH was varied from one sample to another because of paddies type, fertilisers type, rice variety, soil chemistry, geographic and climate location differences [72][73][74].The RH includes some chemical composition such as 1.82% extractives, 8.11% water, 15.5% mineral ash (approximate 96.34% silica content), 21.34%, hemicelluloses, 21.44% lignin, and 32.24% cellulose [15,25].In addition, 2.0-2.5 mm width, 4-10 mm length, 96-160 kg/m 3 bulk densities and 0.1-0.15mm thickness are the physical characteristics of RH [41,75].Although direct usage of RH like other bio-adsorbents derived from agricultural sector cause leaching of its organic compound to the aqueous media [76], but this composition and desirable features make it to be of interest as a appropriate bio-adsorbent towards removal of dyes and heavy metals.

Rice straw
Approximately, 800-1,000 million tonnes of RS is produced worldwide, while RS is the byproduct of paddy crop.India and China produce around 600-800 million tonnes of RS annually [77].The process of rice production, that precedes straw collection, affects to a large extent on how and how much straw will be created in the field.There is a research gap in the literature on the exact lignin and carbohydrate composition of the RS.The lignin and cellulose substance of RS differ widely because of various factors such as genetic makeup of the cultivars, growing area of crop, season of harvesting and condition of storage [78].The overall carbohydrate composition of straw is 0.4% galactose, 1.8% mannose, 2.7-4.5% arabinose, 14-20% xylose, and 40-43% glucose [79].Jin and Chen in 2007 reported that 43 species of rice crop from various parts of China and observed that in straw composition contains moisture 6.9%, cellulose 33%, hemicelluloses 25%, acid insoluble ash 7.8%, total ash 11%, and klason lignin 10% [80].Those compounds provide active-binding sites for heavy metals and dyes [49,81,82].Therefore, RS should be a potential adsorbent for the scale-up treatment of wastewater containing dyes and heavy metals.

Synthesis of various RH and RS based adsorbent
In this section, we discussed two major synthetical parts of RH and RS such as the physical and chemical treatment are shown in Table S1a & S1b and Table S2a & S2b (Supplementary Materials).All methods show different particle size and surface area.In the physical method, first char is prepared then carbonised of RH and RS at temperatures (400-700°C) in presence of air or at inert atmosphere [83][84][85][86][87][88].The resulting char product exhibits low adsorption capacity because during carbonisation tarry products remain blocked in the pores.To open the pores it was required to provide high temperatures (600-1000°C) in presence of suitable oxidising agent such as steam, carbon dioxide, air or their mixtures [83,[86][87][88][89], this process is known as activation process.After the separation of the tarry products, the diameter of the pores is enlarged, while at the same time, a new porosity is formed.Moreover, a readily accessible and well-developed pore structure with a relatively high surface area is formed after the activation process.In the activation process, carbon is lost so the weight of the host carbon decreases.As compared to other materials, weight loss during physical activation of RH and RS char increases linearly with activation time and temperature.Activation of RH and RS char using air and inert atmosphere are the most commonly employed activating agents through physical activation as summarised in Table 1.Although in presence air or oxygen the activated RH and RS may be problematic because excessive burning of the char occurs due to the exothermic nature of the reactions, since it is difficult to control.Hence, a more developed microporous structure is developed when RH and RS char are activated in the presence of an inert atmosphere as compared to air or oxygen.However, in chemical method, the carbonised and chemical activation perform in a single-stage process, by employing chemical activating agents may be acids or bases at relatively low temperature 300-700°C [90][91][92][93][94][95].A single-step method employed to suitable for chemical activation, the process is only efficient when activating the RH and RS using acidic activating agents [84,85,96,97].But the single-step process is not efficient, when activating the RH and RS using basic activating agents [98].This is because the activating agent is unable to enter inside the bio-char and break down the cross-linkages or react with the resultant carbon sufficiently to produce abundant pores.So, the surface area is decreased [84,85,96].These are the demerits of the single-step process of basic activating agents.To overcome such problems, we have to proceed with a two-step process.In the first step, the precursor material is carbonised then chemically activated with basic activating agents [91,92,96].The surface area of the two-step process is more as compare to single-step process basic activating agents [91,92,96,99].A literature survey observed that the type of activating agents, pre-carbonised conditions,, impregnation ratio, heating rate, inert environment, activation temperature and time influence the quality of RH and RS materials and control the adsorption properties.

Characterisation of rice husk and rice straw
In order to demonstrate the adsorption capacity of adsorbent, various analyses were conducted.The essential characterisation techniques such as FTIR, XRD, SEM, and BET surface area analysis were presented to demonstrate the efficiency of adsorbent and uptake capacity.Therefore, it is essential to understand the adsorbent characteristics to know the affinity of adsorbent-adsorbate materials to achieve the desired removal of dyes and heavy metals.The FTIR analysis revealed the existence of a number of surface functional groups on adsorbent surface.In the region of 400-4000 cm − 1 , the FTIR samples were recorded.The most intense bands and their vibration modes of RH and RS are summarised in Table 1 [62,88,[106][107][108].Most of the common vibration modes were observed between RH and RS.After base treatment with RH, the Si-O-Si peak was less intense as compared to raw RH and the carboxylic ester group converted into the carboxylate and alcohol groups [107].
X-ray diffraction analysis (XRD) is a method for investigating the adsorbent materials crystallographic structure.XRD technique was used to determine the spacing between lattice planes, identify the crystalline phase present in the adsorbent materials, and also examine the epitaxial growth and preferential growth of crystallites within the materials [109].Purwaningsih et al. (2019) studied the XRD pattern of RH and showed a diffraction peak at 2θ = 16º and 34º, this angle attribute to the diffraction peak for cellulose ICDD#00-003-0226 while 2θ = 23º gives a intense peak that corresponds to silica phase obey ICDD#00-001-0424 [109,110].Ahmaruzzaman et al. (2011) reported that quartz is the only crystalline phase present in RH ash [111].
Scanning Electron Microscopy (SEM) and Field Emission Scanning Electron Microscopy (FE-SEM) are used to analyse the surface morphology of adsorbent materials, which focus on three dimensional solid adsorbent samples by generating high-resolution images.Lakshmi et al. (2009) showed the SEM image (surface texture and porosity) of RH ash for adsorption [112].Sharma et al. (2010) observed that the SEM image of RH showed fibrous character with 40-100 μm particle size and the RH ash had comparatively spherical smaller particle size [42].Liu et al. (2012) showed a higher number of microporous structure in the SEM images of RS ash and HCl treatment of RS bio-char and also reported RS ash contained amorphous flakes with few pores while RS bio-char had a more fibrous porous structure [113].Swelam et al. (2016) reported the SEM images of untreated and alkali (NaOH) treated RS, while untreated RS showed surface roughness of the inner and outer surfaces and after alkali treatment with RS showed porous and the surface structure is smoother [114].Moreover, small beds on RS are removed, which corresponds to the decrease in most hemicelluloses, lignin and other impurities.In the same year, Tan and their co-workers reported the SEM image of RS in CO 2 and N 2 atmosphere [104].In both atmospheres, it is shown that porous structure of RS and porous structure had a great contribution towards surface area and uptake capacity.
Surface area plays an important role for adsorption because higher surface area can provide higher pore volume for higher interaction between the adsorbent and adsorbate materials, which effects significantly on efficient and effective adsorption.Commonly, Brunauer-Emmett-Teller (BET) equation is used to examine the specific surface area which is known as BET surface area.Lakshmi et al. (2009) calculated that the surface area of RH ash was 36.44 m 2 /g and adsorption/desorption surface area of pores is 27.45/22.18m 2 /g [112].The adsorption and desorption pore distribution showed that the mesopores account for about 78% and 99%.So the 99% desorption mesopores predominance on mesopores in adsorption process.El-Sonbati et al. (2015) [115] and Hussien et al. (2016) [116] reported that the surface area of RS fly ash was 67.4 m 2 /g and also suggested RS fly ash consists of macropores, mesopores and micropores.Furthermore, Liu et al. (2012) characterised the BET surface area of RS ash (257 m 2 /g) which is comparatively higher than RS fly ash and RS bio-char (954 m 2 /g) [113].Recently, Abd-Elhamid et al. (2020) calculated that the surface area of bio-char prepared after pyrolysis step (p-Biochar) was 223.4 m 2 /g and activated p-Biochar using wet attrition process to give highly active materials (m-Biochar) was 104 m 2 /g, respectively [88].The pore sizes of p-Biochar and m-Biochar were 5-150 μm and 0.5-5.0μm, respectively.After activation both the surface area and pore size are reduced.

Rice husk and rice straw based adsorbent for removal of heavy metals and dyes
The application of RH and RS-based adsorbents towards heavy metals and dyes removal have been studied in details.The basic categories of heavy metals and dyes are discussed in details.The isotherm model, kinetic model, maximum uptake capacity (qmax), optimum pH and contact time are represented in Tables 2-5.The detailed study of various isotherm and kinetic models was very limited in almost all the reported articles but most of the authors fitting Langmuir or Freundlich in isotherm and pseudo first order and pseudo second order in kinetic model.

Amin et al. (2006) removed
As(III) and As(V) from aqueous solution using waste RH (WRH) [117].They implement column methods to check the concentration of As(III) and As(V) removal under the following conditions such as waste RH amount, 6 g; initial Arsenic concentration, 100 μg/L; pH, 6.5 and 6.0; and treatment flow rate, 6.7 and 1.7 mL/min, respectively.The desorption efficiency were in the range 71-96% after treatment of groundwater checked with 1 M KOH.Furthermore, Lee et al. (2010) [118] investigate the adsorption of As(V) from aqueous solution using quaternized RH (QRH) via both batch and column studies.The maximum uptake capacity was reported 18.98 mg/g at pH 7.5 and 28 ± 2°C.To enhance the uptake capacity of RH  [119].The experimental data were examined with various experimental parameters and found to be optimum pH 5, contact time 60 minutes, adsorbent dosage 5 g/ L. The negative Gibbs free energy value (∆Gº = −12.36)with respect to temperature 313 K showed the adsorption process spontaneous and feasible.The adsorption process was followed Freundlich isotherm model and the adsorption capacity was found to be 91.74mg/g.To enhance the adsorption capacity of Pb(II), Dada et al. (2013) [120] modified RH with orthophosphoric acid (PRH) and reported the adsorption capacity 138.89 mg/g.Kumar et al. (2006) investigated the removal of Cd(II) by RH with different modifications such as NaOH treated RH (NRH), sodium bicarbonate treated RH (NCRH), epichlorohydrin treated RH (ERH) [71].They observed the increasing sorption capacity of RH which was found to be 20.24,11.12 and 16.18 mg/g in equilibrium time 4, 2 and 1 h, respectively, for NRH, ERH and NCRH while the raw RH showed a sorption capacity of 8.58 mg/g.In this experiment they studied the sorption kinetics, pH as well as isotherms in batch experiments.The experiment was well established for pseudo-second-order kinetic model and Langmuir adsorption isotherm.Considering all the three modifications the NCRH was found to be the most efficient alternative for adsorbing Cd(II).
Ghorbani et al. ( 2011) prepared a polyaniline (PAn) modified RHA nanocomposite for the removal of toxic Hg(II) metal from the waste water [121].The removal studies were compared in a separate manner for un-treated RHA, polyaniline and PAn coated RHA, respectively.Among all, it was seen that the PAn-modified RHA showed greater efficiency in the removal of mercury.The batch adsorption was carried out with various parameters, such as contact time, pH, adsorbent dosage and rotating speed.The optimum condition for the removal of Hg(II) was found to be adsorbent dosage 10 g/L, pH 9, rotating time 400 rpm and equilibrium time 20 min.Chang et al. (2007) adsorbed Cu(II) from industrial wastewater using rice husk ash (RHA) successfully [122].To check the regeneration efficiency of adsorbent, they washed the copper-loaded adsorbent with nitric acid and sodium thiosulphate and reported sodium thiosulphate (Na 2 S 2 O 3 ) is an effective reagent as compared to HNO 3 .But XAS result showed that this treatment resulted in the transformation of Cu(II) to Cu(I).So, a more convenient way is to use nitric acid (HNO 3 ) in washing the copper loaded RHA.Again in 2012, Ye et al. prepared the modified RH (RHT298) to increase the removal efficiency for the adsorption of Cu(II) [123].According to the report, the adsorption capacity was seen to be 45.5 mg/g at concentration 400 mg/L and pH 7.0.The batch equilibrium and kinetic processes were well fitted to both Langmuir adsorption isotherm and Elovich kinetic model.Mohan et al. (2006) prepared the phosphate treated RH (RHT333) for the removal of toxic heavy metals like Copper (Cu), Lead (Pb), Zinc (Zn) and Manganese (Mn) [124].They also compared the adsorption capacity of untreated (RH) and treated RH (PRH) by batch equilibrium studies in contaminated wastewater.The batch adsorption including optimum pH, dose of adsorbent, contact time were also calculated by conducting the process separately for both RH and PRH.It was observed that the equilibrium was established after 30 min (pH = 6), 40 min (pH = 7) and 50 min (pH = 7) for Pb, Cu, and both Zn and Mn, respectively.The experiment was observed to be well fitted for Freundlich adsorption isotherm.Furthermore, in 2008, Mohan et al. and their co-workers modified the process with a fixed bed column and adsorbed these heavy metals along with a new heavy metal ion such as manganese [125].In the same year, Srivastava et al. studied the competitive removal capacity of RHA adsorbent on the ions of nickel (Ni(II)) and Cadmium (Cd(II)) present in wastewater [62].In their experiment, it was observed that the adsorption capacity of RHA was greater for Ni (II) as compared to Cd(II).The experiment was well established for Freundlich adsorption isotherm.After that, in 2018, Ei-said AG et al. reported the adsorption of Cd(II), Hg(II) and Zn(II) metal ions on the surface of RHA adsorbent [126].The results of this experiment revealed that the adsorption efficiency of RHA for the heavy metals was in the order Hg (II) < Cd (II) < Zn (II).The experiment was well-established for both Langmuir and Freundlich isotherms.

Single dye removal
P.K. Malik (2003) prepared an activated carbon (RHT673) for the treatment of acid yellow 36 dye present in sewage water [86].The result inferred that at pH 3 it adsorbed 86.9 mg/g dye with proper rate limiting intraparticle diffusion constant [127].The isotherm was well favoured for both Langmuir and Freundlich adsorption isotherm with Lagergren first order kinetic model.
Another researcher, Mane et al. (2006), adsorbed Brilliant Green Dye (BG) using RHA by batch adsorption studies [16].They performed various parameters of batch adsorption such as pH, adsorbent dosage, contact time, concentration for the adsorption of BG dye.The optimum condition in this experiment was found to be at pH 3.0, and the initial concentration 50-300 mg/L with adsorbent dosage 6 g/L the batch equilibrium was obtained after a time interval of 5 h.The experiment was well influenced by Langmuir adsorption isotherm and showed the properties of feasibility and spontaneity with high negative ∆G 0 value.In 2013, Tavlieva et al. adsorbed the same BG dye on white RHA adsorbent (WRHA) with the same batch adsorption studies [128].In this case, the concentration was in the range 3-100 mg/L at temperature 290-320 K and the maximum adsorption ability was found to be 85.56 mg/g.The experiment was well suited for pseudo-second order kinetic model.
Mohanty et al. reported a comparative adsorption study of crystal violet dye with adsorbent of RH (RHT373) modified with H 2 SO 4 and ZnCl 2 in a separate manner [129].The experiment was well established for both Langmuir and Freundlich adsorption isotherms and intraparticle diffusion model.The q max value for H 2 SO 4 and ZnCl 2 modified RH were found to be 64.875 and 61.57mg g −1 , respectively.
Lakshmi et al. in 2008 prepared a RH adsorbent (RHT473) for the extraction of Indigo Carmine Dye [112].In this experiment, various parameters such as initial pH value, adsorbent dosage, contact time and initial concentration were determined by batch adsorption studies.The optimum conditions were observed to be at pH = 5.4, dosage = 10 g/L and equilibrium time = 8 h.The process was favoured by Freundlich adsorption isotherm with q max value 65.9 mg/g at 323 K temperature.Han et al. (2007) conducted a fixed bed study for the adsorption of Congo red dye (CR) from wastewater [89].The batch adsorption was studied to determine the various parameters such as pH, initial concentration, flow rate as well as the bed depth.The beddepth/ service time analysis (BDST) model was well influenced by the experiment.Sharma et al. (2010) compared the adsorption study of pre-treated RH and RHA for the removal of methylene blue dye (MB) from wastewater [42].The data was well-fitted for both Langmuir and Freundlich adsorption isotherm with q max value 1347.7 mg/g and 1455.6 mg/g at temperature 323 K.
Sivakumar et al. (2012) suggested a novel nanocomposite using RH based bioadsorbent (RHT378) for the removal of direct red 23 dye present in toxic sewage water [130].The raw RH was washed with 0.6 M citric acid at 120°C temperature.This experiment showed a greater efficiency for the removal of direct red 23 dyes.In the same year, Taha et al. studied the adsorption of Janus Green B dye by taking in the consideration the adsorbent as RH ash (RHA) [131].The experiment followed both Langmuir and Freundlich adsorption isotherm with pseudo-second order kinetics.

Multi dyes removal
In the year 2016, Li et al. suggested a batch adsorption process to describe the removal of Methylene blue, Methyl orange and Neutral red dye [132].The process involved various optimum conditions such as pH, initial concentration and contact time.The process followed pseudo-second order kinetics with optimum pH = 3, initial concentration 50-450 mg/L.In 2020, daRocha et al. modified the RH adsorbent with PMMA for the removal of tatrazine (E102) and indigo carmine (IC) [133].The experiment was followed by a batch adsorption process with optimum pH = 2 and contact time 150 min and the adsorption capacity was found to be 165.7 mg/g and 142.9 mg/g for E102 and IC, respectively.The process was well-fitted for Langmuir adsorption isotherm with pseudo-second order kinetics.Amer et al. (2017) studied the removal of Pb 2+ ion from waste water [134].They prepared the RS bio-adsorbent (RST353) and experimented with the batch adsorption process to study the feasibility of the method.Various operating parameters such as pH = 5.5, contact time 30 min, initial concentration 40 mg/L and particle size 75-150 μm and a dose of 4 g/L.The reaction was well-fitted to the Langmuir adsorption isotherm with q max value 42.55 mg/g and 94% adsorption efficiency.Elmolla et al. (2015) compared the adsorption capacity of raw RS and its activated form for the removal of Cr(III) metal separately in the tannery sewage water [94].They performed various batch adsorption processes to study the feasibility of the reaction individually for rice straw (FRS), its carbonised form (RSC) and activated carbon form (RSAC).The best operating condition was found to be at pH = 2, and dosage 8-20 g/L and contact time 4-8 h for all the adsorbents.The study followed Langmuir adsorption isotherm with q max value (for RSAC) 40.32 mg/g.But this study showed the efficiency upto 97% adsorption of chromium ion only for a limited ratio of adsorbent and adsorbate.Swelam et al. (2018) adsorbed hazardous Co(II) from waste water using RS adsorbent [114].They enhanced the removal capacity of the rRS by modifying with alkali (AOH functional group).The batch adsorption was carried out by the amount of adsorbent, pH, concentration of adsorbate, isotherms and kinetics.The equation was well established for pseudo second-order kinetics and Langmuir adsorption isotherm.The maximum adsorption capacity of the adsorbent was found to be 46.1 mg/g.Again, in the same year, Tan et al. suggested a novel bio-adsorbate for the removal of most toxic and concealed heavy metal Cd(II) [104].They proved that the adsorbent as the best one for the removal of cadmium and the process was studied with pyrolysis conditions with iodine absorption tests.The biochar was seen to be very efficient towards the removal of Cd(II) ion with an efficiency of 91%.

Multi metals removal
Alia et al. (2018) reported a CoFe 2 O 4 modified RS for an efficient adsorption of Cd, Cu, Zn, Mn, Fe, Pb and Ni [135].A spinel ferrite nanocomposite was used in the modification of RSbiochar in this process.In this experiment, the removal percentage of Mn and Fe are 92.45% and 84.25%, respectively, and the rest listed metals showed 100% efficiency.Nawar et al. (2013) compared the removal efficiency of heavy metals like Fe, Zn, Cd, Mn and Pb [87].The experiment showed the result as Fe(III)< Zn(II)< Mn(II)< Cd(II)<Pb(II).They investigated various batch equilibrium processes such as pH, adsorbent dosage, initial concentration of the adsorbate and contact time.The removal efficiency of all the metal ions was found to be 40 g/L excluding Fe (60 g/L).The experiment was well-fitted to Langmuir adsorption isotherm with an efficiency of 99% at pH value equal to 1. Mostafa et al. (2012) studied the removal of a number of toxic heavy metal ions like Cu(II), Cd(II), Hg(II) and Pb(II) using a DMAEM modified RS biochar [136].The pH taken in the batch process was found to be 8 and the dosage varied from 0.50 to 2.0 g at time interval 20 min showing greater removal efficiency.The adsorption study of Cu (II) and Pb(II) using magnetic nanocomposites of RS (RS/Fe 3 O 4 -NCs) prepared by the method of co-precipitation were studied by Khandanlau et al. (2015) [105].They utilised the batch adsorption process by investigating the initial ion concentration, pH value, removal time and adsorbent dosage.The experiment was well established for Langmuir adsorption isotherm and pseudo second order kinetics with the removal percentage of 75.54% and 96.25% for Cu(II) and Pb(II) respectively.The removal time was observed to be 59.35 and 41.96, respectively, for both the heavy metals, and the dosage was of 0.13 g.Brahmaiah et al. (2016) the efficiency of removal of Ni and Cr using the RS adsorbent [137] studied.The batch adsorption was well-established for Freundlich adsorption isotherm with a greater efficiency of removal.Again in the year of 2008, Krishnani et al. removed a number of heavy metals like Pb, Cd, Hg, Co, Ni, Mn, Zn, and Cu by batch adsorption method and the removal efficiency was found to be 0.286, 0.147, 0.226, 0.181, 0.196, 0.189, 0.219, and 0.248, respectively [138].

Single dye removal
El-Bindary et al. ( 2016) reported the removal of reactive blue 19 (RB19) using RSFA [139].The batch equilibrium study was made with respect to contact time, pH, adsorbent dosage, concentration of adsorbate.The optimum pH was taken as 7 and the efficiency was found to be more than 85%.Firdaus et al. (2017) reported a modified form of nanocomposite of RS for the removal of Naphtol AS-G dye [140].The optimum pH was found to be 6 at a contact time of 60 min.The adsorption capacity was found to be 69.44 mg/g with an efficacy percentile of 95%.The adsorption followed Langmuir adsorption and pseudo-second order kinetics.
Sonbati et al. (2015) studied the potential of RSFA for the removal of azorhodanine dye (AR) from wastewater [141].Different parameters of the batch method such as contact time, pH, adsorbent dosage, concentration of adsorbate and temperature were determined and optimal experimental conditions were as certain.0.05 g for initial dye concentration of 20-100 mg/L at pH 2. The experiment was well-established by Freundlich adsorption isotherm with greater efficiency.[143,144].The reaction followed Langmuir adsorption isotherm with maximum adsorption capacity of 20.2 and 16.56, respectively.Gong et al. (2006) studied the removal of various organic dyes such as BR5 and BB9 using a phosphoric acid modified RS [145].The effect of various parameters like pH, dye concentration, sorbent dosage, ion strength and contact time were also studied.

Multi dyes removal
It was inferred that at pH = 4, the 1.5 g/L sorbent removed 250 mg/L dye concentration with 96% efficiency.The data was well-fitted to Langmuir adsorption isotherm and pseudo-first order rate kinetics.In the same year, Gong and his co-workers, modified the RS with citric acid and adsorbed the MG dye at pH = 4 with 93% of efficiency [146].
Elhamid et al. (2020) studied the conversion of agricultural bio-waste (rice straw) into ecological cleaning material (biochar) with the application of airless pyrolysis followed by eco-friendly activation [147].The biochar formed from direct pyrolysis was poorly activated (p-biochar) which was again subjected to wet attrition process to give highly activated material (m-biochar).Both the m-biochar and p-biochar were used as adsorbent for crystal violet (CV) and methylene blue (MB) dye adsorption.The investigation was made on different parameters like adsorption dosage, temperature, concentration, contact time, pH, NaCl dose, etc. which affect the adsorption.The adsorption isotherm was fit for Langmuir adsorption isotherm and pseudo-second order kinetics.Adsorption capacity was 44.64 and 90.91 for CV and MB respectively.Comparing the contact time it had been observed that p-biochar was a poor adsorbent.But in case of m-biochar the equilibrium reached at 15 min (MB dye) and 20 min (CV dye) and the percentage of removal was 94.45% and 92.70%, respectively, for MB and CV.Moreover, they observed that m-Biochar highly adsorbs the MB dye as compared to the CV dye over all investigational conditions.

Conclusion
This article recapitulates the adsorption of different pollutants present in waste water which could be decontaminated through RH and RS.RH and RS being low-cost adsorbents are proved themselves as the best potential for this purpose.Not necessarily to say, these are less expensive and eco-friendly as well.Experiments have shown that locally accessible agricultural wastes could easily be transformed to their respective carbonised forms (charcoal) or activated carbon.An extensive study has been carried out in this review about the adsorption of dye and heavy metal contaminants from sewage water using agriculture based activated carbon (particularly using RH and RS).For this study, more than 130 research papers related to RH and RS as adsorbent are collected and discussed.A comparison of RH and RS-based material versus some other adsorbents are also discussed.From the results, it is observed that RH and RS are more effective as compared to other adsorbents due to their huge amount of production.The effect of other factors like pH, contaminant concentration, particle size, temperature, dosage of RH and RS, and contact time on adsorption has been analysed to assess the performance.Furthermore, various pretreatment procedures for RH and RS were examined which is the major challenge in this ground.As far as the results are concerned, RH and RS have shown greater efficiencies after the pre-treatments when compared to un-treated forms.Right now, as compared to other bio-adsorbents, RH and rice straw (RS) have given away their potential in the direction of being a hopeful eco-friendly, cost-effective as well as renewable adsorbent material in working out the problem of water contamination.Hence, in the literature Chemical methods of treatments are used more than thermal treatments.In general, this review spotlights the advantages of RS and RH for removal of many textile dyes and heavy metals.But, more investigation is necessary for the potential of these adsorbents in large scale.

Future scope
After cautious contemplation of the described review articles in past decades for the adsorption of carcinogenic dyes and heavy metals, this comprehensive analysis also predicted a few research gaps for further exhaustive and systematic studies as well as technological progress that are required for current systems.The suggested recommendations are as follows: In major cases of review analysis, only a few attempts are carried out to study the characterisation results by means of their performances towards adsorbate under various conditions.Many expensive and sophisticated analytical types of equipment are used for the characterisation of these adsorbent materials.However, in many cases, they become unable to yield the necessary information which the evaluator needs for the concerned adsorption study.Hence, a significant, systematic and detailed assessment is required to evaluate their morphology, surface area, adsorption capacity, functional group, etc.This will improve the percentage of adsorption of target dye and heavy metal contaminants.
The appliance of untreated solid bio-wastes as adsorbents can lead to the production of a large amount of organic compounds.These organic compounds which are leached in the experimental analysis must be studied carefully otherwise it may infer some erroneous results.
Until now, adsorption processes are at the step of laboratory-scale batch studies.So, furthermore study is needed in this area to stretch its extent upto industrial scale by designing various techniques.
Most of the research techniques are focused on removal and adsorptive efficiency of the adsorbent.But more focus is needed on sustainable valorisation of post-sorption materials which can be taken as the alternative for chemical products like catalyst, fertilisers, feed additives, etc.
The feasible study on the cost effectiveness of bio-adsorbent for waste water treatment must be conducted for the practical use in industrial scale.

Figure 1 .
Figure 1.Classification of dye on the basis of their application.

Table 3 .Table 4 .Table 5 .
List of isotherm, maximum adsorption capacity, optimum pH/time, and kinetic models for describing the removal of dyes on RH-based adsorbent.List of isotherm, maximum adsorption capacity, optimum pH/time and kinetic models for describing the removal of heavy metals on RS-based adsorbent.)/ Hg(II)/ Co (II)/ Ni (II)/ Zn (II)/ Cu(II)/ Cd(II) Langmuir 0.286/0.147/0.226/0.181/0.196List of isotherm, maximum adsorption capacity, optimum pH/time and kinetic models for describing the removal of Dyes on RS-based adsorbent.
Jain et al. (2007) obtained a novel nanocomposite of RS for the removal of Rhodamine B dye from sewage water [142].The experiment was well-suited for Freundlich adsorption isotherm with pH = 2 and contact time 20 min.Again in Liu et al. (2012) and Zhang et al. (2016) adsorbed the MB dye from sewage water

Table 1 .
FT-IR peaks of RH and RS.

Table 2 .
List of isotherm, maximum adsorption capacity, optimum pH/time and kinetic models for describing the removal of heavy metals on RH based adsorbent.
[75]al et al. (2009)011)[102]modified polyaniline on RH (PAn/RH) and obtained the uptake capacity 34.48 mg/g.They also estimated some thermodynamic parameters such as enthalpy (∆Hº) and entropy (∆Sº) were −22.49kJ/mol and −0.066 kJ/(mol K), respectively.The negative Gibbs free energy value (∆Gº = −2.13)withrespect to temperature 30°C showed the adsorption process spontaneous and feasible.Bansal et al. (2009)prepared pre-treatments RH by boiling and formaldehyde treatment, which represented BRH and FRH[75].Both the adsorbent removed Cr(VI) i.e. 71.0% (BRH) and 76.5% (FRH), respectively, at pH 2.0.The experimental values reveal that the Freundlich and D-R models best fit for Cr(VI) adsorption.Compare the adsorption capacity of BRH and FRH was not much difference.So, BRH is a potential candidate for the adsorption of Cr(VI) from aqueous solution because no chemical used in this preparation.BRH is attractive preference for small-scale industries.